Flex circuit and electrical communication assemblies related to same

Abstract

Flex circuit embodiments are provided having high signal conductor density and high signal integrity. Electrical communication systems are described that are configured to be placed in electrical communication with the flex circuits. Electrical communication systems are described that include an electrical connector that is selectively intermatable with an electrical connector that is mounted to a flex circuit, and an electrical connector that is mounted to a substrate such as a printed circuit board (PCB).

Description

FLEX CIRCUIT AND ELECTRICAL COMMUNICATION ASSEMBLIES RELATED TO SAMECROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Patent Application Serial Number 63 / 515,158, filed July 24, 2023, the disclosure of which is hereby incorporated by reference as if set forth in its entirety herein.BACKGROUND

[0002] High data rate communication and processing is provided by integrated circuits (ICs), which can generate and process terabytes per second (Tbps) of information. Within the integrated circuit, information is transmitted by narrow (<10nm) electrically conductive traces and processed by thousands or millions of transistors. ICs are typically packaged in the form of an IC die which is mounted on a die package substrate to form a die package or an IC package. In turn, the IC package is mounted to a host substrate. The host substrate has electrical traces, and these electrical traces can produce unwanted, parasitic insertion loss and other undesirable signal transmission qualities.

[0003] An earlier approach to mitigate unwanted and undesirable signal transmission losses in a host or circuit board substrate is disclosed in United States Patent No. 6,971,887, hereby incorporated by reference in its entirety. This patent discloses using an external substrate to couple first and second socket elements. The external substrate has a dielectric with a lower electrical loss tangent value than a dielectric that comprises the circuit board substrate. Signals may transfer through the external substrate at a rate of 12 gigatransfers per second (GT / s)+ at a distance of about six inches. In general, United States Patent No. 6,971,887 teaches connecting central processing unit (CPU) sockets with an external substrate so that high-rate signals bypass the host or circuit board substrate.

[0004] Another approach at mitigating unwanted and undesirable signal transmission losses in host substrates is described at pages 26 and 27 of the book “Flexible Circuit Technology”, Third Edition, Joseph Fjelstad, BR Publishing, Inc. (2006). Mr. Fjelstad writes, “While the historical role of flex circuits was most often as a wire harness replacement, the technology has grown well beyond such mundane applications. Today, flexible circuits are continuing to increase the breadth of their application. Electronic packaging engineers around the world are devising newer ways of using flex circuits and areexpanding on the basic promise of the technology by developing ever more fanciful, yet practical, electronic interconnection structures. It is worth exploring briefly some of flexible circuit technology’s unique abilities to increase electronic circuit packaging density and performance in terms of some of the many novel applications that are either in use or in development. Some of the new applications and approaches to the use of flexible circuit technology have further demonstrated the ability of the technology to increase circuit density in unusual ways, such as in IC packaging where the new package structures typically occupy a small fraction of the volume of more conventional design approaches. High-speed flex circuit assemblies have proven a viable alternative for high-speed applications for board-to- board distances up to 75mm (30 inches) at data rates up to lOGbps with the flex circuit integrated directly into connectors. An example is shown in Figure 2-14 (High speed flex cables can be directly connected from package to connector in order to bypass parasitics and avoid crosstalk issues associated with traditional interconnection design.) Commonly available high-speed flex circuit products are available in pitches down to 0.5mm (0.020") and less for both differential pair and single-ended configurations. With the move to ever- higher data transmission speeds, these types of flexible circuit applications will become increasingly important. High-speed structures made possible by high-speed cables will be discussed in more detail later.”

[0005] In general, instead of providing a jumper between at least two CPUs or at least two CPU sockets, Mr. Fjelstad discloses using flexible circuit material to bypass the host substrate and define a flex cable connection between a differential pair of a right-angle backplane connector and a die package substrate for signaling up to lOGbps.

[0006] United States Patent No. 8,353,708, entitled, “Independent Loading Mechanism Facilitating Interconnections for Both CPU and Flexible Printed Cables” generally discloses electrically connecting a CPU with a printed circuit board and achieves high-speed signal transmissions between CPUs through cables.

[0007] Moving forward approximately five more years, United States Patent Publication No. 2016 / 0218455, entitled, “Hybrid Electrical Connector For High-Frequency Signals”, filed by the Applicant and hereby incorporated by reference in its entirety, discloses that electrical traces in the host substrate have much higher loss than an optical or shielded cable and are far more susceptible to interference and crosstalk. US Publication 2016 / 0218455 proposes shortening the electrical traces in the host substrate to about 5mm or 10mm from the IC and connecting twin axial cable to the electrical traces in the host substrate.

[0008] United States Patent Publication 2021 / 0265785, entitled, “Cable Connector System”, filed by the Applicant and hereby incorporated by reference in its entirety, discloses, “In total, on both the first and second surfaces of the die package, a die package in the range of approximately 140 mm by 140 mm to approximately 280 mm by 280 mm can carry at least 1024 twin axial pairs or 2048 individual cable conductors which are routed to respective first electrical panel connectors. ..”

[0009] United States Patent Publication No. 2021 / 0289617, entitled, “Alternative Circuit Apparatus For Long Host Routing” and hereby incorporated by reference in its entirety, discloses a circuit assembly. The circuit assembly includes a package comprising a multi-level BGA / chip carrier and a package to board flex circuit. The BGA / chip carrier includes an IC including a first BGA mounted to the chip carrier / interposer board comprising a PCB or substrate that is interposed between a first BGA and a second BGA mounted to a multilayer PCB via a first set of BGA pads patterned on an upper layer of a multilayer PCB. The left end of a flex circuit is mounted to the topside of chip carrier by means of a BGA, while the right end of the flex circuit is mounted to a multilayer PCB by a second set of BGA pads patterned on the upper layer of the PCB. The second set of pads are electrically connected to the connector via wiring in a layer. A high-speed data channel can have a bandwidth of at least 50 Gbps.

[0010] Finally, United States Patent Publication No. 2022 / 0140514, entitled, “Flex Circuit And Electrical Communication Assemblies Related To Same” and hereby incorporated by reference in its entirety and attached hereto as Appendix A, discloses a flex circuit assembly.SUMMARY

[0011] In accordance with one aspect of the present disclosure, a flex circuit and associated interconnects facilitates the routing of at least 1024 differential signal pairs from a single surface of an IC die package, a single surface of a die package substrate, or a signal surface of a communication module; attaching flex circuits to at least two, at least three, or at least four die package sides of a die package substrate; and a hybrid cable assembly that includes a combination of a flex circuit or circuits and cables, alone or in combination with an end of one electrical connector and / or an end of two electrical connectors.BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The foregoing summary, as well as the following detailed description of illustrative embodiments of the electrical communication system of the present disclosure, will be better understood when read in conjunction with the appended drawings. For the purposes of examples of the present disclosure, there is shown in the drawings illustrative embodiments. It should be understood, however, that the present disclosure is not limited to the precise arrangements and instrumentalities shown. In the drawings:

[0013] Fig. 1 A is a top perspective view of an interconnect system including a die package assembly having a die package substrate and an integrated circuit (IC) chip mounted to the die package substrate, and a plurality of flex circuits mounted to the die package substrate;

[0014] Fig. IB is an exploded view of the interconnect system of Fig. 1A.

[0015] Fig. 2A is a perspective view of a portion of a multi-layer flex circuit of Fig. 1 A including signal conductors arranged in pairs;

[0016] Fig. 2B is a side elevation view of a stack-up depicting connections between layers of the flex circuit shown in Fig. 2A;

[0017] Fig. 2C is another side elevation view of a stack-up depicting connections between layers of the flex circuit shown in Fig. 2A

[0018] Fig. 3 is an exploded view of the first ends of a first and second ones of the flex circuits adjacent a die package footprint;

[0019] Fig. 4A is a sectional view along a plane including the transverse and longitudinal directions of the first end of the die package substrate, and first, second, third, and fourth ones of the flex circuits coupled to the die package substrate;

[0020] Fig. 4B is a sectional view along a plane including the transverse and longitudinal directions of the first end of the die package substrate, and first, second, third, and fourth ones of the flex circuits coupled to the die package substrate;

[0021] Fig. 5 is a top perspective view of a second side of the first end of the flex circuit of Fig. 2A;

[0022] Fig. 6 is a partially exploded top perspective view of a portion of the interconnect of Fig. 1A including an IC die coupled to a substrate, and first, second, third, and fourth ones of the flex circuits adjacent the die package substrate;

[0023] Fig. 7 is a bottom perspective view of the second one of the flex circuits;

[0024] Fig. 8 is an enlarged view of the die package footprint of Fig. 1A;

[0025] Fig. 9 is a partially exploded top perspective view of a second end of the flex circuit with a housing coupled to the second end;

[0026] Fig. 10 is an enlarged perspective view of a portion of the second end of the flex circuit;

[0027] Fig. 11 is a top perspective view of a portion of a flex circuit having two grounds between adjacent differential signal pairs;

[0028] Fig. 12 is a top perspective view of a portion of a flex circuit having two grounds between adjacent differential signal pairs and a row of grounds between rows of differential signal pairs; and

[0029] Fig. 13 is a top perspective view of a flex circuit having two grounds between adjacent differential signal pairs and rows and columns of grounds between rows and columns of differential signal pairs.DETAILED DESCRIPTION

[0030] The present disclosure can be understood more readily by reference to the following detailed description taken in connection with the accompanying figures and examples, which form a part of this disclosure. It is to be understood that this disclosure is not limited to the specific devices, methods, applications, conditions or parameters described and / or shown herein, and that the terminology used herein is for the purpose of describing particular embodiments by way of example only and is not intended to be limiting of the scope of the present disclosure. Further, reference to a plurality as used in the specification including the appended claims includes the singular “a,” “an,” “one,” and “the,” and further includes “at least one.” Further still, reference to a particular numerical value in the specification including the appended claims includes at least that particular value, unless the context clearly dictates otherwise.

[0031] The term “plurality”, as used herein, means more than one. When a range of values is expressed, the range extends from the one particular value to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another example. All ranges are inclusive and combinable.

[0032] The term “substantially,” “approximately,” “about,” and derivatives thereof, and words of similar import, when used to described sizes, shapes, spatial relationships, distances, directions, and other similar parameters includes the stated parameter in addition to a range up to 10% more and up to 10% less than the stated parameter, including up to 5% more and up to 5% less, including up to 3% more and up to 3% less, including up to 1% more and up to 1% less. If terms such as “equal”, “perpendicular”, or a numerical value associatedwith a given dimension are used to compare or describe elements of the invention, the terms should be interpreted as referring to within manufacturing tolerances.

[0033] As an overview, with all things being equal, a flex circuit has a higher differential pair density than two coaxial cables or a co-extruded twinaxial cable. However, flex circuit also performs electrically worse than an equal length of coaxial, twin axial or extruded waveguide cable. As the length of the flex circuit increases, the signal integrity performance degrades faster than the coax, twinaxial and waveguide cables. So, many have adopted twinaxial cables over flex for applications where signals are being transmitted at high speeds or data rates, such as 56G NRZ / 112G PAM4 signaling or 112G NRZ / 224G PAM4 signaling.

[0034] A problem with cables, however, is density. For example, a 34 AWG, 100 Ohm twin axial cable with a THV (thermoplastic elastomer) jacket is approximately 1.2mm wide. Center-to-center spacing of two immediately adjacent cable conductor differential pairs is at least 1.5mm with ground terminations and mechanical tolerance. So, a simplified equation to figure out the number of 34 AWG cables that can be attached to one of four sides or edges of the die package substrate is roughly (Side Length - 10mm (keep out)) / 1.5mm / pair. In some examples, a die package substrate has an edge length of about 50 mm.

[0035] As shown in Table 1, the size of fully shielded coaxial cables limits the number of cables that can be mounted to a side of a die package substrate that carries an IC die. In particular, as shown in Table 1, sixty twinaxial cables can be mounted to a side of a die package that is 100 mm in length. Thus, mounting large numbers of twinaxial cables to die packages using conventional techniques can become prohibitively difficult, even when the cables are stacked in multiple rows at each side of the die package substrate. For instance, four rows of twinaxial cables mounted to a 100 mm side of a die package substrate would produce 240 twinaxial cables mounted to each side of the die package substrate, resulting in a total of 960 twinaxial cables mounted to the die package substrate. Thus, the twinaxial cables can support 480 differential signal pairs that can be placed in communication with the IC die through the IC package substrate.Table 1: No. of 34 AWG Twin Ax Cables That Fit on One of Four Die Package Sides

[0036] Making die package substrates larger to accommodate larger cables is not always a practical solution because as the die package substrate sides get longer and the die package major surfaces grow in area, the more likely the die package substrate will warp, ‘potato chip’ or lose coplanarity during reflow. Increasing sizes of die package substrates also present challenges as the different materials of the die, the die package substrate, and the host board can all have different coefficients of thermal expansion. The different coefficients of thermal expansion can lead to stress on the solder balls or ball grid array that couple the components to each other as the components expand and contract at different rates. Further, in some architectures miniaturization of die package substrates can be desirable.

[0037] The present disclosure includes examples that increase the density of electrical cables that can be placed in electrical communication with a die package substrate. The electrical cables can route high-speed differential signal pairs to or from the die package substrate to an electrical component, a communication module or an electrical connector. For instance, the high speeds can be at least 28 gigabits per second (G) NRZ, 56G PAM-4, such as 56G NRZ, 112G PAM-4 and 112G NRZ, 224G PAM-4. In one example discussed in more detail below, one or more rows of flex circuits can be mounted to a die package substrate, thereby providing a greater density of electrical conductors compared to twinaxial cables mounted to the die package substrate. The flex circuits can be geometrically configured to achieve the high density at a first end having a length suitable for mounting to the die package substrate. The flex circuits can have a second increased length at a second end opposite the first end, such that the second end is configured to be attached to electrical cables which can be configured as coaxial or twinaxial cables. The second end of each flex circuit can attach to a greater number of electrical cables compared to the side of the die package substrate. Thus, each of the flex circuits, or a flex circuit assembly that includesmultiple rows of flex circuits, can define a converter that converts the length of the respective side of the die package substrate to the second greater length of the second end of each flex circuit. This solution also provides for the signal integrity benefits of electrical cables over long lengths of signal transmission.

[0038] Referring to Fig. 1 A, an interconnect system 100 can include a die package assembly 100 that includes a die package substrate 74 and an integrated circuit (IC) die 70 mounted to the die package substrate 74. In one example, the IC die 70 can be an application-specific integrated circuit (ASIC). The die package substrate 74 can be configured as a printed circuit board in some examples. The die package substrate 74 can include first and second major surfaces 200, 202 opposite each other in a transverse direction T (Fig. IB). The first and second surfaces 200, 202 can be planar along respective planes defined by a longitudinal direction L and a lateral direction A. The longitudinal direction can be perpendicular to the transverse direction and the lateral direction can be perpendicular to each of the transverse and longitudinal directions. In one example, the IC die 70 can be mounted to the first major surface 200 of the die package substrate 74.

[0039] The IC die 70 can be electrically connected to another component by the die package substrate 74. The die package substrate 74 can include electrical traces in electrical communication with the IC die 70. A flex circuit assembly 21 can include one or more flex circuits 20, such as two or more flex circuits, that can be directly mounted to the die package substrate 74 and thus placed in electrical communication with the electrical traces of the die package substrate 74, and therefore the IC die 70. The flex circuit assembly 21 can include flex circuits 20a, 20b, 20c, and 20d as shown, but the assembly 21 can include any number of flex circuits as desired. The two or more flex circuits can be compressed in the same compression direction, such as toward a major surface of the die package substrate 74. The compression direction can be oriented along the transverse direction T. Each flex circuit 20 can include a first end 134 that is configured to be mounted to the die package substrate 74, and a second end 136 opposite the first end 134 along the longitudinal direction. The first ends 134, and thus the flex circuits 20, can overlay a major surface of the die package substrate 74 along the transverse direction T.

[0040] The second end 136 of the flex circuit 20 can couple to another electrical component, such as an electrical connector 172 that, in turn, is mounted to a plurality of electrical cables 79. Alternatively, the electrical cables 79 can be mounted directly to the second end of the flex circuit 20. For instance, each flex circuit can include a plurality of electrical cable contact pads 41 that are in electrical and physical communication withrespective electrical signal traces of the respective flex circuit 20. In one example, the electrical signal traces of the flex circuits 20 can terminate at the electrical cable contact pads. The electrical cables 79 can be configured as twin axial cables or coaxial cables or dielectric waveguides or optical I / O modules that are in communication with optical engines 176. The optical engine 176 can convert electrical signals to optical signals and convert optical signals to electrical signals. The optical engine 176, in turn, can be coupled to one or more optical fibers 177. In some examples, the second end 136 of the flex circuit 20 can be coupled to a plurality of electrical connectors 172. Respective electrical connectors 172 can each define a separable, electrical connector mating interface. Each electrical connector 172 can be configured to mate and unmate with a cable connector 173 having the electrical cables 79 mounted thereto. Each cable connector 173 can carry one or more of: differential signal pair conductors physically attached, electrically attached, or both to corresponding cable signal conductors of the cables 79; ground conductors physically attached, electrically attached or both to corresponding ground shields or drain wires of the cables 79; and / or dielectric waveguides.

[0041] As described above, the flex circuit assembly 21 can include a plurality of flex circuits 20 with the first ends 134 of each flex circuit 20 stacked on each other along the transverse direction T. Further, the first ends 134 can be coupled to each other. In one example, the electrically insulative bodies 19 (see Fig. 2B) of the flex circuits 20 at their respective first ends 134 can be monolithic or otherwise bonded or joined to each other. The electrically insulative bodies 19 can be spaced from each other at the second ends 136 along the transverse direction T, such that the flex circuits 20 can be spaced from each other in a transverse direction T. The second ends 136 can be spaced from each other such that the second ends are coupled to one or more electrical connectors 172 as described above.

[0042] The flex circuit assembly 210 can include two or more flex circuits, such as the first flex circuit 20a and the second flex circuit 20b. The flex circuits 20 can also be referred to as flex cables. As discussed above, the first flex circuit 20a can be stacked with the second flex circuit 20b along the transverse direction T. For example, the first flex circuit 20a can be disposed such that a major surface of the first flex circuit 20a is adjacent to, or in contact with, a major surface of the second flex circuit 20b. Further, the flex circuits 20 can be compressed toward each other along the transverse direction T.

[0043] The cable contact pads 41a of the first flex circuit 20a can be vertically coincident with respective cable contact pads of the second flex circuit 20b. For example, a particular cable contact pad of the first flex circuit 20a can be immediately adjacent a cablecontact pad of the second flex circuit 20b. In some cases, a particular cable contact pad of the first flex circuit 20a can be vertically aligned with a respective cable contact pad of the second flex circuit 20b. In particular, a straight line that is oriented along the transverse direction T (i.e., normal to a major surface of a cable contact pad 41a of the first flex circuit 20a) extends through both the cable contact pad 41a of the first flex circuit 20a and the cable contact pad 41b of the adjacent second flex circuit 20b. In one example, the straight line can define respective central axes of the contact pads 41a, 41b, 41c, 41d, or a combination thereof. Otherwise stated, the footprints of vertically aligned or vertically coincident contact pads 41a and 41bcan entirely overlap each other in the transverse direction. The footprint can be defined in a plane that is perpendicular to the transverse direction T. Thus, respective cable contact pads of the first and second flex circuits 20a, 20b, can be vertically coincident with each other with or without an intermediate layer 188 present. Likewise the flex circuit assembly 21 can include more than two flex circuits 20. Thus, cable contact pads 41c of a third flex circuit 20c can be vertically coincident with respective cable contact pads 41a of the first flex circuit 20a and respective cable contact pads 41b of the second flex circuit 20b. Similarly, cable contact pads 41d of a fourth flex circuit 20d can be vertically coincident with respective cable contact pads 41a of the first flex circuit 20a, respective cable contact pads 41b of the second flex circuit 20b, and respective cable contact pads 41c of the third flex circuit 20c.

[0044] In some examples, the first and second flex cables are not received in a common flex cable connector (FCC) or a flexible printed circuit (FPC) connector.

[0045] Referring to Figs. 2B and 2C, the flex circuit 20 can be a multi-layer circuit. The flex circuit 20 can include a first flex circuit surface 23 a and a second flex circuit surface 23b opposite the first flex circuit surface 23 a along the transverse direction T. The first and second flex circuit surfaces 23a, 23b can be planar along respective planes defined by the longitudinal and lateral directions L, A. The flex circuit 20 can include first and second electrically conductive layers 22 and 24, respectively opposite each other, and thus spaced from each other, along the transverse direction T. The flex circuit 20 can further include an electrical signal conductive layer 26 that can include electrical signal conductors 26, disposed between the first and second electrically conductive layers 22 and 24 in the transverse direction T.

[0046] As best shown in Fig. 2B, the flex circuit 20 can include a flexible outer dielectric or electrically insulative body 19 that can define a first outer dielectric layer 23 that covers an outer surface of the first electrically conductive layer 22. The first electricallyconductive layer 22 is positioned between the first outer dielectric layer 23 and the plurality of electrical signal conductors 26. The flex outer dielectric or electrically insulative body 19 can further define a second outer dielectric layer 25 opposite the first outer layer 23 along the transverse direction T. The second electrically conductive layer 24 can be positioned between the second outer dielectric layer 25 and the plurality of electrical signal conductors 26. The electrically insulative body 19 can be insert molded, extruded, or otherwise formed around the electrically conductive layers 22 and 24, such that the electrically insulative body 19 coats all surfaces of the first and second electrically conductive layers 22 and 24 as desired. The first and second electrically conductive layers 22 and 24 can define respective outermost electrically conductive members of the flex circuit 20 with respect to the transverse direction T. The first and second outer dielectric layers 23 and 25 may define respective outermost layers of the flex circuit 20 with respect to the transverse direction T.

[0047] The electrically insulative body 19 can also include first and second inner dielectric layers 27 and 28, respectively opposite each other, and thus spaced from each other, along the transverse direction T. The first inner dielectric layer 27 can be situated between the first electrically conductive layer 22 and the plurality of electrical signal conductors 26. The first inner dielectric layer 27 can electrically isolate the first electrically conductive layer 22 from the plurality of electrical signal conductors 26. The second inner dielectric layer 28 can be situated between the second electrically conductive layer 24 and the plurality of electrical signal conductors 26. The second inner dielectric layer 28 can electrically isolate the second electrically conductive layer 24 from the plurality of electrical signal conductors 26. A bond sheet 29 may be situated between the first inner dielectric layer 27 and the plurality of electrical signal conductors 26. The bond sheet 29 may help to adhesively connect layers of the flex circuit 20 together.

[0048] The first electrically conductive layer 22, the second electrically conductive layer 24, and the plurality of electrical signal conductors 26 may be formed from copper or any suitable alternative electrically conductive material, such as a metal. The first and second outer dielectric layers 23, 25 may be formed from polyimide or any suitable alternative dielectric material. The first and second inner dielectric layers 27, 28 may be formed from a liquid crystal polymer. A liquid crystal polymer can have better dielectric properties than polyimide and thus it may be advantageous to use a liquid crystal polymer in an inner region of the flex circuit 20 where electric fields are present during circuit operation. A liquid crystal polymer has a lower dielectric constant and dissipation factor than polyimide. Also, unlike polyimide, it is not hydroscopic, so its dielectric properties are not affected by the presence ofwater. Alternatively, the first and second inner dielectric layers 27, 28 can be formed of any suitable alternative dielectric material as desired.

[0049] The electrical signal conductors 26 can include a plurality of ground conductors, a plurality of electrical signal conductors, or both. In one example, adjacent signal conductors, such as signal conductors SI and S2, can define differential signal pairs. Alternatively, the signal conductors can be single ended. The electrical signal conductors 26 can each be elongate along the longitudinal direction L. The electrical signal conductors 26 can be arranged such that immediately adjacent ones of the electrical signal conductors 26 can be spaced from each other along the lateral direction along a center-to-center conductor pitch that is in a range from approximately 0.3 mm to approximately 0.5 mm. For instance, the conductor pitch can be approximately 0.35 mm. The pitch between the repeating pattern of conductors is thus approximately 0.9 mm to approximately 1.5 mm. For instance, the repeating pattern pitch may be approximately 1.05 mm. In one example, the pitch can be defined along the lateral direction A.

[0050] The electrical signal conductors 26 can be substantially coplanar with each other along a plane that includes the longitudinal direction L and the lateral direction A. Further, the electrical signal conductors 26 can be rectangular or trapezoidal in shape in a plane defined by the transverse direction T and a lateral direction A. The electrical signal conductors 26 can be wider along the lateral direction A than they are tall along the transverse direction T. It should be appreciated that the transverse direction T, the longitudinal direction L, and the lateral direction A, and other spatial relationships are described herein while the flex circuit 20 is in a flat position, it being recognized that the flex circuit 20 can be bent, twisted, or otherwise contorted during use.

[0051] The flex circuit 20 depicted in Figs. 2A and 2B may be referred to as a three- layer flex circuit 20, since there are three layers of metal conductors separated by the electrically insulating dielectric layers. Each of the one or more flex circuits 20 can include at least two layers, at least three layers, at least four layers, at least five layers, only two layers, only three layers, only four layers, only five layers, only six layers, only seven layers, only eight layers, only nine layers, only ten layers, only eleven layers, only twelve layers, three or more layers, four or more layers, five or more layers, or six or more layers. A minimum number of layers for the chosen application are preferred. In a three-layer flex circuit 20 that defines a strip line transmission structure, the first and second layers can each be ground layers, ground planes or first and second electrically conductive layers 22, 24. A third layer, positioned between the first and second layers can be a signal layer that includes only signaltraces or only electrical signal conductors 26 or a combination of signal and ground traces. In flex circuits 20 with more than three layers, other respective conductive layers can be a ground layer or a signal layer as desired.

[0052] The flex circuit 20 may be fabricated by laminating one or more layers of metal / dielectric sheets. The metal of the metal / dielectric sheets may be patterned during an additive manufacturing process such as three-dimensional printing, by using photolithography, or some other means to etch away metal in areas where it is not wanted. The metal can be copper, and the flexible dielectric can be a polyimide or a liquid crystal polymer. Thickness of the metal layer can be very thin (approximately > 0 microns < 0.002 microns) to very thick (approximately > 250 microns) and the dielectric thickness can vary from approximately 10 microns to 220 microns. Thickness of the various layers comprising the flex circuit 20 may be chosen to optimize performance while maintaining adequate flexibility. In some embodiments, the thickness of each of the layers of a three-layer flex circuit 20 may be less than approximately 0.15 mm and the total flex circuit thickness may be less than approximately 0.4 mm. Filled electrically conductive ground or signal vias between the different conductive layers may be made using mechanical or laser drilling and well known plating processes. It should be noted that the manufacturing process for the flex circuit 20 can be different from that of a flat cable, which is made by an extrusion process.

[0053] The size and dimension of the flex circuit 20 can be selected so as to achieve a desired characteristic impedance of the signal conductors SI, S2. The characteristic impedance of the signal conductors SI, S2 can be adjusted by altering at least one of a size and shape of the electrical signal conductors 26, their spatial relationship to ground planes such as the first and second electrically conductive layers 22, 24, and the dielectric properties of the dielectric material surrounding the electrical signal conductors 26. The characteristic impedance may be adjusted to be in the range of approximately 85±5 Ohms to approximately 100±10 Ohms. In particular, the characteristic impedance may be 92.5±5 Ohms. The flex circuit 20 and interconnections at the respective first and second ends 134, 136 of the flex circuit 20, where electrical signals enter and exit the flex circuit 20, can be designed to reduce unwanted significant fluctuations in impedance, to minimize reflections and resonances in the transmission system.

[0054] At least one flex circuit 20 of the flex circuit assembly 21 can include a flex mating region at the first end 134 that is configured to be mounted directly to the first surface 200 of the die package substrate 74. Alternatively, the first end 134 can be configured to bemated or mounted to a complementary electrical component, such as an electrical connector (not shown) that, in turn, is mounted to the die package substrate 74.

[0055] The flex mating region can include electrical pads 30. Referring to Fig. 2A, the electrical pads 30 can define first electrical pads 30a on or adjacent to the first flex circuit surface 23 A. The electrical pads 30 can also be referred to as electrical signal pads. The electrical pads can be electrical signal pads. In one example, adjacent ones of the first electrical pads 30a can define differential signal pairs. The first electrical pads 30a of the flex circuit 20 can each be electrically connected to a respective one of the electrical signal conductors 26. Thus, the electrical pads 30 are in electrical communication with respective ones, and in particular different ones, of the electrical signal conductors 26. In particular, the flex circuit 20 can include a plurality of signal vias 34a that can each extend from the electrical pads 30 to a respective one of the electrical signal conductors 26. In one example, the electrical pads 30 can be aligned with a respective one of the electrical signal conductors 26 along the transverse direction T. The signal vias 34a can extend from a respective one of the electrical pads 30 to an aligned one of the electrical signal conductors 26 along the transverse direction T. In one example, each of the first plurality of electrical pads 30 can be connected to a respective single one of the electrical signal conductors 26 by a single signal via 34, though it should be appreciated that electrical pads 30 can be connected to a single one of the electrical signal conductors 26 by more than one signal via 34 if desired. One or more signal vias 34a can extend into, but not through, a respective one of the electrical pads 30 and a respective electrical signal conductor 26 along the transverse direction T, if desired. Alternatively, the signal via 34 can extend through each of the electrical pads 30 and the electrical signal conductor 26 along the transverse direction T. The first electrical pads 30a can be exposed at an outer surface of the first flex circuit surface 23 a such that the first electrical pads 30a can be electrically coupled to another electrical component.

[0056] The electrical pads 30 can define a plurality of second electrical pads 30b. The second electrical pads 30b can be configured as signal pads. In one example, adjacent ones of the second electrical pads 30b can define differential signal pairs. The second electrical pads 30b can be electrically isolated from the electrical signal conductors 26. The second electrical pads 30b can be exposed at an outer surface of the electrically insulative body 19 such that the second electrical pads 30b can be electrically coupled to another electrical component. The second electrical pads 30b can be on or adjacent to at least one of the first and second flex circuit surfaces 23a, 23b. The second electrical pads 30b can extend from the first flex circuit surface 23 a to the second flex circuit surface 23b of the flex circuit20. In other examples, one of the second electrical pads 30b can be on or adjacent to the first flex circuit surface 23 a, a second one of the second electrical pads 30b can be on or adjacent the second flex circuit surface 23b, and the first and second ones of the second electrical pads 30b can be electrically connected to each other. For example, the first and second ones of the second electrical pads 30b can be electrically coupled to each other by a via.

[0057] The first electrical pads 30a can be arranged in at least one row that extends in the lateral direction A. The first electrical pads 30a can be arranged in a plurality of rows. The first electrical pads 30a can be arranged in two rows. The first electrical pair pads 30a can be arranged in parallel rows. The first electrical pads 30a can be arranged in rows that each extend in the lateral direction A. The rows of first electrical pads 30a can be spaced from each other in the longitudinal direction L. The rows of first electrical pads 30a can be disposed on a first side of a lateral center line that bifurcates the electrically insulative body 19 with respect to the longitudinal direction L.

[0058] The second electrical pads 30b can be arranged in at least one row that extends in the lateral direction A. The second electrical pads 30b can be arranged in a plurality of rows. The second electrical pair pads 30b can be arranged in parallel rows. The second electrical pads 30b can be arranged in rows that each extend in the lateral direction A. The rows of second electrical pads 30b can be spaced from each other in the longitudinal direction L. The rows of second electrical pads 30b can be disposed on a first side of a lateral center line that bifurcates the electrically insulative body 19 with respect to the longitudinal direction L. The first electrical pads 30a can be disposed on a first side of a lateral center line that bifurcates the electrically insulative body 19 with respect to the longitudinal direction L and the rows of second electrical pads 30b can be disposed on a second side of the lateral center line.

[0059] The second electrical pads 30b can be arranged in two rows. The second electrical pads 30b can be arranged in three rows. The second electrical pads 30b can be arranged in four rows. The second electrical pads 30b can be arranged in five rows. The second electrical pads 30b can be arranged in six rows. The second electrical pads 30b can be arranged in seven rows. The second electrical pads 30b can be arranged in eight rows. The second electrical pads 30b can be arranged in nine rows. The second electrical pads 30b can be arranged in ten rows.

[0060] The first and second rows of the second electrical pads 30b can be coplanar in a plane that extends in the lateral and longitudinal directions A, L. The first and second rows of the second electrical pads 30b can be aligned in the transverse direction T. Thenumber of rows of the second electrical pads 30b can be equal to the number of rows of first electrical pads 30a. The number of rows of the second electrical pads 30b can be greater than the number of rows of first electrical pads 30a. The first electrical pads 30a can be positioned closer to the first end 134 than the second electrical pads 30b. The first and second electrical pads 30a, 30b can be coplanar in a plane that extends in the lateral and longitudinal directions A, L. At least a portion of the first and second electrical pads 30a, 30b can be coplanar in a plane that extends in the lateral and longitudinal directions A, L. Each of the first and second electrical pads 30a, 30b can be exposed at the first flex circuit surface 23a.

[0061] The flex circuit 20 can include one or more ground pads that provide electromagnetic interference shielding to the electrical pads 30 during operation. Referring to Fig. 2A, one or more ground pads 35 can be disposed adjacent to the electrical pads 30. The ground pads 35 can be disposed between adjacent pairs of electrical pads 30 along the lateral direction A. The ground pads 35 and electrical pads 30 can be disposed in a G-S-S repeating pattern. The ground pads 35 can be electrically isolated from the electrical pads 30. The ground pads 35 can be exposed at an outer surface of the electrically insulative body 19 such that the ground pads 35 can be electrically coupled to another electrical component. The ground pads 35 can be on or adjacent at least one of the first and second flex circuit surfaces 23 A, 23B. The ground pads 35 can be exposed at the first and second flex circuit surfaces 23 A, 23B.

[0062] The ground pads 35 can be electrically connected to at least one of the first and second electrically conductive layers 22, 24. The ground pads 35 can be electrically coupled to one or more ground vias. Referring to Fig. 2C, the ground via 34b can be electrically connected to at least one of the first and second electrically conductive layers 22, 24. Ground vias 34b can each extend through the first and second electrically conductive layers 22 and 24 along the transverse direction T. Alternatively, the ground vias 34b can extend into, but not through one or both of the first and second electrically conductive layers 22 and 24. Thus, it can be said that the ground vias 34b can extend from respective ones of the ground pads 35 to at least one or both of the first and second electrically conductive layers 22 and 24. Multiple ground vias 34b (or pairs of first and second ground vias 34b) can connect each of the ground pads 35 to the first and second electrically conductive layers 22 and 24. It should be appreciated that the first and second electrically conductive layers 22 and 24, and the ground pads 35, can be placed in electrical communication with each other through the ground vias 34b. The flex circuit 20 can have any number of ground vias 34b. For example, the flex circuit 20 may have no ground vias 34b or only a small number ofground vias 34b, such as less than 2, 4, 6, 8, or 10 ground vias 34b per flex differential signal pair SI, S2.

[0063] The first plurality of electrical pads 30 can be spaced apart from one another such that a row of at least sixty -four first electrical pads 30a fit along a flex circuit 21 having a width in the lateral direction A greater than 50mm but not more than approximately 75mm or having a width greater than 55mm but not more than approximately 80mm or having a width greater than 60mm but not more than approximately 85mm or having a width greater than 65mm but not more than approximately 90mm or having a width greater than 70mm but not more than approximately 95mm or having a length width than 75mm but not more than approximately 100mm, 105mm or 110mm. The first differential flex signal pair pads 30a, the second differential flex signal pair pads 30b, or both, can be spaced apart from one another such that at least two-hundred and fifty-six of the first differential flex signal pair pads 30a, the second differential flex signal pair pads 30b or both fit, within an area of approximately 500 square millimeters or approximately 550 square millimeters or approximately 600 square millimeters or approximately 650 square millimeters or approximately 700 square millimeters or approximately 750 square millimeters or approximately 800 square millimeters.

[0064] Each of the flex circuits 20 can define a width along the lateral direction A. The width of each flex circuit 20 can increase along the lateral direction A as the flex circuit extends in a direction from the first end 134 to the second end 136. Thus, it can be said that the flex circuit 20 can flare out along a direction from the first end 134 to the second end 136. Therefore, the electrical signal conductors 26 carried by the flex circuit 20 can fan or flare out along the direction from the first end 134 to the second end 136 so as to increase a distance between the electrical signal conductors 26 in the lateral direction A. For example, the distance between the electrical signal conductors 26 can fan out from an approximate 300- micron (approximately 0.3 mm) pitch to an approximate 600-micron (approximately 0.6 mm) pitch as the flex circuit 20 extends along the direction from the first end 134 to the second end 136. The signal conductors 26 can be spaced at a pitch suitable to be mounted to electrical conductors of electrical cables that can be configured as coaxial cables or twinaxial cables as desired.

[0065] The present inventors recognize that the electrical conductors of adjacent electrical cables 79 are spaced from each other by the distance defined by the outer electrically insulative jacket. Thus, the electrical conductors of adjacent electrical cables are spaced from each other by a distance that is greater than the pitch of the electrical conductors at the first end 134 of the flex circuits 20. Because the flex circuits 20 flare out as they extendfrom the first end 134 to the second end 136, the pitch of the electrical conductors of the flex circuits 20 can similarly increase in a direction from the first end 134 to the second end 136. Therefore, at the second end 136 of the flex circuit 20, the electrical signal conductors 26 can be spaced from each other a suitable distance so as to be mounted to respective electrical conductors of the electrical cables 79. The second circuit end 136 can have a second width between 95mm and 120mm.

[0066] A flex circuit assembly 21 can include a plurality of flex circuits 20. In one example shown in Fig. 3, the flex circuit assembly 21 can include a pair of flex circuits 20, such as first and second circuits 20a and 20b, respectively, of the flex circuits 20. In another example, the flex circuit assembly 21 can include four flex circuits 20, such as first, second, third, and fourth circuits 20a, 20b, 20c, 20d, respectively, of the flex circuits 20 as shown in Figs. 4A and 4B. It is recognized that as the number of flex circuits 20 of the flex circuit assembly 21 increases, the flexibility of the flex circuit assembly can correspondingly decrease. Thus, a flex circuit assembly that includes e.g., two layers of flex circuits can be more flexible than a flex circuit assembly that includes e.g., six layers. The flex circuit assembly can include any number of flex circuits as desired. For instance, a three-layer flex circuit assembly can include first, second, and third ones of the flex circuits 20. The flex circuits 20 of a five-layer flex circuit assembly can include first, second, third, fourth and fifth ones of the flex circuits 20. The flex circuits 20 of a six -layer flex circuit assembly can include first, second, third, fourth, fifth, and sixth ones of the flex circuits 20. The flex circuits 20 of a seven-layer flex circuit assembly can include first, second, third, fourth, fifth, sixth, and seventh ones of the flex circuits 20. The flex circuits 20 of an eight-layer flex circuit assembly can include first, second, third, fourth, fifth, sixth, seventh, and eighth ones of the flex circuits 20. The flex circuits 20 of a nine-layer flex circuit assembly can include first, second, third, fourth, fifth, sixth, seventh, eighth, and ninth ones of the flex circuits 20. A ten-layer flex circuit assembly can include first, second, third, fourth, fifth, sixth, seventh, eighth, ninth, and tenth ones of the flex circuits 20.

[0067] Referring to Figs. 3 to 7, the flex circuit assembly 21 can be constructed such that the flex circuits 20 of the flex circuit assembly 21 can be stacked on each other along the transverse direction. The electrical pads 30 of the innermost one of the flex circuits can be mounted directly to the die package substrate 74. Among the remaining flex circuits 20 of the flex circuit assembly 21, each flex circuit 20 can be placed in electrical communication with the die package substrate 74 through electrical paths that are established through all flex circuits 20 of the flex circuit assembly 21 that are disposed between the respective flex circuit20 and the die package substrate 74 along the transverse direction T. The electrical paths will now be described in more detail.

[0068] Referring to Figs. 2A and 7, the flex circuits 20 of the flex circuit assembly 21 can have a similar construction, but different arrangements of electrical signal conductors and electrical pads. In particular, each flex circuit 20 can include an electrically insulative body 19, a respective plurality of electrical signal conductors 26 supported by the electrically insulative body 19, and electrical pads 30. For example, each of the first and second flex circuits 20a and 20b, respectively, can include first electrical pads 30a coupled to respective electrical signal conductors 26 (see Figs. 2A and 7). However, the second flex circuit 20b can be devoid of the second electrical pads 30b (see Fig. 7). In other examples, the first, second, third, and fourth flex circuits 20a, 20b, 20c, and 20d, respectively, can include first electrical pads 30a coupled to respective electrical signal conductors 26, but only the first, second, and third and third flex circuits 20a, 20b, and 20c include the second electrical pads 30b.

[0069] Referring to Fig. 4A, the first electrical pads 30a of the first flex circuit 20a can be offset from the first electrical pads 30a of the second flex circuit 20b in the longitudinal direction L. The first electrical pads 30a of the first flex circuit 20a can be offset from the first electrical pads 30a of the second flex circuit 20b in the longitudinal direction L such that a line extending in the transverse direction T will extend through the first electrical pads 30a of one of the first and second flex circuits 20a, 20b without passing through the first electrical pads 30a of the other of the first and second flex circuits 20a, 20b. The first electrical pads 30a of the second flex circuit 20b can be aligned with the second electrical pads 30b of the first flex circuit 20a in the longitudinal direction L. For example, a line extending in the transverse direction can extend through one of the first electrical pads 30a of the second flex circuit 20b and one of the second electrical pads 30b of the first flex circuit 20a. The first electrical pads 30a of the second flex circuit 20b can be electrically coupled to the second electrical pads 30b of the first flex circuit 20a, such that the second flex circuit 20b is in electrical communication with the die package substrate 74. The quantity of first electrical pads 30a of the second flex circuit 20b can be equal to the quantity of second electrical pads 30b of the first flex circuit 20a. The first flex circuit 20a can include a first plurality of second electrical pads 30b in electrical communication with respective ones of the plurality of first electrical pads 30a of the second flex circuit 20b. The die package substrate can put the first and second electrical pads 30a, 30b of the first flex circuit 20a in communication with the IC die 70 through the traces in the substrate.

[0070] The ground pads 35 of the first flex circuit 20a can be electrically coupled to the ground pads 35 of the second flex circuit 20b such that the ground pads 35 of the second flex circuit 20b are in electrical communication with the die package substrate 74. A ground pad 35 of the first flex circuit 20a can be aligned with a ground pad 35 of the second flex circuit 20b along the transverse direction T such that a line extending in the transverse direction T can pass through the ground pad 35 of each of the first and second flex circuits 20a, 20b. Further, the ground pads 35 of the second flex circuit 20b can be placed in electrical communication with the ground pads 35 of the first flex circuit 20a, and all other flex circuits of the flex circuit assembly 21 as desired.

[0071] The first electrical pads 30a of the third flex circuit 20c can be in electrical communication with the second electrical pads 30b of the first and second flex circuits 20a, 20b. The first electrical pads 30a of the third flex circuit 20c can be aligned with the second electrical pads 30b of the first and second flex circuits 20a, 20b such that a line extending in the transverse direction can extend through one of the first electrical pads 30a of the third flex circuit 20c and one of the second electrical pads 30b of each of the first and second flex circuits 20a, 20b. The first electrical pads 30a of the third flex circuit 20c can be electrically coupled to one of the second electrical pads 30b of each of the first and second flex circuits 20a, 20b, such that the third flex circuit 20c is in electrical communication with the die package substrate 74. The second flex circuit 20b can include a first plurality of second electrical pads 30b in electrical communication with respective ones of the plurality of first electrical pads 30a of the third flex circuit 20c. The first flex circuit 20a can include a second plurality of second electrical pads 30b in electrical communication with respective ones of the first plurality of second electrical pads 30b of the second flex circuit and respective ones of the plurality of first electrical pads 30a of the third flex circuit 20c.

[0072] The ground pads 35 of the second flex circuit 20b can be electrically coupled to the ground pads 35 of the third flex circuit 20c such that the ground pads 35 of the third flex circuit 20c are in electrical communication with the die package substrate 74. A ground pad 35 of the second flex circuit 20b can be aligned with a ground pad 35 of the third flex circuit 20c along the transverse direction T such that a line extending in the transverse direction T can pass through the ground pad 35 of each of the second and third flex circuits 20b, 20c. Further, the ground pads 35 of the second flex circuit 20b can be placed in electrical communication with the ground pads 35 of the third flex circuit 20c, and all other flex circuits of the flex circuit assembly as desired.

[0073] The first electrical pads 30a of the fourth flex circuit 20d can be in electrical communication with the second electrical pads 30b of the first, second, and third flex circuits 20a, 20b, 20c. The first electrical pads 30a of the fourth flex circuit 20d can be aligned with the second electrical pads 30b of the first, second, and third flex circuits 20a, 20b, 20c such that a line extending in the transverse direction can extend through one of the first electrical pads 30a of the fourth flex circuit 20d and one of the second electrical pads 30b of each of the first, second, and third flex circuits 20a, 20b, 20c. The first electrical pads 30a of the fourth flex circuit 20d can be electrically coupled to one of the second electrical pads 30b of each of the first, second, and third flex circuits 20a, 20b, 20c, such that the fourth flex circuit 20d is in electrical communication with the die package substrate 74. The first flex circuit 20a can include a third plurality of second electrical pads 30b in electrical communication with respective ones of the plurality of first electrical pads 30a of the fourth flex circuit 20d. The second flex circuit 20b can include a second plurality of second electrical pads 30b in electrical communication with respective ones of the third plurality of second electrical pads 30b of the first flex circuit 20a and respective ones of the plurality of first electrical pads 30a of the third flex circuit 20c.

[0074] The ground pads 35 of the third flex circuit 20c can be electrically coupled to the ground pads 35 of the fourth flex circuit 20d such that the ground pads 35 of the fourth flex circuit 20d are in electrical communication with the die package substrate 74. A ground pad 35 of the third flex circuit 20c can be aligned with a ground pad 35 of the fourth flex circuit 20d in the transverse direction T such that a line extending in the transverse direction T can pass through the ground pad 35 of each of the third and fourth flex circuits 20c, 20d. Further, the ground pads 35 of the fourth flex circuit 20d can be placed in electrical communication with the ground pads 35 of the third flex circuit 20c, and all other flex circuits of the flex circuit assembly 21 as desired.

[0075] The first flex circuit 20a can be positioned between the die package substrate 74 and the second flex circuit 20b in the transverse direction. The second flex circuit 20b can be spaced from the die package substrate 74 in the transverse direction when the flex circuit assembly 21 is coupled to the die package substrate 74. The second flex circuit 20a can be positioned between the first and third flex circuits 20a, 20c in the transverse direction. The third flex circuit 20c can be positioned between the second and fourth flex circuits 20b, 20d in the transverse direction. The first end 134 of each of the first, second, third and fourth flex circuits 20a, 20b, 20c, and 20d can be coplanar when the flex circuit 20 is coupled to the die package substrate 74. The first end 134 of at least two of the first, second, third and fourthflex circuits 20a, 20b, 20c, and 20d can be coplanar when the flex circuit 20 is coupled to the die package substrate 74. The ends of each of the first, second, third and fourth flex circuits 20a, 20b, 20c, and 20d can be aligned in the longitudinal direction L when the flex circuit 20 is coupled to the die package substrate 74 as shown in Fig. 4A. The ends of at least two of the first, second, third and fourth flex circuits 20a, 20b, 20c, and 20d can be coplanar in the longitudinal direction L when the flex circuit 20 is coupled to the die package substrate 74. The ends of at least two of the first, second, third and fourth flex circuits 20a, 20b, 20c, and 20d can be coplanar in a plane that extends in the transverse and lateral directions when the flex circuit 20 is coupled to the die package substrate 74. The ends of each of the first, second, third and fourth flex circuits 20a, 20b, 20c, and 20d can be coplanar in a plane that extends in the transverse and lateral directions when the flex circuit 20 is coupled to the die package substrate 74.

[0076] It should be appreciated that the flex circuits 20a-20d that are increasingly spaced from the die package substrate 74 can have decreasing numbers of electrical pads 30. Further, the flex circuits 20a-20d can be positioned such that the electrical pads 30 of the flex circuit assembly can be stepped. Thus, the flex circuit assembly 21 can define outermost electrical pads 30 along the transverse direction T. Outermost electrical pads 30 can be spaced furthest from the die package substrate 74 along the transverse direction T, such that no other electrical pads can be 1) aligned with the outermost electrical pads 30 along the transverse direction, and 2) spaced further from the die package substrate 74 than the outermost electrical pads 30. Each of the flex circuits 20a-20d can define at least one outermost electrical pad 30. The outermost electrical pads 30 can be offset from each other along the longitudinal direction. Further, along a longitudinal direction of travel, the outermost electrical pads 30 can be decreasingly spaced from the die package substrate 74 along the transverse direction. Further still, for each outermost electrical pad 30 of each flex circuit, the outermost electrical pad 30 can be aligned along the transverse direction with an electrical pad 30 of all flex circuits between itself and the die package substrate 74.

[0077] In some examples, each flex circuit 20 of the flex circuit assembly 21 is directly coupled to an adjacent flex circuit 20. In other examples, an intermediate layer is positioned between adjacent flex circuits 20 of the flex circuit assembly 21. Referring to Fig. 3, an intermediate layer 188 can be positioned between the first and second flex circuits 20a, 20b. The intermediate layer 188 can include first and second surfaces 188a, 188b opposite each other in the transverse direction T. The intermediate layer 188 can transition from a relaxed configuration to a compressed configuration when a compressive force is applied toat least one of the first and second surface 188a, 188b. At least one of the first and second surfaces 188a, 188b can move toward the other of the first and second surfaces 188a, 188b when the compressive force is applied to the intermediate layer 188.

[0078] The intermediate layer 188 can be positioned between the first and second flex circuits 20a, 20b in the transverse direction T. The intermediate layer can have a width in the lateral direction A that is substantially equal to the width of at least one of the first and second flex circuits 20a, 20b. In some examples, the intermediate layer 188 extends less than the entire length from the first end 134 to the second end 136 of the flex circuit assembly 21. In other examples, the intermediate layer 188 extends from the first end 134 to the second end 136 of the flex circuit assembly 21. In some examples, the intermediate layer 188 includes a plurality of second electrical pads 30b so as to electrically couple the first electrical pads 30a of the second flex circuit 20b with the second electrical pads 30b of the first flex circuit 20a. In some examples, an entirety of each of the intermediate layers 188 can be aligned with either or both of respective immediately adjacent flex circuits between which the intermediate layers 188 are disposed. Immediately adjacent flex circuits can be defined by flex circuits that are arranged such that no flex circuits are disposed between the immediately adjacent flex circuits. Thus, in some examples, each of the intermediate layers 188 can be positioned and dimensioned such that a straight line that is oriented along the transverse direction that passes through the intermediate layer 188 also passes through at least one or both of the immediately adjacent flex circuits that the intermediate layer 188 is disposed between along the transverse direction T. It should be appreciated that a respective intermediate layer 188 can be disposed between each of the immediately adjacent pairs of flex circuits that are adjacent each other along the transverse direction T. The immediately adjacent pairs of flex circuits can be defined by flex circuits 20a and 20b, and flex circuits 20b and 20c. In some examples, all straight lines that are oriented along the transverse direction that pass through the intermediate layer 188 can also pass through either or both of the immediately adjacent flex circuits that the intermediate layer is disposed between. Otherwise stated, the intermediate layer 188 can be positioned and dimensioned such that no straight line oriented in the transverse direction exists that passes through the intermediate layer 188 and does not also pass through either or both of the immediately adjacent flex circuits that the intermediate layer 188 is disposed between. In some examples, the intermediate layers 188 can be positioned and dimensioned such that no portion of each of the intermediate layers 188 extend out with respect to either or both of the immediatelyadjacent flex circuits, that the intermediate layers are disposed between, along either or both of the longitudinal direction and the lateral direction.

[0079] In other examples, the intermediate layer 188 includes openings that allow respective ground pads 35 and electrical pads 30 of the first and second flex circuits 20a, 20b to be electrically connected to each other. For example, respective ground pads 35 and electrical pads 30 can be placed in physical contact with each other, can be connected by solder, or an intermediate element can be electrically coupled to respective ground pads 35 and electrical pads 30.

[0080] In other examples, the intermediate layer 188 can be a nanowire or an anisotropic conductive film. Examples of materials contemplate for use as an intermediate layer include, but are not limited to; PariPoser® anisotropic elastomer fabric commercially available from Paricon Technologies, Taunton, MA; ANISOLM® anisotropic conductive film commercially available from Resonac Corporation, Tokyo, Japan; and nanowires commercially available from Nanowired GmbH, Gernsheim, Germany. When the intermediate layer 188 is an anisotropic conductive film, the intermediate layer 188 can include conductive elements that are electrically isolated from each other when the intermediate layer 188 is in the relaxed configuration and in electrical communication with each other in the compressed configuration so as to provide an electrical pathway through the intermediate layer 188. The conductive elements can provide an electrical pathway between the first and second surfaces 188a, 188b so as to electrically connect respective ground pads 35 and electrical pads 30 of the first and second flex circuits 20a, 20b. Each conductive element can be electrically isolated from adjacent conductive elements in the longitudinal and lateral directions L, A such that the electrical pathway is limited to the transverse direction T.

[0081] The flex circuit assembly 21 can be flexible to allow the assembly to be bent or folded along its length. However, it may be desirable for at least some sections of the flex circuit assembly 21 to have increased stiffness to help maintain the connection between the flex circuit assembly 21 and the die package substrate 74. Referring to Fig. 4A, a stiffener 192 can be positioned adjacent to the first end 134 of the flex circuit to increase mechanical stability and durability of the flex circuit 20. The stiffener 192 can also reduce warpage of the die package substrate 74 when the flex circuit assembly 21 is coupled thereto. The stiffener 192 can have a greater rigidity than the flex circuit assembly 21. In some examples, the stiffener 192 is coupled to the flex circuit 20 via adhesive, weld, or fastener. The stiffener 192 can overhang the end of the flex circuit 20 in the longitudinal direction L. A gap can be defined between the stiffener 192 and the die package substrate 74 in the transverse directionT. In other embodiments, the end of the stiffener 192 is coplanar with the end of the flex circuit 20 in a plane that extends in the lateral and transverse directions.

[0082] The die package substrate 74 can include package pads 162 in electrical communication with respective ones of the electrical traces. The package pads 162 can be electrical pads. The package pads 162 can be electrical signal pads. The package pads 162 can be positioned on or adjacent the first surface 200 of the die package substrate 74. Each of the package pads 162 can be configured to be attached to respective ones of the signal pads 30 of the flex circuit assembly 21 directly or indirectly. Referring to Fig. 8, the package pads 162 can be arranged in at least one package row 164. The package pads 162 can be positioned in first and second package rows 164. The package pads 162 can be positioned in first, second, and third package rows 164. The package pads 162 can be positioned in first, second, third, and fourth package rows 164. The package rows 164 can each extend in the lateral direction A. The package rows 164 can be parallel to each other. The first and second package rows 164 can be spaced from each other in the longitudinal direction L. The package rows 164 can be coplanar in a plane that includes the lateral and longitudinal directions. The quantity of package pads 162 can be equal to the quantity of electrical pads 30 of the first flex circuit 20a. Along a respective package row 164, the die package pads 162 may be arranged in a selected pattern of differential signal pair and ground contacts, such as a repeating pattern selected from G-G-S-S, G-S-S, and G-S. Fig. 8 shows an exemplary G-S-S pattern, but other patterns may be used as previously described.

[0083] The die package substrate 74 can have a width in the lateral direction and a length in the longitudinal direction. The width and the length of the die package substrate 74 may be equal, i.e., the die package substrate 74 can be square. The IC die 70 can be approximately 40x40mm square. The width and length of the die package substrate 74 may be at least approximately 50 mm, such as at least approximately 70mm, at least approximately 75mm, at least approximately 80mm, at least approximately 85mm, at least approximately 90mm, at least approximately 95mm, at least approximately 100mm, at least 105mm or at least 110mm.

[0084] The die package substrate 74 can include first, second, third and fourth die packages sides 178, 180, 182, 184. Each of the individual die package sides 178, 180, 182, 184 can be at least 50mm in length, but no longer than approximately 75mm, approximately 80mm, approximately 85mm, approximately 90mm, approximately 95mm, approximately 100mm, approximately 105mm, approximately 110mm, or approximately 115mm. At least one hundred and twenty-eight or at least two hundred and fifty-six package pads 162 can bedefined on each of the respective first, second, third, and fourth die package sides 178, 180, 182, 184. In other embodiments, any number of package pads 162 can be defined on each of the respective first, second, third, and fourth die package sides 178, 180, 182, 184.

[0085] The flex circuit assembly 21 can define a flex circuit assembly interconnect footprint 140 that is configured to be mounted to the die package substrate such that all electrical signal conductors of the flex circuits 20 are in electrical communication with the die package substrate. The footprint 140 can have a size and shape that is defined by a complementary footprint of the die package substrate 74, which can be arranged on or adjacent to the surface 200 of the die package substrate 74, such as the surface of the die package substrate 74 that carries the IC die 70. The flex circuit assembly interconnect footprint 140 may be arranged directly on the surface 200 of the die package substrate 74. The surface 200 of the die package substrate 74 may define the flex circuit assembly interconnect footprint 140. At least two, at least three or at least four of the respective first, second, third and fourth die package sides 178, 180, 182, 184 of the die package substrate 74 may have an adjacent flex circuit assembly interconnect footprint 140 as generally shown in Fig. 8. Each flex circuit assembly interconnect footprint 140 can define a die substrate mating region on the die package substrate 74 where electrical connections to corresponding ones of die package traces may be made. All of the first, second, third and fourth die package sides 178, 180, 182, 184 can have the same length or different lengths. Each flex circuit assembly interconnect footprint 140 may also have a single section (e.g., one or more rows 164 of package pads 162) continuous along the length of the flex circuit assembly interconnect footprint 140. The rows 164 of package pads 162 can be oriented parallel to an adjacent, respective first, second, third and / or fourth die package sides 178, 180, 182, 184 of the die package substrate 74.

[0086] Each flex circuit assembly interconnect footprint 140 may be configured to directly mate with a single flex circuit 20 or with a flex circuit assembly 21. That is, the footprint 140 remains unchanged whether a single flex circuit 20 is mounted to the die package substrate or whether a plurality of flex circuits 20 of the flex circuit assembly 21 is mounted to the die package substrate. The first end 134 of the flex circuit assembly 21 can be configured to mate directly or indirectly with the flex circuit assembly interconnect footprint 140. Alternatively, each flex circuit assembly interconnect footprint 140 can be configured to receive or be received in or on an electrical connector. At least two of the respective first, second, third and fourth die package sides 178, 180, 182, 184 can each be configured to receive a corresponding flex circuit assembly 21 either through direct connectsbetween corresponding electrical pads 30 and corresponding package pads 162. An electrical communication system can include the IC die package 72 described herein and one or more flex circuits 20 physically attached, electrically attached or both to respective ones of the package pads 162. The die package pads 162 can be coupled to the first and second electrical pads 30a, 30b of the first flex circuit 20a by solder balls 33a as shown in Fig. 4A. The ground pads 35 can be coupled to the ground contacts of the die package substrate 74 by solder balls 33b.

[0087] The second end 136 of the flex circuit assembly 21 and its connection to the connector 172 will now be discussed. As show in Fig. 10, the second circuit end 136 of the flex circuit 20 can include electrical pads 130 and ground pads 135. The electrical pads 130 can be configured as described above with respect to electrical pads 30. The electrical pads 130 can be the same as the first electrical pads 30a. For example, the electrical pads 130 can be electrically coupled to the electrical signal contacts 26. Thus, the flex circuit assembly 21 can provide a continuous electrical path between the electrical pads 30 and the electrical pads 130. The ground pads 135 can be similar to the ground pads 35. The ground pads 135 can be the same as the ground pads 35.

[0088] The electrical pads 130 and ground pads 135 can be arranged in a repeating G-S-S-G configuration. Alternatively, the electrical pads 130 and ground pads 135 can be arranged in a repeating G-S configuration, a repeating G-G-S configuration, or any combinations thereof. The electrical pads 130, ground pads 135, or both, can be positioned on or adjacent the first flex circuit surface 23A or the second flex circuit surface 23B. The electrical pads 130, ground pads 135, or both, can be positioned on or adjacent each of the first and second flex circuit surfaces 23 A, 23B so as to provide a double-sided connection at the second circuit end 136. The ability of the flex circuits 20 to have single sided electrical pads 30 at the first end 134 of the flex circuit 20 and a double-sided connection of electrical pads 130 and flex ground pads 135 at the second circuit end 136 of the flex circuit 20 can allow the effective spacing between adjacent electrical pads 130 in the lateral direction at the second circuit end 136 to be twice that on the first end 134 without increasing the lateral space between the electrical signal conductors 26. Increasing the effective spacing between electrical pads 130 can allow a separatable interconnection between the electrical connectors 172 and the cable connector 173 at the second circuit end 136 to be made more reliably with reduced mechanical tolerances.

[0089] The electrical connector 172 can include a connector housing 168 coupled to a flex circuit 20. Respective flex connector housings 168 can be mechanically attached torespective stiffeners 166 to form the electrical connector 172. The connector housing can be coupled to at least one, at least two, at least three, at least four, or at least four or more second circuit ends 136 of the flex circuit assembly 21. Each respective flex connector housing 168 can support, pinch, squeeze or otherwise keep the second circuit end 136 within the confines of the respective connector housing 168. For example, each respective connector housing 168 can grip opposed edges of each respective second circuit end 136. One or more stiffeners 166 can be added adjacent to the second circuit end 136 of a respective flex circuit 20, to increase mechanical stability and durability of the flex circuit 20. The stiffeners 166 may engage with holes in the flex circuit 20 to help position the flex circuit 20 so that it can be properly registered relative to the flex connector housing or housings 168 in at least one of the longitudinal and lateral directions. In other examples, the stiffeners 166 are coupled to the flex circuit 20 by adhesive, weld, or fastener.

[0090] In combination, at least one optional stiffener 166, at least one respective flex connector housing 168 and at least one second circuit end 136 can define the electrical connector 172 shown in Fig. 9. Two or more flex circuits 20 can be carried by a single flex connector housing 168 or two flex connector housings 168 and can form a single electrical connector 172.

[0091] The flex circuit assembly 21 can have a signal insertion loss when transmitting signals along respective electrical signal conductors 26 of less than -5 dB at a frequency from about 0 GHz to about 100 GHz. The flex circuit assembly 21 can have a signal insertion loss when transmitting signals along respective electrical signal conductors 26 of less than -4 dB at a frequency from about 0 GHz to about 100 GHz. The flex circuit assembly 21 can have a signal insertion loss when transmitting signals along respective electrical signal conductors 26 of less than -3 dB at a frequency from about 0 GHz to about 50 GHz.

[0092] The flex circuit assembly 21 may be part of a digital communication system that transmits and / or receives digital information. The digital information may be in many formats, but a commonly used format is a non-return-to-zero (NRZ) format. For this format the information transfer rate, which may be expressed in Gigabits per second (Gbps), may be twice the bandwidth of the transmission system. For example, a system capable of transmitting signals at 50 GHz can support an information transfer rate of approximately 100 Gpbs. It should be appreciated that the flex circuit 20 may be used with different communication formats, such as 112G PAM-4, and is not limited to use with a NRZ format.

[0093] The flex circuit assembly 21 may be used to transmit information at data transfer rates up to approximately 120 Gpbs. The flex circuit assembly 21 may be part of a system in which the data transfer rate is at least approximately 12 gigabits per second up to approximately 112 gigabits per second, including approximately 15 gigabits per second, approximately 20 gigabits per second, approximately 25 gigabits per second, approximately 30 gigabits per second, approximately 35 gigabits per second, approximately 40 gigabits per second, approximately 45 gigabits per second, approximately 50 gigabits per second, approximately 55 gigabits per second, approximately 60 gigabits per second, approximately 65 gigabits per second, approximately 70 gigabits per second, approximately 75 gigabits per second, approximately 80 gigabits per second, approximately 85 gigabits per second, approximately 90 gigabits per second, approximately 95 gigabits per second, approximately 100 gigabits per second, approximately 105 gigabits per second, and approximately 110 gigabits per second.

[0094] Referring to Fig. 11, a flex circuit assembly 120 can include a plurality of flex circuits 20 configured as described above with respect to flex circuit assembly 21 but the flex circuit assembly 120 can additional flex ground pads 35 that can be at least partially or entirely aligned with the electrical pads 30 along the lateral direction A. The ground pads 35 can be electrically connected to ground pads on the die package substrate 74.

[0095] While Fig. 2A and its associated description discloses a flex circuit 20 with a G-S-S repeating pattern, the flex circuit assembly 21 could also have a G-S-S-G repeating pattern as shown in Fig. 11. It should be appreciated that a flex circuit 20 may be arranged to have both types of repeating patterns. For example, it may be beneficial to add an extra flex ground conductor 35 between groups of signal conductors SI, S2. Thus, most signal conductors SI, S2 can be separated by a pair of ground conductors 35, but some signal conductors SI, S2 may be separated by a single ground conductor 35.

[0096] The flex circuit assembly 220 can have a signal insertion loss when transmitting signals along respective electrical signal conductors 26 of less than -2 dB at a frequency from about 0 GHz to about 100 GHz. The flex circuit assembly 220 can have a signal insertion loss when transmitting signals along respective electrical signal conductors 26 of less than -3 dB at a frequency from about 0 GHz to about 100 GHz. The flex circuit assembly 220 can have a signal insertion loss when transmitting signals along respective electrical signal conductors 26 of less than -1 dB at a frequency from about 0 GHz to about 70 GHz. The flex circuit assembly 220 can have a signal insertion loss when transmittingsignals along respective electrical signal conductors 26 of less than -2 dB at a frequency from about 0 GHz to about 70 GHz.

[0097] Referring to Fig. 12, a flex circuit assembly 220 can include a plurality of flex circuits 221 having a construction as described above with respect to the flex circuit 20 of Fig. 2A but further including additional flex ground pads 35, some of which can be at least partially or entirely aligned with the electrical pads 30 such that a line extending along the lateral direction A passes through at least some of the electrical pads 30 and ground pads 35. The ground pads 35 can be electrically connected to ground pads on the die package substrate 74. While Fig. 2A and its associated description discloses a three-layer flex circuit 20 with a G-S-S repeating pattern, the flex circuit 20 could also have a G-S-S-G repeating pattern as shown in Fig. 12. Flex circuit 220 also includes a row 35A of ground pads 35 positioned between rows of the electrical pads 30. It should be appreciated that a flex circuit 221 may be arranged to have both types of G-S-S and G-S-S-G repeating patterns. For example, it may be beneficial to add an extra flex ground conductor 35 between groups of flex differential signal pairs SI, S2. Thus, most flex differential signal pairs SI, S2 can be separated in the lateral direction A by a pair of ground conductors 35, but some flex differential signal pairs SI, S2 may be separated by a single ground conductor 35.

[0098] The flex circuit assembly 220 can have a signal insertion loss when transmitting signals along respective electrical signal conductors of less than -2 dB at a frequency from about 0 GHz to about 100 GHz. The flex circuit assembly 220 can have a signal insertion loss when transmitting signals along respective electrical signal conductors of less than -3 dB at a frequency from about 0 GHz to about 100 GHz. The flex circuit assembly 220 can have a signal insertion loss when transmitting signals along respective electrical signal conductors of less than -1 dB at a frequency from about 0 GHz to about 80 GHz. The flex circuit assembly 220 can have a signal insertion loss when transmitting signals along respective electrical signal conductors of less than -2 dB at a frequency from about 0 GHz to about 80 GHz.

[0099] Referring to Fig. 13, a flex circuit assembly 320 can include a plurality of flex circuits 321 having a similar construction to the flex circuit 20 of Fig. 2 A but further including a row 35 A of ground pads 35 positioned between rows of the electrical pads 30 and a column 35B of ground pads 35 positioned between columns of the electrical pads 30. The row 35 of ground pads can extend in the lateral direction A. The column 35B of ground pads 35 can extend in the longitudinal direction L. While Fig. 2A and its associated description discloses a flex circuit 20 with a G-S-S repeating pattern, the flex circuit 20 could also have aG-S-S-G repeating pattern as shown in Fig. 13 and also including the row and columns 35 A, 35B of ground pads 35. It should be appreciated that a flex circuit 20 may be arranged to have both types of G-S-S and G-S-S-G repeating patterns. For example, signal pairs SI, S2 can be separated by a pair of ground conductors 35, but other flex differential signal pairs SI, S2 may be separated by a single ground conductor 35.

[0100] The flex circuit assembly 220 can have a signal insertion loss when transmitting signals along respective electrical signal conductors 26 of less than -2 dB at a frequency from about 0 GHz to about 100 GHz. The flex circuit assembly 220 can have a signal insertion loss when transmitting signals along respective electrical signal conductors 26 of less than -3 dB at a frequency from about 0 GHz to about 100 GHz. The flex circuit assembly 220 can have a signal insertion loss when transmitting signals along respective electrical signal conductors 26 of less than -1 dB at a frequency from about 0 GHz to about 80 GHz. The flex circuit assembly 220 can have a signal insertion loss when transmitting signals along respective electrical signal conductors 26 of less than -2 dB at a frequency from about 0 GHz to about 80 GHz.

[0101] Referring back to Fig. IB, a method to make a dense, high-speed transmission line can include the steps of providing a flex circuit 20 with a first end 134 configured to attach to a die package substrate 74 or a connector carried by the die package substrate 74 and a second circuit and 136 configured to directly or indirectly couple to cables 79. The method can include coupling a plurality of flex circuits 20 to each other. The method can include coupling a plurality of flex circuits 20 to each other such that the first ends 134 of each of the flex circuits 20 are aligned in a plane that extends in the lateral and transverse directions. The method can include coupling the first end 134 of a fourth flex circuit 20d of the flex circuits 20 to the stiffener 192. The stiffener 192 can extend around a perimeter of the die package substrate 74 such that the stiffener is configured to be positioned on the first, second, third and fourth die packages sides 178, 180, 182, 184 when the stiffener flex circuit 20 is coupled to the die package substrate 74. The method can include coupling the third flex circuit 20c of the flex circuit 20 to the fourth flex circuit 20d. The method can include positioning an intermediate layer 188 between the third and fourth flex circuits 20c, 20d prior to coupling the third flex circuit 20c to the fourth flex circuit 20d. The method can include coupling the second flex circuit 20c to the third flex circuit 20c. The method can include positioning an intermediate layer 188 between the second and third flex circuits 20b, 20c prior to coupling the second flex circuit 20b to the third flex circuit 20c. The method can include coupling the first flex circuit 20a to the second flex circuit 20b. The method caninclude positioning an intermediate layer 188 between the first and second flex circuits 20a, 20b prior to coupling the first flex circuit 20a to the second flex circuit 20b. Once the first, second, third, and fourth circuits 20a, 20b, 20c, 20d are coupled to each other and the stiffener 192, the flex circuit 20 is flipped onto the IC substrate 74. The flex circuit 20 can then be coupled electrically, physically, or both, to the die package substrate. For example, the flex circuit 20 can be soldered to the die package substrate 74.

[0102] It should be appreciated that any of the electrical contacts or conductors of the electrical communication assembly can be made from any suitable electrically conductive material, such as a metal. Any of the electrical connectors described herein can include magnetic absorbing material and / or electrically conductive lossy material as desired. Inclusion of absorptive or lossy material may help reduce cavity resonances in the electrical communication assembly. Inclusion of electrically conductive lossy materials may help reduce resonances that may be present in the assembly. Any electrically insulative elements of the electrical communication assembly can be made from any suitable dielectric material such as a plastic, glass, ceramic or any suitable electrically nonconductive lossy material.

[0103] It should be understood that the foregoing description is only illustrative of the present invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the present invention. In particular, the flex circuit need not be a three-layer flex circuit, but can have two, five, or any number of conductive layers. The present invention is intended to embrace all such alternatives, modifications, and variances that fall within the scope of the appended claims.

[0104] It should be appreciated that the illustrations and discussions of the embodiments shown in the figures are for exemplary purposes only and should not be construed limiting the disclosure. One skilled in the art will appreciate that the present disclosure contemplates various embodiments. Additionally, it should be understood that the concepts described above with the above-described embodiments may be employed alone or in combination with any of the other embodiments described above. It should be further appreciated that the various alternative embodiments described above with respect to one illustrated embodiment can apply to all embodiments as described herein, unless otherwise indicated.

Claims

WHAT IS CLAIMED IS:

1. A flex circuit assembly comprising: a first flex circuit having a plurality of electrical signal conductors, a plurality of first electrical pads in electrical communication with respective ones of the electrical signal conductors, and a respective plurality of second electrical pads electrically isolated from the respective plurality of electrical signal conductors; and a second flex circuit having a plurality of electrical signal conductors and a respective plurality of first electrical pads in electrical communication with respective ones of the electrical signal conductors of the second flex circuit, wherein the first electrical pads of the second flex circuit are in electrical communication with respective ones of the second electrical pads of the first flex circuit.

2. The flex circuit assembly of claim 1, wherein the first electrical pads of the first flex circuit are electrically isolated from the electrical signal conductors of the second flex circuit.

3. The flex circuit assembly of claim 1, wherein the first electrical pads of the first flex circuit are electrically isolated from all electrical signal conductors of the second flex circuit.

4. The flex circuit assembly of any one of claims 1 to 3, wherein the electrical pads are electrical signal pads.

5. The flex circuit assembly of any one of claims 1 to 4, wherein the first and second flex circuits include an electrically insulative body that supports the electrical signal conductors and first signal pads.

6. The flex circuit assembly of any one of claims 1 to 5, wherein the flex circuit assembly includes first and second flex circuit surface opposite each other in a transverse direction, wherein the plurality of first electrical pads conductors are disposed in a row along a lateral direction perpendicular to the transverse direction, and wherein the flex circuit includes first and second circuit ends opposite each other along a longitudinal direction perpendicular to each of the transverse and lateral directions.

7. The flex circuit assembly of claim 6, wherein respective ones of the second electrical pads of the first flex circuit and respective ones of the first electrical pads of the second flex circuit define a path oriented along the transverse direction.

8. The flex circuit assembly of any one of claims 6 to 7, wherein respective ones of the second electrical pads of the first flex circuit and respective ones of the first electrical pads of the second flex circuit define are stacked along the transverse direction.

9. The flex circuit assembly of any one of claims 6 to 8, wherein the electrical signal conductors are at least partially embedded in the respective flex circuit bodies.

10. The flex circuit assembly of any of claims 1 to 9, further comprising an anisotropic conductive film disposed between the first flex circuit and the second flex circuit.

11. The flex circuit assembly of any of claims 1 to 8, further comprising an anisotropic conductive paste disposed between the first flex circuit and the second flex circuit.

12. The flex circuit assembly of any of claims 1 to 8, further comprising a nanowire interconnect disposed between the first flex circuit and the second flex circuit.

13. The flex circuit assembly of any of claims 1 to 12, further comprising a stiffener coupled to the second flex circuit.

14. The flex circuit assembly of claim 13, wherein the second flex circuit is disposed between the stiffener and the anisotropic conductive film in the transverse direction.

15. The flex circuit assembly of claim 13, wherein the stiffener overhangs the second flex circuit so as to define a gap between the stiffener and a printed circuit board in the transverse direction.

16. The flex circuit assembly of any of claims 1 to 12, wherein the first flex circuit includes a plurality of third electrical pads electrically isolated from the plurality of electrical signal conductors, wherein the second flex circuit includes a respective plurality of second electrical pads electrically isolated from the plurality of second electrical signal conductors, theplurality of second electrical pads of the second flex circuit in electrical communication with the plurality of third electrical pads of the first flex circuit, and wherein the flex circuit assembly includes: a third flex circuit having a respective plurality of electrical signal conductors and a respective plurality of first electrical pads in electrical communication with respective ones of the electrical signal conductors of the third flex circuit, wherein the first electrical pads of the third flex circuit are in electrical communication with respective ones of the second electrical pads of the second flex circuit.

17. The flex circuit assembly of claim 16, wherein respective ones of the second electrical pads of the first and second flex circuits and respective ones of the first electrical pads of the third flex circuit define a path oriented along the transverse direction.

18. The flex circuit assembly any one of claims 16 to 17, further comprising at least one of an anisotropic conductive film, an anisotropic conductive paste, and a nanowire interconnect disposed between the second and third flex circuits.

19. The flex circuit assembly of any one of claims 16 to 18, further comprising a stiffener coupled to the third flex circuit.

20. The flex circuit assembly of claim 19, wherein the third flex circuit is disposed between the second flex circuit and the stiffener in the transverse direction.

21. The flex circuit of any of claims 16 to 20, wherein the first flex circuit includes a respective plurality of fourth electrical pads electrically isolated from the respective plurality electrical signal conductors, wherein the second flex circuit includes a respective plurality of third electrical pads electrically isolated from the plurality of electrical signal conductors of the second flex circuit, the plurality of third electrical pads of the second flex circuit in electrical communication with the fourth plurality of electrical signal conductors of the first flex circuit, wherein the third flex circuit includes a respective plurality of second electrical pads electrically isolated from the plurality of electrical signal conductors of the third flex circuit, the plurality of second electrical pads of the third flex circuit in electrical communication with the plurality of third electrical pads of the second flex circuit, andwherein the flex circuit includes: a fourth flex circuit having a respective plurality of electrical signal conductors and a respective plurality of first electrical pads in electrical communication with respective ones of the electrical signal conductors of the fourth flex circuit, wherein respective ones of the first electrical pads of the fourth flex circuit are in electrical communication with respective ones of the second electrical pads of the third flex circuit.

22. The flex circuit assembly of any one of claims 1 to 21 wherein the flex circuit assembly is configured to be physically attached, electrically attached, or both, to an IC die or a die package substrate.

23. The flex circuit assembly of any one of claims 1 to 21 wherein the flex circuit assembly is configured to be physically attached, electrically attached, or both, to respective package pads on a die package surface.

24. The flex circuit assembly of any one of claims 1 to 21 further comprising a plurality of cables positioned adjacent to the second end of the flex circuit.

25. The flex circuit assembly of claim 24 further comprising at least one electrical flex connector positioned adjacent to the second circuit end wherein the at least one electrical flex connector is configured to mate with a cable connector and the cable connector carries the plurality of cables.

26. The flex circuit assembly of any one of claims 24 to 25 wherein each cable of the plurality of cables is physically attached to the flex circuit assembly.

27. The flex circuit assembly of any one of claims 24 to 26, wherein the plurality of cables are optical cables.

28. The flex circuit assembly of any one of claims 24 to 26, wherein the plurality of cables are coaxial cables with coaxial cable conductors.

29. The flex circuit assembly of any one of claims 24 to 28 wherein the plurality of cables are twin axial cables with a pair of cable conductors.

30. The flex circuit assembly of any one of claims 24 to 29 wherein the first and second flex circuits have a shorter end-to-end length than an end-to-end length of one of the plurality of cables.

31. The flex circuit assembly of claim of any one of claims 24 to 29 wherein the end-to- end length of the first and second flex circuits is at least two times less than an end-to-end cable length of one of the plurality of cables.

32. The flex circuit assembly of claim of any one of claims 24 to 29 wherein the end-to- end length of the first and second flex circuits is at least three times less than an end-to-end cable length of one of the plurality of cables.

33. The flex circuit assembly of any one of claims 24 to 29 wherein the end-to-end length of the flex circuit is at least four times less than an end-to-end cable length of one of the plurality of cables.

34. The flex circuit assembly of any one of claims 24 to 29 wherein the end-to-end length of the flex circuit is at least five times less than an end-to-end cable length of one of the plurality of cables.

35. The flex circuit assembly of any one of claims 24 to 29 wherein the end-to-end length of the flex circuit is at least six times less than an end-to-end cable length of one of the plurality of cables.

36. The flex circuit assembly of any one of claims 24 to 29 wherein the end-to-end length of the flex circuit is at least seven times less than an end-to-end cable length of one of the plurality of cables.

37. The flex circuit assembly of any one of claims 24 to 29 wherein the end-to-end length of the flex circuit is at least eight times less than an end-to-end cable length of one of the plurality of cables.

38. The flex circuit assembly of any one of claims 24 to 29 wherein the end-to-end length of the flex circuit is at least nine times less than an end-to-end cable length of one of the plurality of cables.

39. The flex circuit assembly of any one of claims 24 to 29 wherein the end-to-end length of the flex circuit is at least ten times less than an end-to-end cable length of one of the plurality of cables.

40. The flex circuit assembly of any of claims 1 to 39, wherein the first flex circuit includes a first electrically conductive layer positioned adjacent to the first flex circuit surface; and a second electrically conductive layer opposite the first electrically conductive layer, adjacent to the second flex circuit surface.

41. An electrical communication system comprising: the flex circuit assembly of any one of claims 1 to 40; an IC die; and a die package substrate that defines first, second, third and fourth die packages sides, each of the die package sides being no longer than 85 mm or 100mm, wherein at least 128 or at least 256 package pads are defined on each of the first, second, third, and fourth die package sides, each of the package pads configured to be attached directly to the flex circuit assembly, and wherein one of the first and second flex circuits are attached to respective ones of the package pads.

42. The electrical communication system of claim 41 wherein at least 1024 differential signal pair pads are carried on the surface of the die package substrate.

43. The electrical communication system of claim 42 wherein the at least 1024 differential signal pair pads are arranged with at least 256 differential signal pair pads on each of the respective first, second, third and fourth package sides.

44. The electrical communication system of any one of claims 42 to 43 wherein the at least 1024 differential signal pair pads are SMT pads.

45. The electrical communication system of any one of claims 42 to 43 wherein the at least 1024 differential signal pair pads are compression pads.

46. An electrical communication system comprising:an IC die package including an IC substrate and an IC die mounted to the IC substrate; a plurality of first electrical contacts arranged in first and second rows in electrical communication with the IC die, each of the first and second rows extending along a lateral direction, the first and second rows spaced from each other in a longitudinal direction perpendicular to the lateral direction, the first and second rows of first electrical contacts being coplanar in a plane that extends in the longitudinal and lateral directions ; and a flex circuit assembly comprising a first flex circuit and a second flex circuit stacked on the first flex circuit along a transverse direction perpendicular to each of the lateral and longitudinal directions, wherein a first end of the first flex circuit is electrically coupled to the first row of electrical contacts, a first end of the second flex circuit is electrically coupled to the second row of electrical contacts such that the first end of the first flex circuit is positioned between the first end of the second flex circuit and the IC substrate with respect to the transverse direction.

47. The electrical communication system of claim 46 further comprising cables having respective first cable ends and second cable ends, the first cable ends removably or permanently attached to the second circuit end of the flex circuit assembly.

48. The electrical communication system of claim 47 further comprising a panel connector, communications module, substrate or electrical component attached, directly or indirectly, to the second cable ends.

49. The electrical communication system of any one of claims 46 to 48 wherein the first die package side has at least 256 differential signal package pads on a surface of the IC substrate.

50. The electrical communication system of any one of claims 46 to 49 wherein the second die package side has at least 256 differential signal package pads on a surface of the IC substrate.

51. The electrical communication system of any one of claims 46 to 50 wherein the third die package side has at least 256 differential signal packages pads on a surface of the IC substrate.

52. The electrical communication system of any one of claims 46 to 51 wherein the fourth die package side has at least 256 differential signal package pads on a surface of the IC substrate.

53. A flex circuit comprising: first and second electrically conductive layers opposite each other along a transverse direction; a plurality of electrical signal conductors disposed between the first and second electrically conductive layers with respect to the transverse direction; a plurality of first electrical pads in electrical communication with the respective ones of the plurality of electrical signal conductors; and a plurality of second electrical pads electrically isolated from the respective plurality of electrical signal conductors.

54. The flex circuit of claim 53, wherein the flex circuit includes a first and second flex circuit surfaces opposite each other in the transverse direction, wherein the plurality of first electrical pads are disposed in a row along a lateral direction perpendicular to the transverse direction, and wherein the flex circuit includes first and second ends opposite each other in a longitudinal direction perpendicular to each of the transverse and lateral direction.

55. The flex circuit of claim 54, wherein the second plurality of signal pads extend from the first flex circuit surface to the second flex circuit surface.

56. The flex circuit of any one of claims 54 to 55, wherein the first plurality of signal pads extend from the first flex circuit surface toward the second flex circuit surface.

57. The flex circuit of claim 56, wherein the first plurality of signal pads do not extend to the second flex circuit surface.

58. The flex circuit of any one of claims 54 to 57, wherein the first plurality of signal pads are positioned between the second plurality of signal pads and the second end.

59. The flex circuit of any one of claims 54 to 58, wherein at least a portion of the first and second plurality of signal pads are coplanar in a plane including the longitudinal and lateral directions.

60. The flex circuit of any one of claims 54 to 59, wherein the first and second plurality of signal pads are positioned at the first end.

61. The flex circuit of any one of claims 54 to 60, wherein the first and second plurality of signal pads are all positioned on the first flex circuit surface.

62. The flex circuit of any one of claims 54 to 59, wherein the first and second plurality of signal pads are all positioned at the first end and all positioned on the first flex circuit surface.

63. The flex circuit of claim any one of claims 54 to 62, wherein the second plurality of signal pads are positioned in at least one row that extends in the lateral direction.

64. The flex circuit of any one of claims 53 to 63 wherein the first plurality of signal pads comprises differential flex signal pair pads that are spaced apart from one another such that at least two-hundred and fifty-six of the differential flex signal pair pads fit within an area of approximately 500 square millimeters.

65. The flex circuit of any one of claims 53 to 63 wherein the first plurality of signal pads comprises differential flex signal pair pads that are spaced apart from one another such that at least two-hundred and fifty-six of the differential flex signal pair pads fit within an area of approximately 550 square millimeters.

66. The flex circuit of any one of claims 53 to 63 wherein the first plurality of signal pads comprises differential flex signal pair pads that are spaced apart from one another such that at least two-hundred and fifty-six of the differential flex signal pair pads fit within an area of approximately 600 square millimeters.

67. The flex circuit of any one of claims 53 to 63 wherein the first plurality of signal pads comprises differential flex signal pair pads that are spaced apart from one another such that atleast two-hundred and fifty-six of the differential flex signal pair pads fit within an area of approximately 650 square millimeters.

68. The flex circuit of any one of claims 53 to 63 wherein the first plurality of signal pads comprises differential flex signal pair pads that are spaced apart from one another such that at least two-hundred and fifty-six of the differential flex signal pair pads fit within an area of approximately 700 square millimeters.

69. The flex circuit of any one of claims 53 to 63 wherein the first plurality of signal pads comprises differential flex signal pair pads that are spaced apart from one another such that at least two-hundred and fifty-six of the differential flex signal pair pads fit within an area of approximately 750 square millimeters.

70. The flex circuit of any one of claims 53 to 63 wherein the first plurality of signal pads comprises differential flex signal pair pads that are spaced apart from one another such that at least two-hundred and fifty-six of the differential flex signal pair pads fit within an area of approximately 800 square millimeters.

71. The flex circuit of any one of claims 53 to 63 wherein the first plurality of signal pads defines differential flex signal pair pads spaced apart from one another such that a row of at least sixty-four differential flex signal pair pads fit along a first die package side having a length greater than 50mm but not more than approximately 75mm.

72. The flex circuit of any one of claims 53 to 63 wherein the first plurality of signal pads defines differential flex signal pair pads spaced apart from one another such that a row of at least sixty-four differential flex signal pair pads fit within along a first die package side having a length greater than 55mm but not more than approximately 80mm.

73. The flex circuit of any one of claims 53 to 63 wherein the first plurality of signal pads defines differential flex signal pair pads spaced apart from one another such that a row of at least sixty-four differential flex signal pair pads fit within along a first die package side having a length greater than 60mm but not more than approximately 85mm.

74. The flex circuit of any one of claims 53 to 63 wherein the first plurality of signal pads defines differential flex signal pair pads spaced apart from one another such that a row of at least sixty-four differential flex signal pair pads fit within along a first die package side having a length greater than 65mm but not more than approximately 90mm.

75. The flex circuit of any one of claims 53 to 63 wherein the first plurality of signal pads defines differential flex signal pair pads spaced apart from one another such that a row of at least sixty -four differential flex signal pair pads fit within along a first die package side having a length greater than 70mm but not more than approximately 95mm.

76. The flex circuit of any one of claims 53 to 63 wherein the first plurality of signal pads defines differential flex signal pair pads spaced apart from one another such that a row of at least sixty-four differential flex signal pair pads fit within along a first die package side having a length greater than 75mm but not more than approximately 100mm.

77. The flex circuit of any one of claims 54 to 76, further comprising: a plurality of third electrical pads positioned at the second end, the plurality of third electrical pads electrically connected to the respective ones of the plurality of electrical signal conductors, wherein an electrical flex connector is attached to the plurality of third electrical pads at the second end and is configured to receive a mating cable connector.

78. The flex circuit of claim 77, wherein respective coaxial and / or twin axial cables are directly attached to the third plurality of signal pads.

79. The flex circuit of any one of claims 54 to 78, further comprising flex ground pads positioned at the first end on the first flex circuit surface.

80. The flex circuit of any one of claims 54 to 79, further comprising flex ground pads position at the second end on the second flex circuit surface.

81. The flex circuit of any one of claims 53 to 80, wherein the signal pads are all devoid of fusible elements prior to use and during use.

82. The flex circuit of any one of claims 53 to 81, wherein the flex circuit is made from liquid crystal polymer (LCP) material.

83. The flex circuit of any one of claims 53 to 82, wherein the flex circuit is configured to transmit data at frequencies up to 55 GHz while producing no more than -45 dB worst-case multi-active asynchronous cross talk.

84. The flex circuit of any one of claims 53 to 83, wherein the flex circuit is configured to transmit data at frequencies up to 55 GHz while producing no more than -40 dB worst-case multi-active asynchronous near-end cross talk.

85. The flex circuit of any one of claims 53 to 84, wherein the flex circuit is configured to transmit data at frequencies up to 55 GHz while producing no more than -35 dB worst-case multi-active asynchronous far-end cross talk.

86. The flex circuit of any one of claims 49 to 85, wherein the flex circuit is configured to transmit data at frequencies up to 100 GHz while producing no more than -30 dB worst-case multi-active asynchronous cross talk.

87. The flex circuit of any one of claims 53 to 76, further comprising: a plurality of third electrical pads electrically isolated from the respective plurality of electrical signal conductors.

88. The flex circuit of claim 87, further comprising: a plurality of fourth electrical pads electrically isolated from the respective plurality of electrical signal conductors.

89. An interconnect for a die package substrate comprising: two or more flex cables that each comprise a plurality of respective electrical signal traces and a plurality of electrical pads, wherein respective individual ones of the electrical signal traces terminate at respective individual ones of the plurality of electrical pads, wherein the electrical pads of one of the two or more flex cables are vertically stacked over an immediately adjacent another one of the two or more flex cables, with respective individual ones of the electrical pads of one of the two or more flex cables vertically coincident with respective individual ones of the electrical pads of the immediately adjacent another one of the two or more flex cables.

90. The interconnect of claim 89, wherein the respective individual ones of the electrical pads of one of the two or more flex cables are electrically connected, physically connected or both to the respective individual ones of the electrical pads of the immediately adjacent another one of the two or more flex cables.

91. The interconnect of any one of claims 89 and 90, wherein the two or more flex cables are not received in a common flex cable connector (FCC) or a flexible printed circuit (FPC) connector.

92. The interconnect of any one of claims 89-91, wherein the two or more flex cables are compressed in the same direction.

93. The interconnect of any one of claims 89-92, wherein the two or more flex cables overlay a major surface of a die package substrate.

94. The interconnect of any one of claims 89-93, wherein the electrical pads of the one of the two or more flex cables are electrically isolated from all electrical pads of the another one of the two or more flex cables.

95. The interconnect of any one of claims 89-94, wherein the electrical pads are electrical signal pads.

96. The interconnect of any one of claims 89-95, further comprising an anisotropic conductive film disposed between the one of the two or more flex cables and the another one of the two or more flex cables.

97. The interconnect of any one of claims 89-96, further comprising an anisotropic conductive paste disposed between the one of the two or more flex cables and the another one of the two or more flex cables.

98. The interconnect of any one of claims 89-97, further comprising a nanowire interconnect disposed between the one of the two or more flex cables and the another one of the two or more flex cables.

99. A flex circuit assembly comprising: a first flex circuit having a plurality of electrical signal conducting means, a plurality of first electrical conducting means in electrical communication with respective ones of the electrical signal conducting means, and a respective plurality of second electrical conducting means electrically isolated from the respective plurality of electrical signal conducting means; and a second flex circuit having a plurality of electrical signal conducting means and a respective plurality of first electrical conducting means in electrical communication with respective ones of the electrical signal conducting means of the second flex circuit, wherein the first electrical conducting means of the second flex circuit are in electrical communication with respective ones of the second electrical conductive means of the first flex circuit.

100. The flex circuit assembly of claim 99, wherein the first electrical conducting means of the first flex circuit are electrically isolated from the electrical signal conducting means of the second flex circuit.

101. The flex circuit assembly of claim 99, wherein the first electrical conducting means of the first flex circuit are electrically isolated from all electrical signal conducting means of the second flex circuit.

102. The flex circuit assembly of any one of claims 99 to 101, wherein the electrical conducting means are electrical signal pads.

103. The flex circuit assembly of any one of claims 99 to 102, wherein the first and second flex circuits include electrically insulating means that supports the electrical signal conducting means and first electrical conducting means.

104. The flex circuit assembly of any one of claims 99 to 103, wherein the flex circuit assembly includes first and second flex circuit surfaces opposite each other in a transverse direction,wherein the plurality of first electrical conducting means are disposed in a row along a lateral direction perpendicular to the transverse direction, and wherein the flex circuit includes first and second circuit ends opposite each other along a longitudinal direction perpendicular to each of the transverse and lateral directions.

105. The flex circuit assembly of claim 104, wherein respective ones of the second electrical conducting means of the first flex circuit and respective ones of the first electrical conducting means of the second flex circuit define a path oriented along the transverse direction.

106. The flex circuit assembly of any one of claims 104 to 105, wherein respective ones of the second electrical conducting means of the first flex circuit and respective ones of the first electrical conducting means of the second flex circuit define are stacked along the transverse direction.

107. The flex circuit assembly of any one of claims 104 to 106, wherein the electrical signal conducting means are at least partially embedded in the respective flex circuit bodies.

108. The flex circuit assembly of any of claims 99 to 107, further comprising an anisotropic conductive film disposed between the first flex circuit and the second flex circuit.

109. The flex circuit assembly of any of claims 99 to 108, further comprising an anisotropic conductive paste disposed between the first flex circuit and the second flex circuit.110 The flex circuit assembly of any of claims 99 to 109, further comprising a nanowire interconnect disposed between the first flex circuit and the second flex circuit.

111. An electrical communication system comprising: an IC die package including an IC substrate and an IC die mounted to the IC substrate; a plurality of first electrical conducting means arranged in first and second rows in electrical communication with the IC die, each of the first and second rows extending along a lateral direction, the first and second rows spaced from each other in a longitudinal directionperpendicular to the lateral direction, the first and second rows of first electrical conducting means being coplanar in a plane that extends in the longitudinal and lateral directions ; and a flex circuit assembly comprising a first flex circuit and a second flex circuit stacked on the first flex circuit along a transverse direction perpendicular to each of the lateral and longitudinal directions, wherein a first end of the first flex circuit is electrically coupled to the first row of electrical conducting means, a first end of the second flex circuit is electrically coupled to the second row of electrical conducting means such that the first end of the first flex circuit is positioned between the first end of the second flex circuit and the IC substrate with respect to the transverse direction.

112. The electrical communication system of claim 111 further comprising cables having respective first cable ends and second cable ends, the first cable ends removably or permanently attached to the second circuit end of the flex circuit assembly.

113. A flex circuit comprising: first and second electrically conductive layers opposite each other along a transverse direction; a plurality of electrical signal conducting means disposed between the first and second electrically conductive layers with respect to the transverse direction; a plurality of first electrically conducting means in electrical communication with the respective ones of the plurality of electrical signal conducting means; and a plurality of second electrically conductive means electrically isolated from the respective plurality of electrical signal conducting means.

114. The flex circuit of claim 113, wherein the flex circuit includes a first and second flex circuit surfaces opposite each other in the transverse direction, wherein the plurality of first electrically conductive means are disposed in a row along a lateral direction perpendicular to the transverse direction, and wherein the flex circuit includes first and second ends opposite each other in a longitudinal direction perpendicular to each of the transverse and lateral direction.

115. The flex circuit of claim 114, wherein the second plurality of electrically conductive means extend from the first flex circuit surface to the second flex circuit surface.

116. The flex circuit of any one of claims 114 to 115, wherein the first plurality of electrically conducting means extend from the first flex circuit surface toward the second flex circuit surface.

117. The flex circuit of claim 116, wherein the first plurality of electrically conductive means do not extend to the second flex circuit surface.

118. An interconnect for a die package substrate comprising: two or more flex cables that each comprise a plurality of respective electrical signal tracing means and a plurality of electrical contacting means, wherein respective individual ones of the electrical signal tracing means terminate at respective individual ones of the plurality of electrical contacting means, wherein electrical contacting means of one of the two or more flex cables are vertically stacked over an immediately adjacent another one of the two or more flex cables, with respective individual ones of the electrical contacting means of one of the two or more flex cables vertically coincident with respective individual ones of the electrical contacting means of the immediately adjacent another one of the two or more flex cables.

119. The interconnect of claim 118, wherein the respective individual ones of the electrical contacting means of one of the two or more flex cables are electrically connected, physically connected or both to the respective individual ones of the electrical contacting means of the immediately adjacent another one of the two or more flex cables.

120. The interconnect of any one of claims 118 and 119, wherein the two or more flex cables are not received in a common flex cable connector (FCC) or a flexible printed circuit (FPC) connector.

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