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 201780068352.2 entitled "Method and apparatus for connecting flexible circuit cables", filed on November 3, 2017. Technical Field

[0002] This invention generally relates to flexible circuit cables, and more particularly to the connection between flexible circuit cables and a substrate. Background Technology

[0003] Flexible circuit cables are widely used in chip-to-printed circuit board (PCB) interconnects, chip-to-substrate interconnects, optical sub-assembly-to-PCB interconnects, and PCB-to-PCB interconnects. They provide high-density signal routing capabilities in limited space and with flexible methods. However, previous methods for connecting flexible circuit cables have various drawbacks.

[0004] For example, when flexible circuit cables do not overlap, it is possible or preferred to directly solder a single flexible circuit cable connection or multiple flexible circuit cable connections. However, when flexible circuit cables overlap, the thermal reflow of the solder will affect the chips or flexible circuit cables already connected to the substrate. For example, as... Figure 1 As shown, when attempting to connect the 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 cases where the sequential flexible circuit cable needs to overlap with the pre-connected cable or flexible circuit cable (Flex#1), ​​it is nearly 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) interferes with the placement of the second flexible circuit cable (Flex#2). Solder struggles to flow between the flexible circuit cable and the substrate to form an electrical connection and a durable, reliable bond.

[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 in liquid crystal display manufacturing. These processes also have some limitations. For example, these processes require bonding pads to be imprinted (raised) from the surfaces of the flexible circuit cable and the substrate, allowing the conductive particles in the ACF or ACP to form an electrical connection in the Z-direction through compression contact. The ACF process also requires high thermal temperatures to cure the film to form a bond. This high temperature can affect chips or flexible circuit cables already bonded to the substrate, thus making it difficult to connect overlapping flexible circuit cables if gaps exist due to previously bonded flexible circuit cables or chips. Furthermore, the epoxy resin used to fill the conductive particles in ACPs typically has high resistance, resulting in limited radio frequency (RF) bandwidth.

[0006] Conductive epoxy resin can also be used to complete flexible circuit cable connections. Before placement, conductive epoxy resin can be applied to the pads of the flexible circuit cable or substrate. Several issues exist with epoxy resin bonding, including variations in the flexibility / flatness between the flexible circuit cable and the substrate, excessive / inadequate epoxy volume, proper pressure control, limited reprocessability, bond strength, and higher resistance than solder. Furthermore, the increased complexity due to overlapping flexible circuit cables makes conductive epoxy resin more attractive.

[0007] In summary, connecting overlapping flexible circuit cables to substrates, interposers, or other structures presents numerous challenges. The connection of flexible circuit cables should not affect the assembled chips on the substrate or interposer. Any proposed method needs to overcome 1) wetting problems between the flexible circuit cable and the substrate due to gaps formed by previously assembled flexible circuit cables or chips, or due to the design of the flexible circuit cable, and 2) coplanarity problems 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 within limited space, taking into account signal RF bandwidth and achieving low signal crosstalk to realize high bandwidth and high-density signal traces on a single flexible cable circuit. For example, flexible circuit cable connections are used in small component scenarios, such as optical subassemblies (OSAs) used in pluggable transceivers (Small pluggable transceivers (SFP, SFP+, QSFP), Type-C pluggable transceivers (CFP, CFP2+, QSFP), Type-C pluggable transceivers (CFP, CFP2) etc.). Summary of the Invention

[0008] This invention describes a method and apparatus for connecting flexible circuit cables. Gold bumps are bonded to interconnect pads on a substrate and solder, or conductive epoxy resin is printed or dispensed onto the flexible circuit cable. Multiple gold bumps can be bonded to each interconnect pad to form pillars that restrict the path of the solder or epoxy resin. The substrate and flexible circuit cable are aligned and pressed together by 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 resin. The solder wets the interconnect pads (assisted by the gold bumps) and reduces the bridging risk of pillars created by the multiple gold bumps on the interconnect pads. If conductive epoxy resin is printed or dispensed onto the flexible circuit cable, heat, ultraviolet (UV) light, or both can be applied to cure the epoxy resin between the flexible circuit cable and the substrate. Due to the gaps and volumes created by the gold bumps, the risk of epoxy resin being squeezed and smeared onto adjacent pads is reduced. To increase mechanical strength, a non-conductive underfill epoxy resin can be applied. Heat and capillary effects will absorb the underfill epoxy between the flexible circuit cable and the substrate. Gold bumps form a support height, which helps the underfill path, solder wicking, and reduces bridging risk. Furthermore, for high-frequency applications, multiple gold bumps provide improved radio frequency (RF) performance. Attached Figure Description

[0009] A more detailed understanding can be obtained by referring to the following description, which is given by way of example in conjunction with the accompanying drawings, wherein:

[0010] Figure 1 This is a side view of a flexible circuit cable being directly soldered onto a substrate using solder.

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

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

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

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

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

[0016] Figure 7 These are examples of different patterns for placing gold bumps on a substrate according to various embodiments;

[0017] Figure 8 It is a photograph of a pattern of six gold bumps on a substrate according to one embodiment;

[0018] Figure 9A An example of an uncleaned flexible circuit cable according to one embodiment is shown;

[0019] Figure 9B An example of a clean, flexible circuit cable according to one embodiment is shown;

[0020] Figure 10 This is an example of a template for printing solder on a flexible circuit cable according to one embodiment;

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

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

[0023] Figure 13A An example of a flexible circuit cable connected to a substrate according to one embodiment is shown;

[0024] Figure 13B An example of a substrate connected to a flexible circuit cable according to one embodiment is shown;

[0025] Figure 14 This is an exemplary schematic diagram illustrating a hot air rework system according to one embodiment;

[0026] Figure 15 This is an example photograph showing the bottom filler according to one embodiment;

[0027] Figure 16 An example of a first flexible circuit cable connected to a substrate according to one embodiment is shown;

[0028] Figure 17 This is an exemplary schematic diagram illustrating a bending tool according to one embodiment;

[0029] Figure 18 An example of a second flexible circuit cable connected to a substrate according to one embodiment is shown;

[0030] Figure 19 Examples of first and second flexible circuit cables connected to a substrate according to an embodiment of gold bumps having two stacks are shown.

[0031] Figure 20AThis is an exemplary schematic diagram illustrating a return fixture according to one embodiment;

[0032] Figure 20B This is an exemplary schematic diagram showing a disassembled first flexible circuit cable assembly according to one embodiment;

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

[0034] Figure 21 This is an exemplary schematic diagram showing a return clamp with a thermocouple according to one embodiment;

[0035] Figure 22A This is an exemplary schematic diagram illustrating a pickup platform and a weight according to one embodiment;

[0036] Figure 22B This is an exemplary schematic diagram illustrating a pickup platform, a weight, and an assembled flexible circuit cable and substrate according to one embodiment.

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

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

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

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

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

[0042] Figure 28 This is an illustrative temperature profile for a bottom-fill curing method according to one embodiment. Detailed Implementation

[0043] It should be understood that the accompanying drawings and descriptions of embodiments of the methods and apparatus for flexible circuit cable connections have been simplified to illustrate the elements relevant to clear understanding, while many other elements in typical vehicle systems have been omitted for clarity. Those skilled in the art will recognize that other elements and / or steps are expected and / or necessary in carrying out the present invention. However, because these elements and steps are well known in the art and because they do not facilitate a better understanding of the invention, a discussion of these elements and steps is not provided herein.

[0044] The non-limiting embodiments described herein relate to methods and apparatus for flexible circuit cable connections. While remaining within the spirit and scope of the claims, the methods and apparatus for flexible circuit cable connections can be modified for various applications and uses. The embodiments and variations described herein and / or shown in the accompanying drawings are presented as examples only and are not intended to limit the scope or spirit. The description herein is applicable to all embodiments of the 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 sub-assembly-to-PCB interconnects, and PCB-to-PCB interconnects.

[0045] Referring now to the accompanying drawings, in which similar reference numerals denote similar elements across several views, methods and apparatus for connecting flexible circuit cables are described. The embodiments described herein provide solutions for connecting flexible circuit cables, and in some embodiments, such as when multiple flexible circuit cables overlap.

[0046] This describes an illustrative method that provides multiple gold bumps on interconnect pads located on a substrate. When multiple flexible circuit cables are attached to the substrate, particularly when the flexible circuit cables overlap, the gold bumps overcome gaps or variations between the substrate and the flexible circuit cables. Furthermore, the gold bumps provide increased wettability of the solder from the flexible circuit cables to the substrate, a reduced risk of bridging or smearing, and minimal solder gaps, which allows for underfill to increase mechanical strength. In some embodiments, the flexible circuit cables also utilize dual solder bumps to increase solder volume and help compensate for gaps between the flexible circuit cables and the substrate. Weights are used to maintain tight contact between the flexible circuit cables and the substrate during reflow and to reduce the risk of open solder joints or pincushion defects. Multiple fixtures are designed and developed as described herein.

[0047] Typically, and as further described below, interconnect pads on the substrate are provided with one or more gold (or copper) bumps. For example, each interconnect pad may have 3 to 9 gold (or copper) bumps. For illustrative purposes, the bumps may be circular or circular in shape 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 conventional PCB, another flexible circuit, or even a chip with pads that can be plated with gold (copper). Stencil printing is used to deposit solder or dispense conductive epoxy onto the flexible circuit cables to be connected. The flexible circuit cables and the substrate are aligned with each other and placed together. Pressure is applied by force or weight on the top of the substrate or the flexible circuit cable (whichever is on top).

[0048] Heat is applied to reflow the solder or cure the epoxy. The solder will flow to the substrate with the aid of gold bump wetting, and the risk of bridging from pillars created by multiple gold (or copper) bumps on the substrate is reduced. If conductive epoxy is printed 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 epoxy being squeezed and smeared onto adjacent pads is reduced due to the gaps created by the gold (or copper) bumps. Once cooled, a non-conductive underfill epoxy is applied to increase mechanical strength. Heat and capillary effects will absorb the underfill epoxy between the flexible circuit cable and the substrate. After the underfill epoxy has cured properly, the applied weight and / or pressure is removed.

[0049] The above steps can be repeated for each flexible circuit cable to be attached to the substrate. Additional flexible circuit cables can be pre-bent using a bending jig to reduce the complexity of downstream components. Gold bumps on the substrate or flexible circuit cables can compensate for the support height created by the first flexible circuit cable, aiding in bottom fill path, solder wicking, and reducing the risk of bridging. X-ray imaging, connectivity testing, and shear and tensile testing can be performed to verify the mechanical integrity of multiple flexible circuit cable attachments.

[0050] Figure 2A This is an illustrative example of substrate 200. Substrate 200 may include component 205 and interconnect pads 210. The substrate may be, but is not limited to, a single fused silicon (Si) glass substrate or any other type of similar material. Although the description herein is about a substrate, the method can be applied to interposers, silicon or glass interposers, liquid crystal display (LCD) glass, printed circuit boards (PCBs), other flexible circuits, chips with gold (copper) plated pads, and other interface modules.

[0051] Figure 2B This is an illustrative example of a flexible circuit cable 220, and particularly a short flexible circuit cable. The flexible circuit cable 220 has interconnecting pads 225. Figure 2C This is another illustrative example of a flexible circuit cable 230, and particularly a long flexible circuit cable. The flexible circuit cable 230 has interconnecting pads 235. Figure 2B and 2C The flexible circuit cable shown is illustrative and may be used in other forms and shapes without departing from the scope of the claims.

[0052] Figure 3 This is an overall flowchart 300 of connecting a flexible 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 bonded to the substrate. Any bonding method can be used, which causes the bump material to diffuse into the substrate. Although the description herein relates to gold bumps, other materials, such as copper and aluminum, can also be used. Figure 4 A substrate 400 having gold bumps 405 on interconnect pads 410 is shown according to one embodiment. Figure 5 This is an example of a substrate 500 having a layer of gold-stacked bumps 505 according to one embodiment. Figure 6 This is an example of a substrate having two layers of gold bumps 600 according to one embodiment. The first layer of gold bumps 605 is wire-bonded to interconnect pads, corresponding to the placement of first and second flexible circuit cables, and the second layer of gold bumps 610 is wire-bonded to interconnect pads, corresponding to the placement of a second flexible circuit cable. As described herein, the gold bumps provide additional volume and support height to help provide an underfill path, solder wicking, and reduce the risk of bridging between interconnect pads. Additionally, the bumps can be used to set a "support height" between the substrate and the flexible circuit cables to ensure that solder is not extruded under connection pressure.

[0053] Figure 7 Different patterns for placing gold bumps on a substrate are illustrated 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 circular patterns. The number of gold bumps is illustrative and may depend on the nature of the substrate, available area, interconnect pad size, and other similar considerations. Figure 8 This is a photograph of a pattern of six gold bumps on a substrate according to one embodiment.

[0054] Return to reference Figure 3Clean the flexible circuit cables (310). Place or immerse each flexible circuit cable in denatured alcohol for an appropriate time to remove contaminants. For example, placement in denatured alcohol may last about 15 seconds. Then, place the flexible circuit cables in a stain remover for an appropriate time. For example, placement in a stain remover may last about 3 minutes. The stain remover containing the flexible circuit cables may be agitated to facilitate the cleaning process. Then, place the flexible circuit cables in a bag of denatured alcohol, and then place the bag containing the flexible circuit cables in an ultrasonic bath for an appropriate time. For example, placement in an ultrasonic bath may last about 10 minutes. Then, remove the flexible circuit cables and allow them to air dry. Figure 9A An example of an uncleaned flexible cable circuit is shown. Figure 9B An example of a clean, flexible circuit cable according to one embodiment is shown.

[0055] Return to reference Figure 3 Solder bumps are printed on the first flexible circuit cable (315). For illustrative purposes only, the first flexible circuit cable is a small flexible circuit cable. In one embodiment, the solder paste is tin-bismuth with a low 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 herein relates 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, if a diffusion bonding process such as thermal compression or ultrasonic welding is used, solder or conductive epoxy is not required.

[0056] Use a template to apply solder paste to the flexible circuit cable. Figure 10 This is an example of a stencil for printing solder on a flexible circuit cable according to one embodiment. The flexible circuit cable with solder paste is then placed on a heating plate at a predetermined temperature. For example, the predetermined temperature could be 250°C. Once the solder paste has completely reflowed (i.e. melted) and allowed to cool, the flexible circuit cable can be removed from the heating plate. The flexible circuit cable is then immersed in a denatured alcohol and ultrasonic bath for a predetermined or appropriate time. For example, the predetermined or appropriate time could be 10 minutes. In embodiments requiring double bumps, a stencil is used to apply solder paste onto the reflowed solder paste to increase the solder amount. Figure 11 This is an example photograph of solder printed on a flexible circuit cable after the first reflow, according to one embodiment. Figure 12 This is an example photograph of solder printed on a flexible circuit cable after a second reflow, according to one embodiment.

[0057] Return to reference Figure 3The first flexible circuit cable is placed on the Hot Air Recirculation System (HARS) fixture (320). The substrate and weight are placed on the pick-up table. The flexible circuit cable and substrate are positioned as follows: Figure 13A Or as shown in 13B. Specifically, Figure 13A An example is shown where a flexible circuit cable 1300 is connected to a substrate 1305, with the flexible circuit cable 1300 above the substrate 1305. Clamps 1310 and 1315 are used to support the flexible circuit cable 1300 and the substrate 1305, respectively, and press them together. Figure 13B An example of a substrate 1320 connected to a flexible circuit cable 1325 is shown, with the substrate 1320 positioned above the flexible circuit cable 1325. Clamps 1330 and 1335 are used to support the flexible circuit cable 1320 and the substrate 1325, respectively, and press them together. In one embodiment, a weight is cantilevered off the flexible circuit cable to increase compression and bonding. After proper placement, the first HARS sequence (325) of the flexible circuit cable and substrate is then initiated. Regarding... Figure 24 A more detailed description of the HARS process is given.

[0058] Figure 14 This is an example schematic diagram illustrating the HARS clamp 1400 according to an embodiment. Typically, and as further described below, the HARS clamp 1400 is designed to control the temperature within a predetermined temperature range, such that the first flexible circuit cable and the substrate are bonded together without affecting the integrity of any electronic components or substrate.

[0059] Specifically, in embodiments using solder, the HARS sequence will use appropriate heat as described below to reflow the solder, and the solder will flow through gold bumps to the substrate and be contained by pillars formed by a plurality of gold bumps on the substrate.

[0060] Although the description here concerns solder, conductive epoxy resin can be used. If conductive epoxy resin is printed or dispensed onto a flexible circuit cable, heat, ultraviolet (UV) light, or both, can be applied to cure the epoxy resin between the flexible circuit cable and the substrate. The risk of epoxy resin being squeezed and smeared onto adjacent pads is reduced due to the gaps created by the gold bumps. That is, the epoxy resin is contained within pillars formed by multiple gold bumps on the substrate.

[0061] After the HARS sequence is completed, the first flexible circuit cable and substrate are allowed to cool. Then, an underfill is applied and cured to increase mechanical stability (330). In one embodiment, the underfill is a non-conductive underfill epoxy resin. The underfill is applied to the edge of the flexible circuit cable near the substrate. Heat and capillary effects absorb the underfill epoxy resin between the flexible circuit cable and the substrate. Application of the underfill epoxy resin is stopped if rounding is formed around the edge of the flexible circuit cable. Heat is applied either by a heating block built into the fixture or by conventional heating. Figure 15 This is an example photograph showing the bottom filler according to one embodiment. The connection (335) to the first flexible circuit cable of the substrate can then be inspected using, for example, X-ray.

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

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

[0064] The second flexible circuit cable needs to be bent at a predetermined angle relative to the interconnect pads and the remainder of the second flexible circuit cable. This predetermined angle is sufficient to clear the second flexible cable circuitry relative to the first flexible circuit cable or other component carrying module. In one embodiment, the predetermined angle may be 35°. In another embodiment, the predetermined angle is between 35° and 60°. This can be achieved using methods such as... Figure 17 The bending tool 1700 shown is used to complete this process. Specifically, the flexible circuit cable 1705 is placed in the bending tool 1700. Then, the flexible circuit cable and the bending tool are placed in an oven at a predetermined temperature for a predetermined time. In the illustrative example, the predetermined temperature is 60°C and the predetermined time is 1 hour. As mentioned earlier, if double bumps are required, a stencil is used to apply solder paste onto the reflow solder paste to increase the solder volume. As mentioned earlier, the solder paste is exemplary and conductive epoxy resin can be used.

[0065] Return to reference Figure 3 The second flexible circuit cable is placed into the Hot Air Recirculation System (HARS) fixture (345). The substrate and weight are placed on the pick-up table. The flexible circuit cable and substrate are positioned as follows: Figure 13A Or as shown in 13B. Figure 18 An example of a second flexible circuit cable 1800 connected to a substrate 1805 according to one embodiment is shown. Specifically, a clamp 1810 is placed on top of the second flexible circuit cable 1800, and a clamp 1815 is used to support and compress the substrate 1805.

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

[0067] Figure 19 Examples of first and second flexible circuit cables connected to a substrate according to one embodiment are shown. Specifically, the first flexible circuit cable 1900 is connected to a portion of the substrate 1905 using a single-layer gold-stacked bump 1910, and the second flexible cable circuit 1915 is connected to a portion of the substrate 1905 using a double-layer gold-stacked bump 1920.

[0068] Return to reference Figure 14 Referring also to Figures 20-23, the HARS fixture 1400 is described here, along with various fixtures required to perform the HARS process for the first and second flexible circuit cables. The HARS fixture 1400 includes a top heater 1405, a bottom heater 1410, fixture support legs 1415, a metal plate 1420, and polyimide tape 1425. The polyimide tape 1425 is used to cover all air holes on the bottom heater 1405 except those near the center. This forces all airflow towards the center of the bottom heater 1405. The metal plate 1420 contains hot air by creating a chimney-like effect and directs the air to a return fixture 1430, which also includes the flexible cable circuit. The return fixture 1430 is supported by the fixture support legs 1415, which are vertical beams suspended above the bottom heater 1410. The top heater is located at a predetermined distance above the return fixture 1430 and provides hot N2 gas. The height and temperature of the top heater 1405 are necessary for controlling the solder temperature, and N2 helps with solder wetting. The predetermined distance of the top heater 1405 can vary depending on the application and material, and may be 25 mm for illustrative purposes only.

[0069] Figure 20A This is an exemplary schematic diagram illustrating a return fixture 2000 according to one embodiment. Furthermore, Figure 20A The diagram illustrates how a first flexible circuit cable 2005 and a second flexible circuit cable 2010 are positioned on a reflow clamp 2000. The reflow clamp 2000 has a vacuum wire 2015 for securing the first flexible circuit cable 2005 and the second flexible circuit cable 2010 during reflow. Various holes 2020 are made to assist in clamp preheating from a bottom heater. The reflow clamp 2000 can be made of many suitable materials, including aluminum for illustrative purposes. Contact between the substrate and all heated airflow is prevented to minimize displacement during reflow. The reflow clamp 2000 is also designed to fit a HARS clamp position clamp (i.e.,... Figure 14 The clamp support leg 1415 in the middle is used to ensure consistent placement and position, such as Figure 14 As shown. Figure 20B This is an exemplary schematic diagram showing an exploded first flexible circuit cable assembly 2030 according to one embodiment. The first flexible circuit cable assembly 2030 includes a return clamp 2035, a flexible circuit cable 2040, a substrate 2045, and a weight 2050 as described above. Figure 20C This is an exemplary schematic diagram showing a first flexible circuit cable assembly 2060 assembled using the above-described components according to one embodiment.

[0070] Figure 21 This is an exemplary schematic diagram illustrating a return clamp 2100 having a thermocouple 2105 according to one embodiment. Specifically, Figure 21 Each location marked with an "X" indicates the thermocouple position used for thermal profile control. To simulate the thermal mass of the component, a substrate and a tungsten block (which serves as a weight and is further described below) are placed when determining the thermal distribution pattern. Figure 20B and 20C (As shown). The thermocouple is placed on the reflux clamp 2100 as an adjustment mark to trigger changes in the bottom heater and top heater.

[0071] Figure 22A This is an exemplary schematic diagram showing a HARS pickup stage 2200, a weight 2205, and a substrate 2210 according to one embodiment. The HARS pickup stage 2200 is designed to prepare the substrate 2210 and the weight 2205 (e.g., a tungsten block) for simultaneous pickup and placement in a consistent position. Figure 22B This is an exemplary schematic diagram showing a pickup stage 2250, a weight 2255, and an assembled flexible circuit cable 2265 and a substrate 2260 according to one embodiment.

[0072] Figure 23This is an exemplary schematic diagram illustrating a weight 2300 according to one embodiment. The weight is applied to the substrate during the hot air reflow process, in addition to securing the flexible circuit cable to the vacuum channel of the reflow fixture. The main portion of the weight 2300 may be, but is not limited to, a tungsten block. Other metals may be used depending on the application. The weight 2300 consists of alternating layers of double-sided polyimide 2305 and polytetrafluoroethylene (PTFE) 2310, which are added to provide thermal insulation between the tungsten block 2315 and the substrate. The outer layer consists of a single-sided polyimide layer 2320. To provide suction, holes 2325 are drilled through the tungsten block 2300 and the insulating layers 2305, 2310, and 2320. The holes may be, but are not limited to, 2 mm in diameter. This allows the weight 2300 and the substrate to be simultaneously picked up and placed by a HARS pick-up tube (not shown), as is well known. In one embodiment, the weight 2300 may have a 33g tungsten block, a 0.14mm thick double-sided polyimide strip (2x), a 0.14mm thick PTFE strip (2x), and a 0.03mm thick single-sided polyimide strip (1x).

[0073] Figure 24 This is an overall flowchart 2400 of connecting a first flexible circuit cable to a substrate according to one embodiment. The substrate and weight (2405) are picked up from the HARS pick-up table. The substrate and flexible circuit cable (e.g., a small flexible circuit cable) are aligned and placed in the HARS fixture (2410). Referring now also to… Figure 14 Bottom heater 1410 and top heater 1405 are turned on and stabilized at 225°C and 150°C respectively (2415). These temperatures are illustrative; other temperatures may be appropriate depending on the application. When the reflux fixture 1430 reaches a predetermined temperature (which may be 166°C for illustrative purposes, but may vary depending on the application), the temperature of bottom heater 1410 drops and stabilizes at another predetermined temperature (which may be 178°C, but may vary depending on the application) (2420). A timer is set to two minutes to increment the liquidus time (2425). The timer duration is illustrative, and other times may be used based on the application and materials used. After the timer expires, bottom heater 1410 and top heater 1405 are set to room temperature, and a cooling boost is initiated from top heater 1405 (2430). When the temperature of reflux fixture 1430 reaches 138°C, the external cooling fan is turned on (2535). When the temperature of the return fixture 1430 reaches 80°C, turn off the external cooling fan, bottom heater 1410 and top heater 1405 (2440).

[0074] Figure 25This is 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 rapid attainment of reflow temperature before the flux burns in the solder. The flux reduces oxides, which need to be very low to achieve sufficient bonding. Moreover, as Figure 25 As shown, the method described herein allows operation within a narrow thermal window to achieve bonding without damaging the components. The temperatures described in the embodiments herein are illustrative of the applications and materials used herein. Other temperatures may be used without departing from the scope of the claims.

[0075] Figure 26 This is an overall flowchart 2600 of connecting a second flexible circuit cable to a substrate according to one embodiment. The substrate and weight (2605) are picked up from the HARS pick-up table. The substrate and flexible circuit cable (e.g., a large flexible circuit cable) are aligned and placed in the HARS fixture (2610). Referring now also to… Figure 14 Bottom heater 1410 and top heater 1405 are switched on and stabilized at 225°C and 150°C, respectively (2615). These temperatures are illustrative and may vary depending on the application. When reflux fixture 1430 reaches a predetermined temperature (which may be 162°C for illustrative purposes, but may vary depending on the application), the temperature of bottom heater 1410 drops and stabilizes at another predetermined temperature (which may be 182°C, but may vary depending on the application) (2620). A timer is set to two minutes to increment the liquidus time (2625). The timer duration is illustrative and may vary depending on the application and materials used. After the timer expires, bottom heater 1410 and top heater 1405 are set to room temperature, and a cooling boost is initiated from top heater 1405 (2630). When the temperature of reflux fixture 1430 reaches 138°C, the external cooling fan is activated (2535). When the temperature of the return fixture 1430 reaches 80°C, turn off the external cooling fan, bottom heater 1410 and top heater 1405. Figure 27 This 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 for the purpose of illustrating the applications and materials used herein. Other temperatures may be used without departing from the scope of the claims.

[0076] As described above, an epoxy underfill is applied immediately after each flexible circuit cable connection to increase mechanical strength. The epoxy underfill can cure while mounted on the HARS fixture to reduce the risk of failure in the second flexible circuit cable connection. For the first or small flexible circuit cable assembly, a single drop of underfill material can be used; for the second or large flexible circuit cable assembly, two drops can be used. The number of drops is illustrative and can vary depending on the application and materials used. Return to Reference 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 more than 8 minutes. The materials, times, and temperatures are illustrative, and other values ​​may be used without departing from the scope of the claims. It should be noted that a top heater may not be required for underfill curing.

[0077] Even when flexible circuit cables have irregular shapes, the methods and apparatus described herein can achieve flexible circuit cable connections. For example, when the interconnect pads on the flexible circuit cable are located in a U-shaped region. These methods overcome the coplanarity problem of the pads on the flexible circuit cable if the flexible circuit cable is pre-bent and loses coplanarity. These methods provide multiple flexible circuit cable connections, and the flexible circuit cables can overlap each other. Moreover, the method can overcome gaps created by pre-connected chips or flexible circuit cables.

[0078] The connection temperature is low and does not affect pre-connected chips or flexible circuit cables. These methods reduce the connection resistance between the flexible 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 technologies. For example, the solderable conductive epoxy SMT138 E has a resistance of 1030 μΩ·cm, which is approximately 10 times that of a gold resistor of 2.44 μΩ·cm.

[0079] Instead of routing signals via a single flexible circuit cable, these methods can help improve crosstalk performance by using multiple flexible circuit cables. Crosstalk occurs between signal paths on the same flexible circuit cable when high-speed signals are too close to each other on the flexible circuit cable. Therefore, it is advantageous to use more flexible circuit cables to transmit signals that require very low crosstalk, such as high-speed signals driving optical transmitters and high-speed signals from optical receivers. These methods provide high-density, high-throughput, wide-bandwidth signal fan-out solutions from small substrates or interposers. These methods can reduce the pad size and spacing on the substrate and flexible circuit cables, thus increasing interconnect density more significantly than any existing flexible circuit cable connection solution.

[0080] These methods improve connection reliability because the solder or conductive epoxy is contained within the gold bumps and does not spread to adjacent pads. These methods also reduce connection processing time because soldering and underfill can be performed simultaneously.

[0081] In summary, gold bumps allow for: 1) flexible circuit cable connections with finite coplanarity; 2) connections of irregularly shaped flexible circuit cables; 3) connections of multiple flexible circuit cables to the same substrate (overcoming the spacing created by stacking flexible circuit cables); 4) the use of low-temperature, high-pressure processes; 5) reduced resistance between the substrate and the flexible circuit cables; 6) increased design flexibility; and 7) improved RF performance for high-frequency applications.

[0082] Typically, methods for connecting flexible circuit cables include: attaching a plurality of gold bumps to each of a plurality of interconnect pads on a substrate to create a pillar at each interconnect pad; dispensing adhesive material onto a first flexible circuit cable; aligning and pressing the substrate and the first flexible circuit cable together, wherein the pillar restricts the dispersion of the adhesive material; and applying a first predetermined level of heat to promote bonding of the adhesive material 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; attaching a further plurality of gold bumps to each of a further plurality of interconnect pads on the substrate to create a pillar at each interconnect pad; dispensing adhesive material onto the second flexible circuit cable; aligning and pressing the substrate and the second flexible circuit cable together; and applying a second predetermined level of heat to promote bonding of the adhesive material 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 between the substrate and the second flexible circuit cable without affecting electronic components. In one embodiment, a second predetermined level of heat is controlled to affect the bonding between the substrate and the second flexible cable without affecting the bonding between the electronic components and the substrate and the first flexible circuit cable. In one embodiment, a weight is used to forcibly press the substrate and the first flexible circuit cable together, or to forcibly press the substrate and the second flexible circuit cable together. In one embodiment, the weight extends cantilevered from at least the first flexible circuit cable to enhance compression and bonding. In one embodiment, adhesive material is applied at least twice on the first flexible circuit cable. 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, a plurality of gold bumps are arranged in a predetermined pattern on each interconnect pad.

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

[0084] It should be understood that the present invention is not limited to the embodiments described above, but includes any and all embodiments within the scope of the appended claims. Furthermore, although features and elements of this application are described in specific combinations in the exemplary embodiments, each feature or element may be used alone (without the other features and elements of the exemplary embodiments), or in various combinations with or without the other features and elements of this application.

Claims

1. A system for connecting flexible 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 columnar structure; at least one flexible circuit cable, wherein an adhesive material is dispensed on the at least one flexible 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 flexible circuit cable on the hot air reflow system fixture; the weight configured to force the substrate and the at least one flexible circuit cable together; and a hot air reflow system (HARS) configured to apply a first predetermined level of heat packs to facilitate bonding of the adhesive material between the substrate and the at least one flexible circuit cable located in the hot air reflow system fixture with the weight, wherein the columnar structure restricts dispensing of the adhesive material.

2. The system of claim 1, wherein a further plurality of gold bumps are bonded to a further set of interconnect pads to create an additional columnar structure, further comprising: a bending tool; at least another flexible circuit cable, wherein the bending tool is configured to bend the at least another flexible circuit cable relative to the at least one flexible circuit cable, and wherein the adhesive material is dispensed on the bent at least another flexible circuit cable; the hot air reflow system fixture and the pick and place table configured to align the weight, the substrate, and the bent at least another flexible circuit cable; the weight configured to force the substrate and the bent at least another flexible circuit cable together such that the bent at least another flexible circuit cable covers the further set of interconnect pads; and the HARS to apply a second predetermined level of heat packs to facilitate bonding of the adhesive material between the substrate and the bent at least another flexible circuit cable located in the hot air reflow system fixture with the weight.

3. The system of claim 2, wherein the at least one flexible circuit cable and the bent at least another flexible circuit cable overlap.

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

5. The system of claim 4, wherein the second predetermined level of heat is controlled to affect bonding between the substrate and the bent at least another flexible cable without affecting electronic components and bonding between the substrate and the at least one flexible circuit cable.

6. The system of claim 2, wherein the bent at least one flexible circuit is pressed against the first set of interconnect pads on the substrate, and the weight is cantilevered at least from the at least one flexible circuit cable and the bent at least another flexible circuit cable to enhance compression and bonding while in the HARS during application of heat by the HARS. ​ 7. The system of claim 2, wherein the adhesive material is dispensed at least twice on at least one of the at least one flexible circuit cable and the at least one other flexible circuit cable that is bent.

8. The system of claim 2, wherein the at least one flexible circuit is pressed against the first set of interconnect pads on the substrate and a bottom fill is located between the substrate and the at least one flexible circuit cable and between the substrate and the at least one other flexible circuit cable that is bent to provide mechanical strength.

9. The system of claim 2, wherein the plurality of gold bumps and the other plurality of gold bumps are arranged in a predetermined pattern.

10. The system of claim 2, wherein the HARS includes a plurality of heaters to provide the first predetermined level of heat group and the second predetermined level of heat group.

11. A method of connecting a flexible circuit cable, 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 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 group with a thermal air reflow system when aligned with the weight on the thermal air reflow system fixture to facilitate bonding of the adhesive material between the substrate and the at least one flexible circuit cable, wherein the columnar structure limits dispensing of the adhesive material.

12. A method of connecting a flexible circuit cable, the method comprising: creating a plurality of columns for each of a first set of interconnect pads on a substrate, each column including 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 group with 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 column limits dispensing of the adhesive material.

13. A system for connecting a flexible circuit cable, comprising: a substrate including interconnect pads, each interconnect pad having bonded gold bumps to create a columnar structure; a thermal air reflow system fixture 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 thermal air reflow system fixture; and a thermal air reflow system configured to apply a predetermined level of heat group when aligned with the weight on the thermal air reflow system fixture to facilitate bonding of the adhesive material between the substrate and the flexible circuit cable. a thermal vacuum reflow system configured to apply a predetermined set of levels of heat to facilitate bonding of an adhesive material between the substrate and the flexible circuit cable when aligned with the weight on the thermal vacuum reflow system fixture, wherein the columnar structures restrict dispensing of the adhesive material.

14. A system for connecting a flexible circuit cable, the system comprising: a substrate comprising interconnect pads each having at least two bonded gold bumps to create a column; a thermal vacuum reflow system fixture 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 thermal vacuum reflow system fixture; and a thermal vacuum reflow system configured to apply a predetermined set of levels of heat to facilitate bonding of an adhesive material on the flexible circuit cable between the substrate and the flexible circuit cable when aligned with the weight on the thermal vacuum reflow system fixture, wherein each column restricts dispensing of the adhesive material.

15. A thermal vacuum reflow fixture comprising: an alignment area configured to support a flexible circuit cable having an adhesive material; a vacuum line configured to secure the flexible circuit cable to the alignment area; the alignment area configured to align the flexible circuit cable, a substrate, and a weight, wherein the substrate comprises interconnect pads having bonded gold bumps to create columnar structures, and each columnar structure is configured to restrict dispensing of the adhesive material; and a plurality of holes configured to direct a predetermined set of levels of heat to facilitate bonding of the adhesive material between the substrate and the flexible circuit cable when aligned with the weight on the thermal vacuum reflow fixture.

16. The thermal vacuum reflow fixture of claim 15, further comprising: the alignment area configured to cantilever the weight from the flexible circuit cable to enhance compression and bonding when the thermal vacuum reflow fixture directs the predetermined set of levels of heat to the flexible circuit cable.

17. The thermal vacuum reflow fixture of claim 15, further comprising: another alignment area configured to support a curved flexible circuit cable having an adhesive material; the vacuum line configured to secure the curved flexible circuit cable to the other alignment area; the other alignment area configured to align the curved flexible circuit cable, the substrate, and the weight, wherein the substrate comprises additional interconnect pads having bonded gold bumps to create additional columns; and a plurality of holes configured to direct another predetermined set of levels of heat to facilitate bonding of the adhesive material between the substrate and the curved flexible circuit cable when aligned with the weight on the thermal vacuum reflow fixture.

18. The thermal vacuum reflow fixture of claim 17, wherein the flexible circuit cable and the curved flexible circuit cable overlap on the substrate.

19. The thermal vacuum reflow fixture of claim 17, further comprising: The other alignment region is configured to cause the weight to cantilever from the curved flex circuit cable to enhance compression and bonding when the hot air reflow fixture directs the other predetermined level of heat group to the curved flex circuit cable.

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