A method for manufacturing a bump pad flexible plate supporting a chip package contact

Abstract

The application discloses a method for manufacturing a flexible plate with a bump pad supporting a chip package, and aims at solving the problems of bump pad edge warping and surface crack. The method comprises the following steps: performing selective electroplating processing on a flexible copper-clad plate with a rolled copper layer, so that the surface copper layer of the corresponding area of the bump pad to be manufactured is kept as a rolled copper layer; providing a cover film layer, the window size of the cover film layer corresponding to the position of the bump pad is smaller than the planar size of the bump pad, and the cover film layer is attached to the surface of the flexible copper-clad plate, so that the cover film is pressed and covered on the edge of the bump pad; providing a stamping die, the stamping die comprises a top die and a bottom die, the upper surface of the bottom die is provided with a thimble corresponding to the position of the bump pad, and the lower surface of the top die is provided with a groove corresponding to the position of the thimble; the flexible copper-clad plate with the attached cover film layer is placed in the stamping die, and stamping processing is performed, so that the bump pad is formed in a protruding mode. The application realizes the manufacturing of the bump pad with no warping, no crack and high precision.

Description

Technical Field

[0001] This invention relates to the field of circuit board processing, and more particularly to a method for manufacturing a flexible board with raised pads that supports chip package contact. Background Technology

[0002] Flexible printed circuits (FPCs) are widely used in various electronic devices due to their thinness and flexibility. In applications with contact module packages (such as contact modules for bank cards, SIM cards, social security cards, etc.), the corresponding insertion and removal devices (such as POS machines, SIM card slots, and social security card readers) are required to have raised pad surfaces on their contact points to achieve effective insertion and connection.

[0003] As electronic devices become smaller and more integrated, the use of flexible circuit boards as carriers for raised pads is becoming increasingly common. At the same time, the requirements for the safety and anti-interference capabilities of electronic devices are constantly increasing, necessitating the application of shielding films to the surface of flexible circuit boards to achieve electromagnetic shielding.

[0004] In the prior art, raised pads are typically fabricated using the following two methods: One approach involves designing planar pads and then using a stamping die with ejector pins to create a raised pad area. However, due to the flawed design of the existing pad, cover film, and shielding film stack structure, the edges of the raised pads are prone to warping during stamping or subsequent use, affecting contact reliability. Furthermore, during the processing of raised pads, issues related to processing methods, die design, and stamping techniques can easily lead to stamping cracks in the raised pads, resulting in physical raised pads that appear... Figure 1 As shown, the left side represents a raised pad without cracks, while the right side represents a raised pad with cracks.

[0005] Another method involves electroplating tapered solid bumps onto planar pads using multiple electroplating processes. However, due to the significant expansion and contraction of the flexible circuit board's substrate (polyimide), and the complexity and long processing time of the multiple electroplating steps, problems such as pad misalignment and copper layer misalignment are easily caused. Furthermore, the electrolytic copper layer formed by electroplating has poor ductility, increasing the risk of cracking during stamping.

[0006] Therefore, how to provide a method for manufacturing raised pads on flexible circuit boards that can effectively prevent the edges of raised pads from lifting, significantly reduce the risk of pad cracking, and has high processing accuracy and stable process has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0007] This invention aims to solve the problems of easy edge lifting and surface cracking of pads during stamping in the fabrication of raised pads for flexible boards, which leads to unreliable contact and low product yield. It provides a method for fabricating flexible boards with raised pads that support chip package contact. By selectively electroplating to retain the rolled copper layer and avoiding the brittleness of electrolytic copper, the cracking rate of pads during stamping is reduced by more than 50%. A cover film pressing over the pad edges and a shielding film with a window larger than the pad effectively prevent edge lifting. Combined with a reverse mold structure featuring bottom mold ejector pins and top mold grooves, stamping stress is evenly released. These three elements work synergistically to achieve high-precision, crack-free raised pad fabrication without lifting or cracking.

[0008] This invention provides a method for fabricating a flexible board with raised pads that supports chip package contact, the method comprising: Selective electroplating is performed on the flexible copper-clad laminate with rolled copper layer so that the copper layer on the surface of the corresponding area where the raised pad is to be made remains rolled copper layer. A cover film is provided, wherein the opening size of the cover film at the position of the raised pad is smaller than the planar size of the raised pad, and the cover film is attached to the surface of the flexible copper clad laminate so that the cover film covers the edge of the raised pad. A stamping die is provided, the stamping die including a top die and a bottom die, the upper surface of the bottom die is provided with an ejector pin corresponding to the position of the protruding solder pad, and the lower surface of the top die is provided with a groove corresponding to the position of the ejector pin; A flexible copper-clad laminate with a covering film is placed in the stamping die and stamped to make the raised pads protrude.

[0009] Furthermore, the method also includes: An extended copper layer is provided around the raised pad. The extended copper layer is a copper layer pattern that is enlarged towards the periphery of the raised pad, thereby increasing the size of the pattern of the raised pad.

[0010] Furthermore, the single-sided extension width of the copper layer in the extension region is greater than or equal to 0.1 mm.

[0011] Furthermore, the width of the cover film pressed onto the edge of the raised pad on one side is 0.05mm to 0.15mm.

[0012] Furthermore, the method also includes: After the cover film is applied and before the stamping process, a shielding film is provided, wherein the opening size of the shielding film at the position of the raised pad is larger than the planar size of the raised pad, and the shielding film is applied to the surface of the cover film.

[0013] Furthermore, the window size corresponding to the raised pad position of the shielding film layer also satisfies: The single-sided dimension of the shielding film opening is ≤ the distance from the center to the edge of the raised pad + the width of the blank area around the raised pad - 0.05mm, wherein the width of the blank area is greater than 0.05mm.

[0014] Furthermore, the method also includes: When the shielding film on the back of the raised pad has an adverse effect on the stamping process, a window is provided on the shielding film on the back of the raised pad corresponding to the position of the raised pad.

[0015] Furthermore, the stamping die is a steel die with a Shore hardness of 85 to 97.

[0016] Furthermore, the head of the ejector pin has a spherical or conical structure, and the height of the ejector pin in the stamping die is greater than the protrusion height of the required protruding pad, and the depth of the groove is greater than the height of the ejector pin.

[0017] Furthermore, the method also includes: Before stamping, the flexible copper-clad laminate with a cover film and a shielding film is pre-baked; the pre-baking temperature is 100 to 150°C and the time is 20 to 40 minutes.

[0018] The main beneficial effects of this technical solution are: The present invention discloses a method for fabricating a flexible board with raised pads supporting chip package contact. Firstly, selective electroplating is used to maintain the copper layer on the surface of the area to be fabricated as a rolled copper layer, avoiding the problem of poor ductility caused by electrolytic copper layer formation in existing technologies. Rolled copper has excellent ductility and can withstand large deformations during stamping without easily cracking. Furthermore, the cover film opening size is set smaller than the raised pad size, and after application, the cover film is pressed onto the edge of the pad, forming a wrapping structure. During stamping, the cover film effectively constrains the edge of the pad, preventing it from warping outwards. Finally, the stamping die adopts a reverse structure with ejector pins on the bottom die and grooves on the top die. During stamping, the raised surface faces upwards, and the top die presses down to press the pad into the groove, ensuring uniform stress distribution and full release. Compared to the traditional ejector pin method, this significantly reduces the probability of pad cracking. This significantly improves the forming quality, reliability, and production yield of the raised pads. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0020] Figure 1Comparison of actual images of the finished raised pads with and without cracks, provided for the present invention. Figure 2 A flowchart illustrating a method for fabricating a flexible board with raised pads supporting chip package contact, provided for an embodiment of the present invention. Figure 3 This is a schematic diagram of a structure for bonding a cover film and a shielding film in the prior art. Figure 4 This is a schematic diagram of another structure for bonding a cover film and a shielding film in the prior art; Figure 5 This is a schematic diagram of a stamping die structure for machining raised pads, which is used in the prior art. Figure 6 This is a schematic diagram of a stamping die structure for processing raised solder pads, used in an embodiment of the present invention. Figure 7 This is a partial structural diagram of an actual bottom mold according to an embodiment of the present invention; Figure 8 This is a schematic diagram of the graphic design of raised pads used in existing technologies; Figure 9 This is a schematic diagram of a graphic design for setting an extended copper layer around a raised pad, as used in an embodiment of the present invention. Figure 10 A schematic diagram of the structure of a flexible plate after the cover film and shielding film layers are laminated according to an embodiment of the present invention; Figure 11 This is a schematic diagram illustrating the specific processing steps of a flexible board with raised pads that supports chip package contact, as provided in an embodiment of the present invention.

[0021] Explanation of reference numerals in the attached diagram: 10. Flexible copper clad laminate; 11. Raised pad; 12. Shielding film; 13. Covering film; 14. Surface treatment layer; 21. Top mold; 22. Bottom mold; 23. Ejector pin; 24. Groove; 25. Positioning pin.

[0022] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0023] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0024] It should be noted that if the embodiments of the present invention involve directional indicators (such as up, down, left, right, front, back, etc.), the directional indicators are only used to explain the relative positional relationship and movement of the components in a specific posture. If the specific posture changes, the directional indicators will also change accordingly.

[0025] Furthermore, in this invention, descriptions involving "first," "second," etc., are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0026] Furthermore, the technical solutions of the various embodiments of the present invention can be combined with each other, but only if they are feasible for those skilled in the art. If the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such combination of technical solutions does not exist and is not within the scope of protection claimed by the present invention.

[0027] In this application, the term "raised pad" refers to a pad whose raised structure is ultimately formed through stamping. Before stamping, this term refers to the pattern of the pad to be formed into a raised structure, and its planar dimensions are the design dimensions of the pad; after stamping, this term refers to the already formed raised structure. Those skilled in the art will understand from the context that in pre-stamping steps such as cover film application and shielding film application, "raised pad" refers to its planar pattern.

[0028] To better understand the above technical solution, a detailed explanation of the technical solution is provided below with reference to the accompanying drawings.

[0029] Figure 2 A flowchart illustrating a method for fabricating a flexible board with raised pads supporting chip package contact, as provided in this embodiment of the invention, is shown below. Figure 2 As shown, the method includes: Step S100: Selectively electroplate the flexible copper-clad laminate with rolled copper layer to keep the copper layer on the surface of the corresponding area of ​​the raised pad to be made as rolled copper layer. Step S200: Provide a cover film layer, wherein the opening size of the cover film layer corresponding to the raised pad position is smaller than the planar size of the raised pad, and attach the cover film layer to the surface of the flexible copper clad laminate so that the cover film covers the edge of the raised pad. Step S300: Provide a stamping die, the stamping die including a top die and a bottom die, the upper surface of the bottom die is provided with an ejector pin corresponding to the position of the protruding solder pad, and the lower surface of the top die is provided with a groove corresponding to the position of the ejector pin; Step S400: Place the flexible copper-clad laminate with the covering film attached into the stamping die and perform stamping to make the raised pads protrude and form.

[0030] In existing technologies, there are two methods for bonding the cover film and the shielding film. The first method involves bonding structures such as... Figure 3 As shown, a cover film 13 and a shielding film 12 are bonded to the flexible copper-clad laminate 10. The opening on one side of the shielding film 12 is larger than the opening on the cover film 13, and the opening size of the cover film 13 is larger than the size of the raised pad 11. At this point, the shielding film 12 does not cover the edge of the cover film 13, and the raised pad 11, cover film 13, and shielding film 12 form a stepped transition in height. Because the copper layer on the raised pad 11 can only rely on its own bonding force with the substrate to resist the external force during stamping, the raised pad is very prone to lifting, resulting in delamination of the copper layer and polyimide (PI). The second bonding structure is as follows... Figure 4 As shown, the opening size of the shielding film 12 is larger than the size of the raised pad 11, and the opening size of the shielding film 12 is smaller than the opening size of the cover film 13. In this case, the shielding film 12 presses against the edge of the cover film 13. In this way, the shielding film 12 is attached to the cover film 13, which will result in a larger height difference. Moreover, the height difference does not form a stepped transition, so the shielding film is prone to lifting after stamping. In addition, the shielding film 12 wraps the cover film 13, and the stress of the cover film 13 is not easily released during the stamping process, which makes the edge of the shielding film 12 more prone to lifting. The difficulty in releasing stress also affects the adjacent raised pad 11, making the edge of the raised pad 11 also prone to lifting.

[0031] Meanwhile, the existing protruding pad stamping die is designed with the top die pressing downwards (rather than the bottom die pressing upwards, or both dies operating simultaneously), and the structure used in the existing die is as follows: Figure 5 As shown, the lower surface of the top mold 21 (corresponding to the raised pad) is provided with ejector pins 23, while the upper surface of the bottom mold 22 is provided with grooves 24 corresponding to the ejector pins. During the stamping process of the flexible circuit board, it is placed on the upper surface of the bottom mold 22 and positioned and fixed by positioning pins 25. The raised pad of the flexible circuit board needs to be placed with the raised side facing the lower surface. Since the pressure required during stamping is large and the time for a single stamping is short, the back of the raised pad is first subjected to concentrated force during stamping, resulting in more pad cracks and a greater probability of fine wrinkles on the board body at the edge of the raised pad.

[0032] Based on this, the purpose of the present invention is to provide a method for manufacturing a flexible board with raised pads that supports chip package contact, so as to solve the technical problems of edge lifting and surface cracks that are easy to occur during stamping of raised pads in the prior art, while avoiding the problems of pad misalignment and low processing accuracy caused by multiple electroplating, and realizing the manufacturing of raised pads that do not lift, have no cracks, and have high reliability.

[0033] In this embodiment, a single-sided or double-sided flexible copper-clad laminate is used as the substrate. The copper layer of the copper-clad laminate is a rolled copper layer, and the substrate is a polyimide material. The rolled copper layer has good ductility, which is the basis for preventing cracks during subsequent stamping.

[0034] Taking double-sided flexible copper-clad laminate as an example, the specific processing procedure of the flexible board in this embodiment is as follows: First, positioning holes are made on the flexible copper-clad laminate to serve as alignment references for subsequent processes.

[0035] Then, selective electroplating is performed. Specifically: a dry film is attached to both sides of the flexible copper-clad laminate, and a dry film pattern is formed through exposure and development processes. This dry film pattern covers the corresponding area of ​​the raised pad to be made, so that the area is not plated with copper in the subsequent electroplating process; copper plating is performed to form a conductive layer in the uncovered area; electroplating of the entire board is performed to form an electrolytic copper layer in the uncovered area; due to the protection of the dry film, no electrolytic copper layer is formed on the surface of the corresponding area of ​​the raised pad to be made, only the original rolled copper layer is retained; after the dry film is removed, etching is performed to complete the selective electroplating process and pattern making; the copper layer on the surface of the corresponding area of ​​the raised pad to be made remains a rolled copper layer.

[0036] The raised pad area retains the rolled copper layer, avoiding the problem of cracks caused by poor ductility of the electrolytic copper layer during stamping.

[0037] Next, a cover film is applied, providing a cover film layer composed of polyimide and adhesive material, typically with a thickness between 12.5 μm and 75 μm. Windows are created on the cover film layer, corresponding to the raised pad locations, with the window size smaller than the planar dimensions of the raised pad. In this embodiment, the cover film is pressed onto the edge of the pad on one side, requiring control of the adhesive overflow to ensure the formed pad size. The windowed cover film layer is then attached to the surface of the flexible copper-clad laminate and subjected to lamination and curing. Rapid lamination is generally used, with parameters readily available for existing cover film lamination, such as a lamination temperature of 160°C to 180°C and a pre-pressing time of 5-10 seconds at 100-130 kgf / cm² to initially soften the adhesive layer and remove air bubbles; subsequently, molding is performed at 175-180°C under the same pressure for 90-150 seconds to allow the adhesive layer to fully flow and completely cure. After lamination, baking curing is performed at 150°C for 30-90 minutes.

[0038] By pressing a cover film onto the edge of the pad, a wrap-around structure is formed. During stamping, the cover film effectively restrains the edge of the pad, preventing it from lifting outwards, and significantly improving the edge reliability of the raised pad.

[0039] After the cover film is laminated, surface treatment is performed, such as electroplating with gold, to form a gold-plated plate. If a shielding film is to be laminated, the shielding film is laminated, and then a shaping process is performed.

[0040] Then, the raised pads are stamped using a stamping die, which includes a top die and a bottom die, with the structure as follows: Figure 6 As shown, the upper surface of the bottom mold 22 is provided with ejector pins 24 corresponding to the position of the raised pad, and the lower surface of the top mold 21 is provided with grooves 23 corresponding to the position of the ejector pins; the raised pad of the flexible circuit board needs to be placed with the raised side facing the upper surface, and the raised pad is formed after stamping.

[0041] In this embodiment, the upper surface of the bottom mold is set as an ejector pin, and the lower surface of the top mold is set as a groove. This is because the protrusion height of the raised pad is small, and the diameter of the bottom surface formed by the protrusion is also small. When using the first type of existing mold ( Figure 5 As shown), the flexible board is placed on the surface of the groove. During stamping, the back of the raised pad is initially subjected to concentrated force. During the stamping and extrusion process, the material and copper layer around the raised pad are pulled, causing excessive stress concentration in the material. This stress cannot be released in time, resulting in a high probability of cracking (similar to piercing a piece of paper with an awl; it is easily pierced). When using the method of this embodiment, i.e., the second type of mold ( Figure 6 As shown), the flexible plate is placed on the surface of the ejector pin. When the groove of the ejector die is pressed downward, the contact surface between the ejector die and the flexible plate is large. The downward pressing die presses the raised pad into the groove, giving the raised pad a larger stress release space and effectively reducing the probability of cracking (similar to placing a piece of paper on an uneven board and tapping the paper with a board, the paper is not easily punctured).

[0042] During the stamping operation, the flexible copper-clad laminate is placed in the stamping die with the side to be formed as the raised pad facing the ejector pin of the bottom die. The stamping process is performed, and the stamping speed can use existing parameters. During stamping, the top die presses down, and the ejector pin of the bottom die pushes the pad area into the groove of the top die, causing the pad area to bulge and form a raised pad. Before stamping, a cover film is pressed onto the edge of the pad (or the edge of the extended copper layer); after stamping, the cover film is pressed onto the root of the raised pad (or the edge of the extended copper layer), forming a stable covering structure.

[0043] Please see Figure 7 , Figure 7 This is a partial structural diagram of an actual bottom mold in this embodiment. The bottom mold has ejector pins 23 and locating pins 25. Through the innovative design of the mold structure (bottom mold ejector pins + top mold grooves), the stress is evenly distributed and fully released during stamping, significantly reducing the probability of solder pad cracking compared to the traditional ejector pin punching method. Simultaneously, the covering film effectively prevents edge lifting. Experimental verification shows that the design of this invention can reduce the probability of solder pad lifting and cracking by more than 50% compared to existing methods.

[0044] The flexible board manufacturing method of this embodiment first uses selective electroplating to maintain the copper layer on the surface of the area to be fabricated as a rolled copper layer, avoiding the problem of poor ductility caused by electrolytic copper layer formation in existing technologies. Rolled copper has excellent ductility and can withstand large deformations during stamping without easily cracking. Furthermore, the opening size of the cover film is set smaller than the size of the raised pad, and after application, the cover film is pressed onto the edge of the pad, forming a wrapping structure. During stamping, the cover film effectively constrains the edge of the pad, preventing it from warping outwards. Finally, the stamping die adopts a reverse structure with ejector pins on the bottom die and grooves on the top die. During stamping, the raised surface faces upwards, and the top die presses down to press the pad into the groove, ensuring uniform stress distribution and full release. Compared with the traditional ejector pin punching method, this significantly reduces the probability of pad cracking. This significantly improves the forming quality, reliability, and production yield of the raised pad.

[0045] Furthermore, the method also includes: An extended copper layer is provided around the raised pad. The extended copper layer is a copper layer pattern that is enlarged towards the periphery of the raised pad, thereby increasing the size of the pattern of the raised pad.

[0046] Existing raised pads are generally isolated pad designs, meaning that the pad to be raised has no surrounding extension area, and the pad itself is circular. A typical design is as follows: Figure 8 As shown, red represents the copper layer circuit pattern, and yellow represents the pad pattern where raised pads need to be created. This invention uses a cover film to overlay the pads, which may affect the pad size, especially when the pad size is small. To solve this problem, a preferred embodiment of this invention involves setting an extended copper layer around the raised pads, as shown in the design... Figure 9 As shown, an extended copper layer pattern is created around the pad. Depending on the wiring space, it can be designed as a square pad, a circular pad, etc. The extended copper layer is a square or circular copper layer pattern enlarged around the pad, and it is the same copper layer as the pad body, formed in one step through an etching process. A cover film is cut into a circular window and pressed onto the pad to form a circular raised pad. The extended copper layer increases the bonding area between the copper layer and the substrate, significantly enhancing the adhesion between the pad and the substrate. When the cover film is subsequently pressed onto the edge of the extended area, it avoids directly pressing onto the pad body, which would reduce the effective contact area, and further improves the anti-lifting capability.

[0047] Furthermore, the single-sided extension width of the copper layer in the extension region is greater than or equal to 0.1 mm.

[0048] Single-sided extension width: refers to the direct distance from the edge of the original pad to the outer edge of the copper layer in the extension area. For square pads, it refers to the width extending outward from one side of the pad; for round pads, it refers to the width extending outward from the circumference of the pad. In circuit design, the pad pattern is designed as a composite structure including the extension area. For example, if the diameter of a round pad is 0.8mm and the extension width is 0.1mm, then the outer diameter of the extension area is 1.0mm; if the square pad is 1.0mm × 1.0mm and the extension width is 0.15mm, then the outer edge of the extension area is 1.3mm × 1.3mm. By creating the pad pattern containing the extension area in photoplotting film (or direct laser imaging), and then forming an etched pattern through exposure and development, after etching, the copper layer of the extension area and the original pad are a single copper layer without seams. When the routing space allows, the extension width can be appropriately increased (e.g., 0.15mm, 0.20mm) to further enhance the bonding strength. However, an excessively large extension width may encroach on the space of adjacent lines, which needs to be weighed in the design.

[0049] The extension width of 0.1 mm or greater designed in this embodiment is sufficient to accommodate the alignment tolerance (±0.05 mm) of the cover film attachment and the etch side etching amount (0.01-0.02 mm). Even with slight deviations, the cover film can still be reliably pressed onto the extension area without problems such as insufficient pressing or pressing onto the pad body.

[0050] Furthermore, the width of the cover film pressed onto the edge of the raised pad on one side is 0.05mm to 0.15mm.

[0051] The single-sided coverage width of the cover film refers to the vertical distance from the edge of the pad (or the edge of the copper layer in the extension area, when an extension area is provided) to the edge of the cover film opening. This width is the width of the annular area of ​​the cover film covering the edge of the pad. The cover film opening size is the original pad + extension area overall size minus the single-sided coverage amount of 0.05mm to 0.15mm.

[0052] By designing a reasonable window size, the edge of the cover film can be reliably prevented from lifting. The cover film is pressed onto the edge of the pad (or the edge of the extension area), forming a wrapping structure. During stamping, the cover film exerts a radial restraining force on the edge of the pad, preventing the copper layer of the pad from flipping outward. The pressing width of 0.05-0.15mm provides sufficient anchoring area to ensure sufficient restraint and prevent failure. During stamping, the ejector pin pushes the center area of ​​the pad upward, and the copper layer of the pad is subjected to an outward tensile force, causing the edge area to tend to lift upward. The cover film pressing onto the edge is equivalent to applying downward pressure on the edge of the pad, counteracting the lifting tendency.

[0053] The overlay width is controlled within 0.15mm to ensure that the opening of the cover film is not excessively reduced. For a circular pad with a diameter of 0.8mm, when the overlay width is 0.10mm, the exposed pad diameter is 0.60mm, and the effective area is retained by approximately 56% (reduced from 0.50mm² to 0.28mm²). Although the area is reduced, for the electrical contact of the contact module, the top area of ​​the raised pad (the highest point protruding after stamping) is the main contact surface. The area of ​​this area is determined by the size of the ejector pin, not the original pad area, so the impact on contact reliability is controllable.

[0054] The 0.05-0.15mm coverage width range fully considers the cover film attachment alignment tolerance (±0.05mm) and etching side etching (0.01-0.02mm). Even with process fluctuations, it can still ensure that the cover film effectively covers the pad edges and there will be no insufficient or excessive coverage.

[0055] The copper layer in the cover film lamination area (within 0.05-0.15mm of the pad edge) is rolled copper (due to selective electroplating, no electrolytic copper is formed in this area). The adhesion between rolled copper and the cover film is better than that of electrolytic copper (the surface of rolled copper is smoother and stress-free), further enhancing the reliability of the lamination structure.

[0056] The 0.15mm upper limit effectively restricts the amount of adhesive extruded from the cover film. During lamination, the adhesive softens and flows under high temperature and pressure. Excessive lamination width would cause more adhesive to be extruded into the window area, potentially contaminating the solder pad surface and affecting subsequent gold plating or contact performance. The 0.15mm upper limit keeps the adhesive overflow within an acceptable range.

[0057] When a shielding film is used, the opening size of the shielding film is larger than the pad size. The area covered by the cover film (a ring-shaped band with a width of 0.05-0.15mm) is covered by the opening of the shielding film, preventing additional stress concentration caused by the cover film. At the same time, the presence of the cover film provides buffer support for the edge of the shielding film opening, preventing the shielding film from directly contacting the edge of the pad.

[0058] Furthermore, the method also includes: After the cover film is applied and before the stamping process, a shielding film is provided, wherein the opening size of the shielding film at the position of the raised pad is larger than the planar size of the raised pad, and the shielding film is applied to the surface of the cover film.

[0059] The shielding film is a composite film layer with electromagnetic shielding function, usually composed of a conductive adhesive layer and a metal layer (copper foil or aluminum foil), used to prevent electromagnetic interference (EMI) and signal leakage. When a shielding film needs to be laminated, it is generally applied after surface treatment.

[0060] The window size of the shielding film layer is the area on the shielding film where material has been removed from the corresponding raised pad position. This area exposes the underlying cover film and pad area after application. The size requirement is larger than the planar size of the raised pad, meaning the edge of the window is outside the pad, and the window area is larger than the pad's own area, ensuring that the shielding film does not press against the pad edge.

[0061] The opening of the shielding film should fall on the surface of the cover film and not touch the copper layer of the pads. Since the cover film is already pressed against the edge of the pads (or the edge of the extension area), the area between the edge of the shielding film opening and the edge of the pads is covered by the cover film to ensure that no short circuit occurs.

[0062] The shielding film must be grounded to achieve its shielding effect. This is usually achieved in the following ways: a grounding copper layer is set at the edge of the flexible board or at a specific location, with the shielding film in direct contact with the grounding copper layer; or a window is opened on the cover film at the location corresponding to the grounding copper layer, allowing the conductive adhesive of the shielding film to make contact and conduction with the grounding copper layer.

[0063] A preferred structure after the covering film and shielding film layers are bonded together is as follows: Figure 10 As shown, the cover film is pressed onto the raised pads, and the shielding film layer is adhered to the cover film layer. The opening size is larger than the planar size of the raised pads, and also necessarily larger than the opening size of the cover film. When the opening image is circular, the opening radius must meet the requirements of the raised pad radius plus a safety margin to compensate for the alignment deviation (usually ±0.10mm) during shielding film attachment, ensuring that even with deviation, the shielding film will not press against the edge of the pads. The attachment and lamination processes can use existing technical parameters, which will not be elaborated here.

[0064] In this embodiment, the metal layer (copper foil or aluminum foil) of the shielding film effectively reflects and absorbs electromagnetic waves, preventing external electromagnetic interference from affecting circuit operation and preventing internal signal leakage. This function is crucial for applications with stringent security and anti-interference requirements, such as smart car cameras and financial POS machines. By setting the window size larger than the pad plane size, it ensures that the shielding film does not press against the pad edge. This avoids the stress concentration problem caused by the shielding film pressing against the edge of the cover film or the pad edge in existing technologies, eliminating the risk of warping caused by the shielding film at its source. The edge of the shielding film window rests on the surface of the cover film, and the cover film itself has formed a stable wrapping structure by pressing against the pad edge (or the edge of the extension area). There is a cover film as a buffer layer between the shielding film window edge and the pad edge. During stamping, the stress is transferred through the cover film and will not act directly on the pad, further reducing the probability of warping and cracking.

[0065] Furthermore, the window size corresponding to the raised pad position of the shielding film layer also satisfies: The single-sided dimension of the shielding film opening is ≤ the distance from the center to the edge of the raised pad + the width of the blank area around the raised pad - 0.05mm, wherein the width of the blank area is greater than 0.05mm.

[0066] The single-sided dimension of the shielding film opening is the radius of the shielding film opening (circular) or half the side length (square). The width of the blank area around the raised pad refers to the minimum distance from the edge of the pad to the adjacent copper layer pattern or board edge. The opening of the shielding film is larger than the pad size, and when the width of the copper area etched away around the pad is ≥0.05mm, the opening size of the shielding film is smaller than the size of "pad size + width of the two sides of the etched-away area" (pad size + blank area size on both sides). That is, the edge of the shielding film is located within the blank area (effect: it can provide good shielding while avoiding the shielding film from affecting the stamping and application of the raised pad)), and maintains a safe distance of at least 0.05mm from the outer boundary of the blank area (edge ​​of the adjacent copper layer). Applicable conditions: blank area width > 0.05mm, that is, there is enough space to accommodate the edge of the shielding film opening.

[0067] By strictly confining the edge of the shielding film opening within the blank area surrounding the pads and maintaining a safe distance of 0.05mm from adjacent copper layers, short circuits and lifting issues caused by the shielding film pressing against adjacent copper layers or pad edges are fundamentally avoided. The 0.05mm safety margin accommodates both shielding film alignment tolerances and the expansion and contraction of the flexible board, providing ample process windows for mass production. This feature, in conjunction with other features such as selective electroplating, cover film lamination, extended areas, and reverse molds, achieves a raised pad fabrication effect characterized by "no lifting, no cracks, high precision, and anti-interference."

[0068] Furthermore, the method also includes: When the shielding film on the back of the raised pad has an adverse effect on the stamping process, a window is provided on the shielding film on the back of the raised pad corresponding to the position of the raised pad.

[0069] The back side of the raised pad refers to the side opposite to the formed surface of the raised pad (i.e., the side facing away from the ejector pin during stamping). Adverse effects on stamping refer to situations where the presence of the shielding film leads to a decrease in stamping quality, including but not limited to: the shielding film being punctured by the ejector pin, delamination of the shielding film and the cover film, insufficient raised pad forming height, increased edge warping of the raised pad, and abnormal stamping pressure. Setting a window on the back side means removing material from the shielding film at the position corresponding to the raised pad to form a through hole or opening, making the shielding film discontinuous in that area, allowing the ejector pin to directly contact the back side of the flexible copper-clad laminate.

[0070] During the design phase, based on the stamping direction, raised pad layout, and shielding film application area, it's crucial to determine whether the back shielding film is located on the ejector pin stamping path. This helps to anticipate whether the back shielding film will negatively impact the stamping process. When uncertainty exists, a "pre-windowing" approach can be used as a preventative measure to avoid subsequent defects. Windowing should be completed before or during shielding film application. The window size should be slightly larger than the raised pad size, taking into account the ejector pin contact area during stamping.

[0071] During stamping, the ejector pin (located in the bottom mold) pushes the flexible plate upwards, causing the pad area to bulge. If the back shielding film completely covers the area corresponding to the raised pad, it will lead to: uneven pressure transmission: the ejector pin pressure acts on the shielding film first, and then is transmitted to the pad through the cover film. The elastic deformation of the shielding film will absorb some pressure, resulting in insufficient pressure actually acting on the pad, insufficient bulge height, and stress concentration: the metal layer (copper foil or aluminum foil) of the shielding film has limited ductility. During stamping, it is stretched by the ejector pin, which easily causes stress concentration at the edges, leading to cracking; interlayer delamination: the adhesive force between the shielding film and the cover film is limited, and the shear force during stamping may cause delamination. After the back shielding film is opened, the opened area loses its shielding function; however, since the opened position corresponds to the raised pad, and the raised pad itself is a signal connection point, this area usually does not need shielding (the signal is transmitted through the pad). Therefore, the impact of the opening on the overall shielding effect is negligible.

[0072] Furthermore, the stamping die is a steel die with a Shore hardness of 85 to 97.

[0073] Compared to using wooden or hard plastic molds (Shore hardness between 65 and 88), steel molds (Shore hardness between 85 and 97) offer better stamping results. Wooden and hard plastic molds have lower hardness than steel molds, making them more prone to deformation, misalignment, and cracking during stamping. Furthermore, prolonged processing can lead to irregular protrusions, requiring timely mold replacement, thus increasing overall costs. Steel molds, on the other hand, have higher hardness, resulting in less deformation during stamping, longer service life, and better stamping performance. The mold precision requirements in this embodiment are: ejector pin position accuracy: ±0.02mm; groove and ejector pin coaxiality: ≤0.01mm; mold closing height tolerance: ±0.01mm. Steel molds ensure no plastic deformation under stamping pressure, preventing rapid loss of fit between the ejector pin and groove due to wear. Within the hardness range, excessive hardness avoids increased brittleness, preventing chipping of the ejector pin or groove edges. Sufficient toughness is maintained while ensuring wear resistance.

[0074] Furthermore, the head of the ejector pin has a spherical or conical structure, and the height of the ejector pin in the stamping die is greater than the protrusion height of the required protruding pad, and the depth of the groove is greater than the height of the ejector pin.

[0075] The ejector pin head is the top part of the ejector pin on the bottom die of the stamping mold. It directly contacts the back of the flexible copper-clad laminate, lifting the pad area to form a protrusion. A spherical ejector pin head refers to a hemispherical or spherical crown-shaped curved surface without sharp edges; the radius of curvature is typically 0.5-1.0 times the ejector pin diameter. A conical ejector pin head refers to a conical ejector pin head with a cone angle typically 30°-60°, and may have a small rounded corner at the top. The required protrusion height of the raised pad refers to the designed height of the raised pad above the surface of the flexible board in the final product, typically 0.15mm-0.30mm. Generally, ejector pin height = required protrusion height + flexible board thickness + safety margin; groove depth = ejector pin height + clearance margin.

[0076] Spherical or conical ejector pins ensure even pressure distribution, preventing stress concentration and damage to the cover film; the ejector pin height exceeding the protrusion height compensates for the elastic rebound of the flexible board, ensuring consistent protrusion height; the groove depth exceeding the ejector pin height provides stress release space, preventing cracks in the solder pads due to rigid constraints.

[0077] Furthermore, the method also includes: Before stamping, the flexible copper-clad laminate with a cover film and a shielding film is pre-baked; the pre-baking temperature is 100 to 150°C and the time is 20 to 40 minutes.

[0078] Pre-baking is a process where, before stamping, the flexible board with the cover film and shielding film attached is placed in an oven and heated at a set temperature for a certain time to remove moisture adsorbed inside the material, release internal stress, and stabilize dimensions. Pre-baking should be performed after the cover film and shielding film have been attached, pressed, and cured, but before stamping; this removes moisture that may have been absorbed naturally from the air by the flexible copper-clad laminate substrate (polyimide), or that may have remained on the surface of the board after wet processes (electroplation, cleaning). By removing moisture adsorbed inside the flexible board, the risk of delamination, blistering, and cracking caused by moisture vaporization during stamping is eliminated. A temperature of 100 to 150°C balances dehumidification and material stability, while a time of 20 to 40 minutes ensures sufficient moisture diffusion without affecting production efficiency. In a preferred embodiment, the temperature is 120°C and the time is 30 minutes.

[0079] This invention systematically solves the technical problems of edge warping and surface cracking in the fabrication of raised pads on flexible boards through multi-dimensional collaborative innovation in material selection, structural design, process optimization, and mold improvement. It employs a rolled copper layer combined with selective electroplating to retain the rolled copper layer in the raised pad area, avoiding stamping cracks caused by the poor ductility of electrolytic copper, reducing the crack defect rate by more than 50%. The cover film has an opening size smaller than the pad's planar size and covers the pad's edge, forming a wrapping structure that effectively prevents edge warping. Simultaneously, an extension area copper layer is provided for further... The process enhances bonding strength; the shielding film's window size is larger than the pad size and confined within the blank area, preventing stress concentration during lamination; a reverse mold structure with ejector pins on the bottom mold and grooves on the top mold is adopted, with ejector pin heads being spherical or conical and their height greater than the protrusion height, and groove depth greater than the ejector pin height. Combined with a steel mold (Shore hardness 85-97), this ensures uniform stress release during stamping, reducing the crack defect rate to 1-2%; pre-baking treatment (120℃ / 30min) removes moisture absorption from the material, preventing stamping delamination, and controlling protrusion height fluctuations within ±0.01mm. These features work synergistically to achieve high-precision, high-reliability protrusion pad manufacturing without warping or cracking, increasing stamping yield to over 95%, extending mold life to 500,000-1,000,000 cycles, and significantly reducing overall costs. This makes it particularly suitable for applications with stringent requirements for contact reliability and electromagnetic shielding, such as smart car cameras and financial POS machines.

[0080] This invention also provides a specific processing procedure for a flexible board with raised pads that supports chip package contact, such as... Figure 11 As shown: Step S1: Take a double-sided flexible copper-clad laminate 10 (the copper layer of this copper-clad laminate must be rolled copper) → Drill (or punch) holes → Electroplat the whole board (to form through holes on the front and back sides) (specifically: apply dry film to the front and back sides of the board after drilling → expose → develop to form dry film pattern, the dry film pattern covers the raised pads (to be made) → copper plating → electroplating the whole board → film removal (this process ensures that there is no electroplated copper on the surface of the raised pads, that is, no electrolytic copper is formed) → make circuit pattern (to form pattern core board).

[0081] Step S2: Take the cover film layer → Make the windowed cover film → Apply the windowed cover film to the front and back of the above pattern core board (the windowed cover film on the back is for ordinary solder pads).

[0082] Step S3: Electroplating gold processing to form a gold-plated board (If only gold plating is needed for the raised pads, selective gold plating can be used, which means two different surface treatments on one board, forming a surface treatment layer 14 on the raised pads).

[0083] Step S4: Take the shielding film → make a windowed shielding film → align and attach it to the surface of the gold-plated plate (to form a shielding plate).

[0084] The flexible board obtained by the above steps is subjected to stamping processing, that is, a stamping die is made with surface protrusion pins (corresponding to the protrusion pads) → the shielding plate is placed in the stamping die for stamping processing → the flexible board with protrusion pads is formed.

[0085] It should be noted that, due to the high precision of circuit board processing, the high density of circuit distribution, and the thinness of the stacked structure, all reaching the micron level, such as the thickness of the cover film being 25μm to 75μm, and the line width being 25μm, 50μm, etc., if the actual stacked structure is drawn, some layers or some patterns are difficult to represent; therefore, the accompanying drawings in this embodiment are graphic concepts that highlight the technical features, and do not represent the actual circuit board structure in the processing process, nor do they represent a scaled-up enlarged view of the processing structure.

[0086] This embodiment adopts a description method based on technical features as key nodes, and the existing technology in the processing process may not be described in detail. For example, the existing technology for circuit pattern making generally assumes "dry film application → exposure → development → etching → film removal", and the basic process of lamination generally assumes "core board layer extraction → prepreg extraction → stacking → fusion / riveting → layout → lamination → X-ray targeting → burr removal". Therefore, the understanding of this embodiment is mainly the understanding of the process of forming technical features during processing.

[0087] The above description is only a preferred embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural transformations made under the inventive concept of the present invention using the description and drawings of the present invention, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

Claims

1. A method for fabricating a flexible board with raised pads supporting chip package contact, characterized in that, The method includes: Selective electroplating is performed on the flexible copper-clad laminate with rolled copper layer so that the copper layer on the surface of the corresponding area where the raised pad is to be made remains rolled copper layer. A cover film is provided, wherein the opening size of the cover film at the position of the raised pad is smaller than the planar size of the raised pad, and the cover film is attached to the surface of the flexible copper clad laminate so that the cover film covers the edge of the raised pad. A stamping die is provided, the stamping die including a top die and a bottom die, the upper surface of the bottom die is provided with an ejector pin corresponding to the position of the protruding solder pad, and the lower surface of the top die is provided with a groove corresponding to the position of the ejector pin; A flexible copper-clad laminate with a covering film is placed in the stamping die and stamped to make the raised pads protrude.

2. The manufacturing method according to claim 1, characterized in that, The method further includes: An extended copper layer is provided around the raised pad. The extended copper layer is a copper layer pattern that is enlarged towards the periphery of the raised pad, thereby increasing the size of the pattern of the raised pad.

3. The manufacturing method according to claim 2, characterized in that, The single-sided extension width of the copper layer in the extension area is greater than or equal to 0.1 mm.

4. The manufacturing method according to claim 1 or 2, characterized in that, The width of the cover film pressed onto the edge of the raised pad on one side is 0.05 mm to 0.15 mm.

5. The manufacturing method according to claim 1 or 2, characterized in that, The method further includes: After the cover film is applied and before the stamping process, a shielding film is provided, wherein the opening size of the shielding film at the position of the raised pad is larger than the planar size of the raised pad, and the shielding film is applied to the surface of the cover film.

6. The manufacturing method according to claim 4, characterized in that, The opening size of the shielding film layer corresponding to the raised pad position also satisfies: The single-sided dimension of the shielding film opening is ≤ the distance from the center to the edge of the raised pad + the width of the blank area around the raised pad - 0.05mm, wherein the width of the blank area is greater than 0.05mm.

7. The manufacturing method according to claim 4, characterized in that, The method further includes: When the shielding film on the back of the raised pad has an adverse effect on the stamping process, a window is provided on the shielding film on the back of the raised pad corresponding to the position of the raised pad.

8. The manufacturing method according to claim 1, characterized in that, The stamping die is a steel die with a Shore hardness of 85 to 97.

9. The manufacturing method according to claim 1, characterized in that, The head of the ejector pin has a spherical or conical structure, and the height of the ejector pin of the stamping die is greater than the protrusion height of the required protruding pad, and the depth of the groove is greater than the height of the ejector pin.

10. The manufacturing method according to claim 4, characterized in that, The method further includes: Before stamping, the flexible copper-clad laminate with a cover film and a shielding film is pre-baked; the pre-baking temperature is 100 to 150°C and the time is 20 to 40 minutes.

Images