Method and apparatus for flexible circuit cable connection

Abstract

A method and apparatus for multiple flexible circuit cable connections is described. Gold bumps are bonded on the interconnect pads of the substrate to form a columnar structure, and solder or conductive epoxy is dispensed on the flexible circuit cable. The substrate and flexible circuit cable are aligned and pressed together with force or by placing a weight on the substrate or flexible circuit cable. Appropriate heat is applied to reflow the solder or cure the epoxy. With the aid of the gold bumps, the solder wets to the substrate pads and has a reduced risk of bridging due to the columnar structure. Non-conductive underfill epoxy is applied to increase mechanical strength.

Description

[0001] This case is a divisional application of Chinese Invention Patent Application No. 201780068352.2, titled “Method and apparatus for flexible circuit cable connections”, filed on November 3, 2017. TECHNICAL FIELD

[0002] The present invention relates generally to flexible circuit cables, and more particularly to the connection of flexible circuit cables to substrates. BACKGROUND

[0003] Flexible circuit cables are widely used for chip to printed circuit board (PCB), chip to substrate, optical subassembly to PCB, and PCB to PCB interconnections. They provide high density signal routing functionality in a limited space and flexible manner. However, previous methods for connecting flexible circuit cables have various drawbacks.

[0004] For example, when flexible circuit cables do not overlap, direct soldering of a single flexible circuit cable connection or multiple flexible circuit cable connections is possible or preferred. However, in the case of overlapping flexible circuit cables, the thermal reflow of solder will affect chips or flexible circuit cables already connected to the substrate. For example, as shown in FIG. 1, when attempting to connect flexible circuit cable Flex#2 using a direct soldering method, the pre-connected cable or flexible circuit cable Flex#1 will be adversely affected. In the case of sequential flexible circuit cables that need to overlap with a pre-connected cable or flexible circuit cable (Flex#1), it is almost impossible to wet the solder between the substrate pads and the second flexible circuit cable (Flex#2). The gap created by the pre-connected cable or flexible circuit cable (Flex#1) will interfere with the placement of the second flexible circuit cable (Flex#2). Solder will have difficulty flowing between the flexible circuit cable and the substrate to form an electrical connection and a durable reliable bond. Figure 1 Anisotropic conductive film (ACF) and / or anisotropic conductive paste (ACP) methods are widely used for flexible circuit cable to substrate and chip to flexible circuit cable connections for liquid crystal display manufacturing. These processes also have some limitations. For example, these processes require electrical pads to be embossed (raised) from the surface of the flexible circuit cable and substrate so that the conductive particles in the ACF or ACP can make electrical connections in the Z direction by compression contact. The ACF process also requires a high thermal temperature to cure the thin film to form a bond. This high temperature will affect chips or flexible circuit cables already connected to the substrate, so the ACF and / or ACP process makes it difficult to connect overlapping flexible circuit cables if there is a gap created by a previously connected flexible circuit cable or chip. In addition, the conductive particle filled epoxy resin traditionally used for ACPs generally has high resistance and results in limited radio frequency (RF) bandwidth.

[0005] Anisotropic conductive film (ACF) and / or anisotropic conductive paste (ACP) methods are widely used for flexible circuit cable to substrate and chip to flexible circuit cable connections for liquid crystal display manufacturing. These processes also have some limitations. For example, these processes require electrical pads to be embossed (raised) from the surface of the flexible circuit cable and substrate so that the conductive particles in the ACF or ACP can make electrical connections in the Z direction by compression contact. The ACF process also requires a high thermal temperature to cure the thin film to form a bond. This high temperature will affect chips or flexible circuit cables already connected to the substrate, so the ACF and / or ACP process makes it difficult to connect overlapping flexible circuit cables if there is a gap created by a previously connected flexible circuit cable or chip. In addition, the conductive particle filled epoxy resin traditionally used for ACPs generally has high resistance and results in limited radio frequency (RF) bandwidth.

[0006] Flexible circuit cable connections can also be accomplished using conductive epoxy. The conductive epoxy can be dispensed on the pads of the flexible circuit cable or the substrate prior to placement. Epoxy connections present several issues including variation / flatness conditions between the flexible circuit cable and the substrate, over / under volume of the epoxy, proper pressure control, limited reworkability, bond strength, and higher electrical resistance than solder. Additionally, the added complexity of overlapping flexible circuit cables makes conductive epoxy more attractive.

[0007] In summary, there are many challenges in connecting overlapping flexible circuit cables to a substrate, interposer, or other structure. The connection of the flexible circuit cable should not impact the assembled chips on the substrate or interposer. Any proposed method needs to overcome 1) wetting issues between the flexible circuit cable and the substrate due to gaps formed by previously assembled flexible circuit cables or chips, or the design of the flexible circuit cable, and 2) coplanarity issues caused by the pre-bending of the flexible circuit cable, or unique shapes such as U-shaped, S-shaped, or open O-shaped flexible circuit cables. The proposed method must be operable in limited space, consider the signal RF bandwidth, and achieve low signal cross-talk to achieve high bandwidth and high density signal traces on a single flexible cable circuit. For example, flexible circuit cable connections are used in small form factor assembly scenarios such as optical sub-assembly (OSA) used in pluggable transceivers (small form-factor pluggable transceivers (SFP, SFP+, QSFP), C-form factor pluggable transceivers (CFP, CFP2+, QSFP), C-form factor pluggable transceivers (CFP, CFP2), etc.). SUMMARY

[0008] A method and apparatus for flexible circuit cable connection is described. Gold bumps are bonded on the interconnect pads of the substrate and solder, or conductive epoxy is printed or dispensed on the flexible circuit cable. Multiple gold bumps can be bonded to each interconnect pad to form a post to limit the path of the solder or epoxy. The substrate and flexible circuit cable are aligned and pressed together with force or by placing a weight on the substrate or flexible circuit cable. Appropriate heat is applied to reflow the solder or cure the epoxy. The solder wets to the interconnect pads (assisted by the gold bumps) and reduces the risk of bridging of the post created by multiple gold bumps on the interconnect pads. If conductive epoxy is printed or dispensed on the flexible circuit cable, heat, ultraviolet (UV) light, or both can be applied to cure the epoxy between the flexible circuit cable and the substrate. The risk of the epoxy being squeezed and smeared to adjacent pads is reduced due to the gap and volume created by the gold bumps. To increase mechanical strength, non-conductive underfill epoxy can be applied. Heat and capillary effect will draw the underfill epoxy between the flexible circuit cable and the substrate. The gold bumps create standoff height, help the underfill path, solder wicking, and reduce the risk of bridging. In addition, for high frequency applications, multiple gold bumps bring improved radio frequency (RF) performance. BRIEF DESCRIPTION OF DRAWINGS

[0009] A more detailed understanding can be had from the following description, given by way of example in conjunction with the accompanying drawings wherein:

[0010] Figure 1 is a side view of a flexible circuit cable directly soldered to a substrate with solder;

[0011] Figure 2A , 2B and 2C are examples of a substrate, short flexible circuit cable, and long flexible circuit cable according to one embodiment;

[0012] Figure 3 is an overall flowchart of connecting a flexible circuit cable to a substrate according to one embodiment;

[0013] Figure 4 is an example of a substrate with gold bumps according to one embodiment;

[0014] Figure 5 is an example of a substrate with one layer of gold bumps according to one embodiment;

[0015] Figure 6 is an example of a substrate with two layers of stacked gold bumps according to one embodiment;

[0016] Figure 7 is an example of different patterns for placing gold bumps on a substrate according to various embodiments;

[0017] Figure 8 is a photograph of six gold bump patterns on a substrate according to one embodiment;

[0018] Figure 9A shows an example of a flexible circuit cable that is uncleaned according to one embodiment;

[0019] Figure 9B shows an example of a flexible circuit cable that is cleaned according to one embodiment;

[0020] Figure 10 is an example of a stencil used to print solder on a flexible circuit cable according to one embodiment;

[0021] Figure 11 is an example photograph of solder printed on a flexible circuit cable after a first reflow according to one embodiment;

[0022] Figure 12 is an example photograph of solder printed on a flexible circuit cable after a second reflow according to one embodiment;

[0023] Figure 13A shows an example of a flexible circuit cable connected to a substrate according to one embodiment;

[0024] Figure 13B shows an example of a substrate connected to a flexible circuit cable according to one embodiment;

[0025] Figure 14 is an example schematic diagram showing a hot air rework system according to one embodiment;

[0026] Figure 15 is an example photograph showing underfill according to one embodiment;

[0027] Figure 16 shows an example of a first flexible circuit cable connected to a substrate according to one embodiment;

[0028] Figure 17 is an example schematic diagram showing a bending tool according to one embodiment;

[0029] Figure 18 shows an example of a second flexible circuit cable connected to a substrate according to one embodiment;

[0030] Figure 19 shows an example of first and second flexible circuit cables connected to a substrate according to an embodiment of gold bumps having two layer stacks;

[0031] Figure 20Ais an exemplary schematic diagram illustrating a reflow fixture according to one embodiment;

[0032] Figure 20B is an exemplary schematic diagram illustrating a first flexible circuit cable assembly in disassembly according to one embodiment;

[0033] Figure 20C is an exemplary schematic diagram illustrating a first flexible circuit cable assembly in assembly according to one embodiment;

[0034] Figure 21 is an exemplary schematic diagram illustrating a reflow fixture with thermocouples according to one embodiment;

[0035] Figure 22A is an exemplary schematic diagram illustrating a pick-up table and weight according to one embodiment;

[0036] Figure 22B is an exemplary schematic diagram illustrating a pick-up table, weight, and assembled flexible circuit cable and substrate according to one embodiment;

[0037] Figure 23 is an exemplary schematic diagram illustrating a weight according to one embodiment;

[0038] Figure 24 is an overall flow chart of connecting a first flexible circuit cable to a substrate according to one embodiment;

[0039] Figure 25 is an illustrative temperature profile for connecting a first flexible circuit cable to a substrate according to one embodiment;

[0040] Figure 26 is an overall flow chart of connecting a second flexible circuit cable to a substrate according to one embodiment;

[0041] Figure 27 is an illustrative temperature profile for connecting a second flexible circuit cable to a substrate according to one embodiment; and

[0042] Figure 28 is an illustrative temperature profile for a bottom fill curing method according to one embodiment. DETAILED DESCRIPTION

[0043] It should be understood that the drawings and description of embodiments of methods and apparatus for flexible circuit cable connections have been simplified for purposes of explanation and to convey the substance of the present application. Many other elements and / or steps can be desirable and / or necessary in implementing the present application. However, they are not discussed herein in order to avoid obscuring the essence of the present application. Embodiments of methods and apparatus for flexible circuit cable connections can be implemented in hardware, software, firmware, or any combination thereof.

[0044] The non-limiting embodiments described herein are directed to methods and apparatus for flexible circuit cable connections. Methods and apparatus for flexible circuit cable connections can be modified for various applications and uses while remaining within the spirit and scope of the claims. The embodiments and variations described herein and / or illustrated in the drawings are presented by way of example only, and do not limit the scope and spirit of the application. The description herein can apply to all embodiments of methods and apparatus for flexible circuit cable connections, including, for example, but not limited to, chip to printed circuit board (PCB) interconnects, chip to substrate interconnects, optical subassembly to PCB interconnects, PCB to PCB interconnects.

[0045] Referring now to the drawings, in which like reference numerals represent like elements across several views, methods and apparatus for flexible circuit cable connections are described. The embodiments described herein provide solutions for connecting flexible circuit cables, and in some embodiments, for example, when multiple flexible circuit cables overlap.

[0046] Described herein is an illustrative method that provides multiple gold bumps on an interconnect pad located on a substrate. The gold bumps overcome gaps or variations between the substrate and the flexible circuit cable when multiple flexible circuit cables are connected to the substrate, particularly when the flexible circuit cables overlap. In addition, the gold bumps provide increased wetting of solder from the flexible circuit cable to the substrate, a reduced risk of bridging or smearing, and minimal solder joint gaps, which allows for the use of underfill to increase mechanical strength. In some embodiments, the flexible circuit cable also utilizes double solder bumps to increase the amount of solder and help compensate for gaps between the flexible circuit cable and the substrate. Weights are used to keep the flexible circuit cable in close contact with the substrate during reflow and reduce the risk of open solder joints or pillow-shaped defects. Multiple fixtures are designed and developed as described herein.

[0047] Typically and as further described below, the interconnect pads on the substrate are provided with one or more gold (or copper) bumps. For example, each interconnect pad can have 3 to 9 gold (or copper) bumps. For illustrative purposes, the bumps can be in the form or shape of a circle on the interconnect pads. Note that there is no limitation on the type of substrate, it can be a silicon or glass interposer, LCD glass or a regular PCB, another flex circuit, or even a chip with pads that can be gold (copper) plated. Stencil printing is used to deposit solder or dispense conductive epoxy on the flex circuit cable to be connected. The flex circuit cable and the substrate are aligned with each other and placed together. Pressure is applied by force or weight on top of the substrate or flex circuit cable, whichever is on top.

[0048] Heat is applied to reflow the solder or cure the epoxy. The solder will flow by the gold bumps to assist wetting to the substrate and has a reduced risk of bridging of the pillars created by multiple gold (or copper) bumps on the substrate. If conductive epoxy is printed on the flex circuit cable, heat, ultraviolet (UV) light, or both can be applied to cure the epoxy between the flex circuit cable and the substrate. The risk of the epoxy being squeezed and smeared to adjacent pads is reduced due to the gap created by the gold (or copper) bumps. Once cooled, to increase mechanical strength, a non-conductive underfill epoxy is applied. Heat and capillary action will draw the underfill epoxy between the flex circuit cable and the substrate. After the underfill epoxy is properly cured, the applied weight and / or pressure is removed.

[0049] The above steps can be repeated for each flex circuit cable to be connected to the substrate. Additional flex circuit cables can be pre-bent with a bending jig to reduce complexity of downstream assembly. The gold bumps on the substrate or flex circuit cable can compensate for the standoff height created by the first flex circuit cable, help the underfill path, wick the solder, and reduce the risk of bridging. X-ray imaging, connection testing, and shear and tensile testing can be performed to verify the mechanical integrity of the multiple flex circuit cable attachments.

[0050] Figure 2A is an illustrative example of a substrate 200. The substrate 200 can include a component 205 and interconnect pads 210. The substrate can be, but is not limited to, a single fused silicon (Si) glass substrate or any other type of similar material. While the description here is in relation to a substrate, the method can be applied to an interposer, a silicon or glass interposer, a liquid crystal display (LCD) glass, a printed circuit board (PCB), another flex circuit, a chip with gold (copper) plated pads, and other interface modules.

[0051] Figure 2B is an illustrative example of a flex circuit cable 220, and in particular, a short flex circuit cable. The flex circuit cable 220 has interconnect pads 225.Figure 2C is another illustrative example of a flex circuit cable 230, and in particular, a long form flex circuit cable. The flex circuit cable 230 has an interconnect pad 235. Figure 2B and 2C The flex circuit cables shown in

[0052] Figure 3 is an overall flowchart 300 of connecting a flex circuit cable to a substrate according to one embodiment. Gold bumps are wire bonded to the substrate (305). In one embodiment, the bumps can be ultrasonically connected to the substrate. Any connection method can be used that results in the diffusion of the bump material into the substrate. Although the description herein is with respect to gold bumps, other materials can be used, such as copper and aluminum. Figure 4 is shown a substrate 400 with gold bumps 405 on the interconnect pads 410 according to one embodiment. Figure 5 is an example of a substrate 500 with one layer of stacked gold bumps 505 according to one embodiment. Figure 6 is an example of a substrate with two layers of stacked gold bumps 600 according to one embodiment. The first layer of stacked gold bumps 605 are wire bonded to the interconnect pads corresponding to the placement of the first and second flex circuit cables, and the second layer of stacked gold bumps 610 are wire bonded interconnects corresponding to the placement of the second flex circuit cable. As described herein, the gold bumps provide additional volume, standoff height to help provide a bottom fill path, solder wicking, and reduce the risk of bridging between interconnect pads. Additionally, the bumps can be used to set a "standoff height" between the substrate and the flex circuit cable to ensure that the solder is not extruded under connection pressure.

[0053] Figure 7 is shown different patterns for placing gold bumps on a substrate according to various embodiments, including but not limited to one gold bump, two gold bumps, three gold bumps, four gold bumps, five gold bumps, and six gold bumps. The gold bumps can be arranged in various patterns and shapes, including but not limited to a circular pattern. The number of gold bumps is illustrative and can depend on the nature of the substrate, available area, interconnect pad size, and other similar considerations. Figure 8 is a photograph of a six gold bump pattern on a substrate according to one embodiment.

[0054] Referring back to Figure 3, cleaning the flexible circuit cables (310). Each flexible circuit cable is placed or submerged in denatured alcohol for an appropriate time to remove contaminants. For example, the placement in denatured alcohol can last for about 15 seconds. The flexible circuit cable is then placed in a spot remover for an appropriate time. For example, the placement in spot remover can last for about 3 minutes. The spot remover with the flexible circuit cable can be agitated to facilitate the cleaning process. The flexible circuit cable is then placed in a bag of denatured alcohol and the bag containing the flexible circuit cable is then placed in an ultrasonic bath for an appropriate time. For example, the placement in the ultrasonic bath can last for about 10 minutes. The flexible circuit cable is then removed and allowed to air dry. Figure 9A An example of a flexible cable circuit that is not cleaned is shown, Figure 9B An example of a cleaned flexible circuit cable is shown in accordance with one embodiment.

[0055] Referring back to Figure 3 Solder bumps are printed on the first flexible circuit cable (315). For purposes of illustration only, the first flexible circuit cable is a small flexible circuit cable. In one embodiment, the solder paste is tin-bismuth, which has a low temperature melting point. Various solder pastes can be used as the choice of solder paste depends on the application and other similar factors. Although the description here is in relation to a double bump soldering method, the number of solder bumps (including the use of a single solder bump) depends on the area, size, and other similar factors. In another embodiment, no solder or conductive epoxy is needed if a diffusion bonding process such as thermal compression or ultrasonic welding is used.

[0056] The solder paste is applied to the flexible circuit cable using a stencil. Figure 10 An example of a stencil for printing solder on a flexible circuit cable in accordance with one embodiment is shown. The flexible circuit cable with the solder paste is then placed on a hot plate at a predetermined temperature. For example, the predetermined temperature can be 250°C. Once the solder paste is fully reflowed (i.e., melted) and allowed to cool, the flexible circuit cable can be removed from the hot plate. The flexible circuit cable is placed in a denatured alcohol and ultrasonic bath for a predetermined or appropriate time. For example, the predetermined or appropriate time can be 10 minutes. In embodiments where double bumps are needed, a stencil is used to apply solder paste on the reflowed solder paste to increase the amount of solder. Figure 11 An example photograph of solder printed on a flexible circuit cable after a first reflow in accordance with one embodiment is shown, Figure 12 An example photograph of solder printed on a flexible circuit cable after a second reflow in accordance with one embodiment is shown.

[0057] Referring back to Figure 3The first flex circuit cable is placed on a hot air reflow system (HARS) fixture (320). The substrate and weight are placed on a pick-and-place table. The flex circuit cable and substrate are positioned as shown in Figure 13A or 13B. Specifically, Figure 13A An example is shown in which the flex circuit cable 1300 is connected to the substrate 1305 with the flex circuit cable 1300 over the substrate 1305. Fixtures 1310 and 1315 are used to support the flex circuit cable 1300 and substrate 1305, respectively, and press them together. Figure 13B An example is shown in which the flex circuit cable 1325 is connected to the substrate 1320 with the substrate 1320 over the flex circuit cable 1325. Fixtures 1330 and 1335 are used to support the flex circuit cable 1320 and substrate 1325, respectively, and press them together. In one embodiment, weights are cantilevered off the flex circuit cable to increase compression and bonding. After proper placement, the HARS sequence for the first flex circuit cable and substrate is initiated (325). Regarding Figure 24 A more detailed description of the HARS process is given.

[0058] Figure 14 is an example schematic diagram showing a HARS fixture 1400 according to an embodiment. In general, and as further described below, the HARS fixture 1400 is designed to control the temperature within a predetermined temperature range such that the first flex circuit cable and substrate bond together without affecting the integrity of any electronic components or substrate.

[0059] In particular, in embodiments that use solder, the HARS sequence will use appropriate heat to reflow the solder as described below, and the solder will be squeezed through the gold bumps to the substrate and contained by the column formed by the plurality of gold bumps on the substrate.

[0060] Although the description herein is with respect to solder, conductive epoxy can be used. If the conductive epoxy is printed or dispensed on the flex circuit cable, heat, ultraviolet (UV) light, or both can be applied to cure the epoxy between the flex circuit cable and the substrate. Due to the gap created by the gold bumps, the risk of the epoxy being squeezed and smeared to adjacent pads is reduced. That is, the epoxy is contained by the column formed by the plurality of gold bumps on the substrate.

[0061] After the HARS sequence is complete, the first flex circuit cable and the substrate are allowed to cool, and then, an underfill is applied and allowed to cure to increase mechanical stability (330). In one embodiment, the underfill is a non-conductive underfill epoxy. The underfill is applied to the edge of the flex circuit cable proximate to the substrate. Thermal and capillary effects absorb the underfill epoxy between the flex circuit cable and the substrate. The application of the underfill epoxy is stopped if a fillet is formed around the edge of the flex circuit cable. Heat is applied by a heat block built into the fixture or by conventional heating. Figure 15 is an example photograph showing the underfill according to one embodiment. The first flex circuit cable connection to the substrate can then be inspected using, for example, X-ray (335).

[0062] Figure 16 An example of the first flex circuit cable connected to the substrate according to one embodiment is shown. In particular, the first flex circuit cable 1600 is connected to a portion of the substrate 1605 using a single row of gold bumps 1610.

[0063] Referring back to Figure 3 , solder bumps are printed on the second flex circuit cable (340). For illustrative purposes only, the second flex circuit cable is a large flex circuit cable. In one embodiment, the solder paste is tin-bismuth, although various solder pastes can be used, the choice of solder paste depends on the application and other similar factors. The solder paste is applied to the flex circuit cable using a stencil. The flex circuit cable with the solder paste is then placed on a heated plate at a predetermined temperature. For example, the predetermined temperature can be 250°C. Once the solder paste is fully reflowed (i.e., melted) and allowed to cool, the flex circuit cable can be removed from the heated plate. The flex circuit cable is placed in a bath of denatured alcohol and ultrasonic waves for a predetermined or appropriate amount of time.

[0064] The second flex circuit cable needs to be bent at a predetermined angle relative to the interconnect pads and the rest of the second flex circuit cable. The predetermined angle is sufficient to clear the second flex circuit cable relative to the first flex circuit cable or other component carrier module. In one embodiment, the predetermined angle can be 35°. In another embodiment, the predetermined angle is between 35° and 60°. This can be accomplished using a bending tool 1700 as shown in Figure 17 . In particular, the flex circuit cable 1705 is placed in the bending tool 1700. The flex circuit cable and the bending tool are then placed in an oven at a predetermined temperature for a predetermined amount of time. In the illustrative example, the predetermined temperature is 60°C and the predetermined amount of time is 1 hour. As previously mentioned, if double bumps are needed, a stencil is used to apply solder paste on the reflowed solder paste to increase the amount of solder. As previously mentioned, the solder paste is exemplary and a conductive epoxy can be used.

[0065] Referring back Figure 3 The second flex circuit cable is placed into a hot air reflow system (HARS) fixture (345). The substrate and weight are placed on the pick-and-place table. The flex circuit cable and substrate are positioned as shown in Figure 13A or 13B. Figure 18 An example of a second flex circuit cable 1800 connected to a substrate 1805 is shown according to one embodiment. In particular, a fixture 1810 is placed on top of the second flex circuit cable 1800 and a fixture 1815 is used to support and squeeze the substrate 1805.

[0066] After proper placement, a HARS sequence of the second flex circuit cable and substrate is initiated (350). Regarding Figure 26 A more detailed description of the HARS process is given. As mentioned above, after the HARS sequence is completed, the second flex circuit cable and substrate are allowed to cool, then an underfill is applied and cured to increase mechanical stability (355). The second flex circuit cable connection to the substrate can then be inspected using, for example, X-ray (360).

[0067] Figure 19 An example of first and second flex circuit cables connected to a substrate is shown according to one embodiment. In particular, a first flex circuit cable 1900 is connected to a portion of a substrate 1905 using a single layer of stacked gold bumps 1910, and a second flex circuit cable 1915 is connected to a portion of the substrate 1905 using a double layer of stacked gold bumps 1920.

[0068] Referring back Figure 14 And with reference also to FIGS. 20-23, described herein is a HARS fixture 1400, and the various fixtures required to implement the HARS process for the first and second flex circuit cables. The HARS fixture 1400 includes a top heater 1405, a bottom heater 1410, fixture support legs 1415, a metal plate 1420, and Kapton tape 1425. The Kapton tape 1425 is used to cover all the air holes on the bottom heater 1405 except those close to the center. This forces all the air to flow to the center of the bottom heater 1405. The metal plate 1420 contains the hot air by creating a chimney effect and directs the air to a reflow fixture 1430 that also includes the flex circuit cables. The reflow fixture 1430 is supported by the fixture support legs 1415, which are vertical beams that hang above the bottom heater 1410. The top heater is located a predetermined distance above the reflow fixture 1430 and provides hot N2 gas. The height and temperature of the top heater 1405 are necessary to control the solder temperature, and the N2 helps with solder wetting. The predetermined distance of the top heater 1405 can vary depending on the application and materials, and can be 25 mm for illustrative purposes only.

[0069] Figure 20A is an exemplary schematic diagram illustrating a reflow fixture 2000 according to one embodiment. Further, Figure 20A illustrates how the first flex circuit cable 2005 and the second flex circuit cable 2010 are positioned on the reflow fixture 2000. The reflow fixture 2000 has vacuum lines 2015 that are used to hold the first flex circuit cable 2005 and the second flex circuit cable 2010 during reflow. Various holes 2020 are made to assist in fixture pre-heat from the bottom heater. The reflow fixture 2000 can be made of many suitable materials, including aluminum for purposes of illustration. The substrate is prevented from touching all of the heating air flow to minimize shifting during reflow. The reflow fixture 2000 is also designed to fit the HARS fixture position jaws (i.e., the fixture support legs 1415 in Figure 14 is an exemplary schematic diagram illustrating a reflow fixture 2100 with thermocouples 2105 according to one embodiment. In particular, Figure 14 each location marked with an "X" in Figure 20B indicates a thermocouple location for thermal profile control. To simulate the thermal mass of the assembly, the substrate and tungsten block (which is used as a weight and is further described below) are placed (as shown in Figure 20C and ) when determining the thermal profile. The thermocouples are placed on the reflow fixture 2100 as adjustment markers to trigger changes in the bottom heater and the top heater.

[0070] Figure 21 Figure 21 is an exemplary schematic diagram illustrating a HARS pick-up table 2200, a weight 2205, and a substrate 2210 according to one embodiment. The HARS pick-up table 2200 is designed to prepare the substrate 2210 and the weight 2205 (e.g., a tungsten block) to be picked up simultaneously and in a consistent position. Figure 20B is an exemplary schematic diagram illustrating a pick-up table 2250, a weight 2255, and an assembled flex circuit cable 2265 and a substrate 2260 according to one embodiment. 20C

[0071] Figure 22A Figure 22B

[0072] Figure 23 ​​​is an exemplary schematic of a weight 2300 according to one embodiment. In addition to securing the flexible circuit cable to the vacuum channel of the reflow fixture, a weight is applied to the substrate during the thermal air reflow process. The main portion of the weight 2300 can be, but is not limited to, a tungsten slug. Other metals can be used depending on the application. The weight 2300 is composed of alternating double sided polyimide 2305 and polytetrafluoroethylene (PTFE) 2310 layers, which are added to provide thermal insulation between the tungsten slug 2315 and the substrate. The outer layer is composed of a single sided polyimide layer 2320. In order to provide suction, a hole 2325 is drilled through the tungsten slug 2300 and the insulation layers 2305, 2310, and 2320. The hole can be, but is not limited to, a 2mm hole. This allows the weight 2300 and the substrate to be simultaneously picked and placed by a HARS pick tube (not shown), which is well known. In one embodiment, the weight 2300 can have a 33g tungsten slug, 0.14mm thick double sided polyimide tape (2x), 0.14mm thick PTFE (2x), and 0.03mm thick single sided polyimide (lx).

[0073] Figure 24 is an overall flowchart 2400 of connecting a first flexible circuit cable to a substrate according to one embodiment. The substrate and weight are picked from a HARS pick station (2405). The substrate and flexible circuit cable (e.g., small form factor flexible circuit cable) are aligned and placed in a HARS fixture (2410). Now also referring to Figure 14 The bottom heater 1410 and top heater 1405 are turned on and stabilized at 225°C and 150°C, respectively (2415). The temperatures are illustrative, other temperatures can be appropriate depending on the application. When the reflow fixture 1430 reaches a predetermined temperature (for illustrative purposes, this can be 166°C, but can vary depending on the application), the temperature of the bottom heater 1410 is decreased and stabilized at another predetermined temperature (this predetermined temperature can be 178°C, but can vary depending on the application) (2420). A timer is set for two minutes to increase the liquidus time (2425). The timer time is illustrative, and other times can be used based on the application and materials used. After the timer expires, the bottom heater 1410 and top heater 1405 are set to room temperature, and a cool down boost is induced from the top heater 1405 (2430). When the temperature of the reflow fixture 1430 reaches 138°C, the external cooling fan is turned on (2535). When the temperature of the reflow fixture 1430 reaches 80°C, the external cooling fan, bottom heater 1410, and top heater 1405 are turned off (2440).

[0074] Figure 25is an illustrative temperature profile for connecting a first flexible circuit cable to a substrate according to one embodiment. The temperatures and temperature ranges described herein allow for a rapid approach to reflow temperature before the flux burns off in the solder. The flux reduces oxides that need to be very low for adequate bonding. Also, as shown, the methods described herein allow for operation in a narrow thermal window to achieve bonding without damaging the components. The temperatures described in the embodiments herein are illustrative for the applications and materials used herein. Other temperatures can be used without departing from the scope of the claims. Figure 25

[0075] Figure 26 is an overall flowchart 2600 for connecting a second flexible circuit cable to a substrate according to one embodiment. The substrate and weight are picked up from a HARS pick station (2605). The substrate and flexible circuit cable (e.g., a large flexible circuit cable) are aligned and placed in a HARS fixture (2610). Reference is now also made to Figure 14 , the bottom heater 1410 and the top heater 1405 are turned on and stabilized at 225 °C and 150 °C, respectively (2615). The temperatures are illustrative, other temperatures can be appropriate depending on the application. When the reflow fixture 1430 reaches a predetermined temperature (for illustrative purposes this can be 162 °C, but can vary depending on the application), the temperature of the bottom heater 1410 is decreased and stabilized at another predetermined temperature (may be 182 °C, but can vary depending on the application) (2620). A timer is set for two minutes to increase the liquidus time (2625). The timer time is illustrative, and other times can be used based on the application and materials used. After the timer expires, the bottom heater 1410 and the top heater 1405 are set to room temperature, and a cool down boost is induced from the top heater 1405 (2630). When the temperature of the reflow fixture 1430 reaches 138 °C, the external cooling fans are turned on (2535). When the temperature of the reflow fixture 1430 reaches 80 °C, the external cooling fans, the bottom heater 1410, and the top heater 1405 are turned off. Figure 27 is an illustrative temperature profile for connecting a second flexible circuit cable to a substrate according to one embodiment. The temperatures described in the embodiments herein are illustrative for the applications and materials used herein. Other temperatures can be used without departing from the scope of the claims.

[0076] ​As described above, immediately after each flex circuit cable connection, an epoxy underfill is applied to increase mechanical strength. The epoxy underfill can be cured while mounted on the HARS fixture to reduce the risk of flex circuit cable connection for the second flex circuit cable connection. For the first or small flex circuit cable assembly, a single drop of underfill material can be used, and for the second or large flex circuit cable assembly, two drops can be used. The number of drops is illustrative and can vary depending on the application and materials used. Referring back to Figure 15 The arrow indicates the cured underfill material. In the illustrative embodiment, the underfill is a ball grid array (BGA) underfill, which can be applied at 80 °C and cured at 130 °C for over 8 minutes. The materials, times, and temperatures are illustrative and other values can be used without departing from the scope of the claims. It is noted that the underfill curing can not require a top heater.

[0077] The methods and apparatus described herein can enable flex circuit cable connections even when the flex circuit cable has an irregular shape. For example, when the interconnect pads on the flex circuit cable are located on a U-shaped region. These methods overcome the coplanarity issues of the pads on the flex circuit cable if the flex circuit cable is pre-bent and loses coplanarity. These methods provide multiple flex circuit cable connections and the flex circuit cables can overlap each other. Also, the methods can overcome the voids created by pre-connected chips or flex circuit cables.

[0078] The connection temperature is low and does not affect pre-connected chips or flex circuit cables. These methods reduce the connection resistance between the flex circuit cable and the substrate with multiple gold bumps (or copper pillars), thus providing a wider RF bandwidth and better interconnect signal integrity compared to other connection techniques. For example, the solderable conductive epoxy SMT138 E has a resistance of 1030 μΩ.cm, which is about 10 times larger than the gold resistance of 2.44 μΩ.cm.

[0079] Instead of routing signals through a single flex circuit cable, these methods can help improve crosstalk performance by using multiple flex circuit cables. When high-speed signals are too close to each other on a flex circuit cable, crosstalk between the signal lanes on the same flex circuit cable will occur. Therefore, it is advantageous to use more flexible circuit cables to transmit signals that require very low crosstalk, such as high-speed signals to drive optical transmitters and high-speed signals from optical receivers. These methods provide a high-density, high-throughput, wide-bandwidth signal fan-out solution from a small substrate or interposer. These methods can reduce the pad size and pitch on the substrate and flex circuit cable, thus increasing the interconnect density much more than any existing flex circuit cable connection solution.

[0080] These methods improve connection reliability because the solder or conductive epoxy is contained by the gold bumps and does not smear into adjacent pads. These methods reduce connection processing time because soldering and underfilling can be performed simultaneously.

[0081] In summary, gold bumps allow: 1) flexible circuit cable connections with limited coplanarity; 2) connections of irregular shaped flexible circuit cables; 3) multiple flexible circuit cable connections to the same substrate (overcoming pitch created by stacking flexible circuit cables); 4) the use of low temperature high pressure processes can be used; 5) reduction of electrical resistance between the substrate and the flexible circuit cable; 6) increased design flexibility; 7) improved radio frequency performance for high frequency applications.

[0082] Generally, a method for flexible circuit cable connection includes bonding a plurality of gold bumps to each of a plurality of interconnect pads on a substrate to create a post at each interconnect pad. An adhesive material is dispensed on a first flexible circuit cable. The substrate and the first flexible circuit cable are aligned and forced together with the posts limiting the spread of the adhesive material. A first predetermined level of heat is applied to cause the adhesive material to bond between the substrate and the first flexible circuit cable. In one embodiment, the method includes bending a second flexible circuit cable relative to the first flexible circuit cable; bonding a further plurality of gold bumps to each of a further plurality of interconnect pads on the substrate to create a post at each interconnect pad; dispensing an adhesive material on the second flexible circuit cable; aligning and forcing the substrate and the second flexible circuit cable together; and applying a second predetermined level of heat to cause the adhesive material to bond between the substrate and the second flexible circuit cable. In one embodiment, the first flexible circuit cable and the second flexible circuit cable overlap. In one embodiment, the first predetermined level of heat and the second predetermined level of heat are controlled to affect the bonding between the substrate and the first flexible cable and the bonding between the substrate and the second flexible circuit cable without affecting an electronic component. In one embodiment, the second predetermined level of heat is controlled to affect the bonding between the substrate and the second flexible cable without affecting the electronic component and the bonding between the substrate and the first flexible circuit cable. In one embodiment, a weight is used to at least force the substrate and the first flexible circuit cable together or the substrate and the second flexible circuit cable together. In one embodiment, the weight is cantilevered from at least the first flexible circuit cable to enhance compression and bonding. In one embodiment, the adhesive material is dispensed on the first flexible circuit cable at least twice. In one embodiment, the method includes applying an underfill between the substrate and the first flexible circuit cable to provide mechanical strength. In one embodiment, the plurality of gold bumps are arranged in a predetermined pattern on each interconnect pad.

[0083] Generally, a system for connecting flexible circuit cables includes a substrate comprising a first set of interconnect pads, wherein a plurality of gold bumps are bonded to each interconnect pad to form a columnar structure. The system includes at least one flexible circuit cable, wherein an adhesive material is dispensed on the at least one flexible circuit cable; an alignment device for aligning the substrate and the at least one flexible circuit cable; a weight for forcibly pressing the substrate and the at least one flexible circuit cable together; and a hot air reflow system (HARS) for applying a first predetermined level of heat set to facilitate bonding of the adhesive material between the substrate and the at least one flexible circuit cable, wherein the columnar structure restricts dispersion of the adhesive material. In one embodiment, an additional plurality of gold bumps are bonded to an additional set of interconnect pads to create an additional columnar structure. In one embodiment, the system further includes a bending tool; at least another flexible circuit cable, wherein the bending tool bends the at least another flexible circuit cable relative to the at least one flexible circuit cable, and wherein an adhesive material is dispensed on the at least another flexible circuit cable; an alignment device for aligning the substrate and the at least another flexible circuit cable; a weight for forcibly pressing the substrate and the at least another flexible circuit cable together, and a HARS for applying a second predetermined level of heat set to facilitate bonding of the adhesive material between the substrate and the at least another flexible circuit cable. In one embodiment, the at least one flexible circuit cable and the at least another flexible circuit cable overlap. In one embodiment, the first predetermined level of heat and the second predetermined level of heat are controlled to affect the bonding between the substrate and the at least one flexible cable and the bonding between the substrate and the at least another flexible circuit cable without affecting electronic components. In one embodiment, the second predetermined level of heat is controlled to affect the bonding between the substrate and the at least another flexible cable without affecting electronic components and the bonding between the substrate and the at least one flexible circuit cable. In one embodiment, the weight overhangs from the at least one flexible circuit cable and the at least another flexible circuit cable to enhance compression and bonding. In one embodiment, the adhesive material is dispensed at least twice on at least one of the at least one flexible circuit cable and the at least another flexible circuit cable. In one embodiment, an underfill is applied between the substrate and the at least one flexible circuit cable and the substrate and the at least another flexible circuit cable to provide mechanical strength. In one embodiment, the plurality of gold bumps and the additional plurality of gold bumps are arranged in a predetermined pattern. In one embodiment, the HARS includes a plurality of heaters to provide the first predetermined level of heat set and the second predetermined level of heat set.

[0084] It should be understood that the application is not limited to the embodiments described above, but includes any and all embodiments within the scope of the following claims. In addition, although features and elements are described above in particular combinations, each feature or element can be used alone without the other features and elements or in various combinations with or without other features and elements.

Claims

1. A method of connecting flexible circuit cables, the method comprising: bonding a plurality of gold bumps to each interconnect pad on a substrate to create a columnar structure, the substrate having a plurality of interconnect pads; coaligning a weight, the substrate, and at least one flexible circuit cable having an adhesive material on a hot air reflow system fixture configured for alignment; forcing the substrate and the at least one flexible circuit cable together using the weight while aligned on the hot air reflow system fixture; and applying a predetermined level of heat set by a hot air reflow system to facilitate bonding of the adhesive material between the substrate and the at least one flexible circuit cable while aligned with the weight on the hot air reflow system fixture, wherein the columnar structure restricts dispensing of the adhesive material.

2. The method of claim 1, further comprising: bonding a further plurality of gold bumps to each interconnect pad of a further plurality of interconnect pads to create an additional columnar structure; bending another flexible circuit cable having adhesive material thereon using a bending tool to form a bent flexible circuit cable; coaligning the weight, the substrate, and the bent flexible circuit cable on the hot air reflow system fixture; forcing the substrate and the bent flexible circuit cable together using the weight while the bent flexible circuit cable covers the further plurality of interconnect pads; and applying a second predetermined level of heat set by the hot air reflow system to facilitate bonding of the adhesive material of the bent flexible circuit cable between the substrate and the bent flexible circuit cable while aligned with the weight on the hot air reflow system fixture.

3. The method of claim 2, wherein the at least one flexible circuit cable and the bent flexible circuit cable overlap on the substrate.

4. The method of claim 2, wherein a first predetermined level of heat is controlled to affect bonding between the substrate and the at least one flexible cable and between the substrate and the bent flexible circuit cable without affecting electronic components.

5. The method of claim 2, wherein a second predetermined level of heat is controlled to affect bonding between the substrate and the bent flexible cable without affecting electronic components and bonding between the substrate and the at least one flexible circuit cable.

6. The method of claim 2, wherein the at least one flexible circuit cable is pressed against the plurality of interconnect pads on the substrate and the bent flexible circuit cable is pressed against the further plurality of interconnect pads on the substrate, the method further comprising: cantilevering the weight from at least the at least one flexible circuit cable and the bent flexible circuit cable to enhance compression and bonding while in the hot air reflow system fixture during application of heat by the hot air reflow system.

7. The method of claim 2, further comprising: dispensing the adhesive material at least two times on at least one of the at least one flexible circuit cable or the curved flexible circuit cable.

8. The method of claim 2, wherein the at least one flexible circuit is pressed against the plurality of interconnect pads on the substrate and the curved flexible circuit cable is pressed against the another plurality of interconnect pads on the substrate, the method further comprising: dispensing a bottom filler between the substrate and the at least one flexible circuit cable and between the substrate and the curved flexible circuit cable to provide mechanical strength.

9. The method of claim 8, wherein the plurality of gold bumps and the another plurality of gold bumps provide standoff height for dispensing the bottom filler.

10. The method of claim 2, wherein the plurality of gold bumps and the another plurality of gold bumps are in a predetermined pattern.

11. The method of claim 2, wherein the thermal air reflow system comprises a plurality of heaters to provide a first predetermined level of heat set and a second predetermined level of heat set.

12. A method of connecting flexible circuit cables, the method comprising: generating a plurality of posts for each of a first set of interconnect pads on a substrate, each post comprising at least two gold bumps; coaligning a weight, the substrate, and at least one flexible circuit cable on a thermal air reflow system fixture configured for alignment; forcing the substrate and the at least one flexible circuit cable together using the weight when aligned on the thermal air reflow system fixture; and applying a predetermined level of heat set by a thermal air reflow system when aligned with the weight on the thermal air reflow system fixture to facilitate bonding of an adhesive material on the at least one flexible circuit cable between the substrate and the at least one flexible circuit cable, wherein each post restricts dispensing of the adhesive material.

13. The method of claim 12, further comprising: generating another plurality of posts for each of a second set of interconnect pads on the substrate, each post comprising at least two gold bumps; bending another flexible circuit cable having an adhesive material thereon using a bending tool to form a curved flexible circuit cable; coaligning the weight, the substrate, and the curved flexible circuit cable on the thermal air reflow system fixture; forcing the substrate and the curved flexible circuit cable together using the weight when the curved flexible circuit cable covers the second set of interconnect pads; and applying a second predetermined level of heat set by the thermal air reflow system when aligned with the weight on the thermal air reflow system fixture to facilitate bonding of the adhesive material of the curved flexible circuit cable between the substrate and the curved flexible circuit cable.

14. The method of claim 13, wherein the at least one flexible circuit cable and the curved flexible circuit cable overlap on the substrate.

15. The method of claim 13, wherein a first predetermined level of heat is controlled to affect the bond between the substrate and the at least one flex circuit cable and between the substrate and the flexed flex circuit cable without affecting the electronic components.

16. The method of claim 13, wherein a second predetermined level of heat is controlled to affect the bond between the substrate and the flexed flex circuit cable without affecting the electronic components and the bond between the substrate and the at least one flex circuit cable.

17. The method of claim 13, wherein the at least one flex circuit cable is pressed against the first set of interconnect pads on the substrate and the flexed flex circuit cable is pressed against the second set of interconnect pads on the substrate, the method further comprising: cantilevering the weight from at least the at least one flex circuit cable and the flexed flex circuit cable during the application of heat by the hot air reflow system to enhance compression and bonding while in the hot air reflow system fixture.

18. The method of claim 13, further comprising: dispensing the adhesive material at least twice on at least one of the at least one flex circuit cable or the flexed flex circuit cable.

19. The method of claim 13, wherein the at least one flex circuit cable is pressed against the first set of interconnect pads on the substrate and the flexed flex circuit cable is pressed against the second set of interconnect pads on the substrate, the method further comprising: dispensing underfill between the substrate and the at least one flex circuit cable and between the substrate and the flexed flex circuit cable to provide mechanical strength.

20. The method of claim 13, wherein the hot air reflow system comprises a plurality of heaters to provide a first predetermined level of heat set and a second predetermined level of heat set.

21. A system for connecting flex circuit cables, comprising: a substrate comprising a first set of interconnect pads, wherein a plurality of gold bumps are bonded to each interconnect pad to create a post structure; at least one flex circuit cable, wherein an adhesive material is dispensed on the at least one flex circuit cable; a hot air reflow system fixture and a pick and place table configured to co-align a weight, the substrate, and the at least one flex circuit cable on the hot air reflow system fixture; the weight configured to force the substrate and the at least one flex circuit cable together; and a hot air reflow system (HARS) configured to apply a first predetermined level of heat set to facilitate the bonding of the adhesive material between the substrate and the at least one flex circuit cable in the hot air reflow system fixture with the weight, wherein the post structure restricts the dispensing of the adhesive material.

22. A system for connecting flex circuit cables, comprising: a substrate comprising interconnect pads, each interconnect pad having a bonded gold bump to create a post structure; ​ a hot air reflow system clamp configured to align a weight, the substrate, and a flexible circuit cable having an adhesive material; the weight configured to force the substrate and the flexible circuit cable together when aligned on the hot air reflow system clamp; and a hot air reflow system configured to apply a predetermined level of heat to facilitate bonding of the adhesive material between the substrate and the flexible circuit cable when aligned with the weight on the hot air reflow system clamp, wherein the post restricts distribution of the adhesive material.

23. A system for connecting a flexible circuit cable, the system comprising: a substrate comprising interconnect pads, each interconnect pad having at least two bonded gold bumps to create a post; a hot air reflow system clamp configured to align a weight, the substrate, and a flexible circuit cable; the weight configured to force the substrate and the flexible circuit cable together when aligned on the hot air reflow system clamp; and a hot air reflow system configured to apply a predetermined level of heat to facilitate bonding of the adhesive material between the substrate and the flexible circuit cable when aligned with the weight on the hot air reflow system clamp, wherein each post restricts distribution of the adhesive material.

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