Method for processing and assembling soldering flux-free FPC (Flexible Printed Circuit) flexible flat cable for high-density assembly

Abstract

The invention relates to a soldering flux-free FPC (Flexible Printed Circuit) flexible flat cable processing and assembling method for high-density assembly. The method comprises the following steps: designing a carrier plate bonding pad; designing an FPC (Flexible Printed Circuit); carrying out ball mounting; positioning the flexible flat cable; welding a flexible flat cable; and reinforcing. According to the invention, a ball mounting process and a flexible printed circuit (FPC) process are used, and high-density interconnection between different carrier plates is realized through process design and product design. By using a ball mounting process, the welding size is greatly reduced, and the minimum size of a bonding pad is 0.2 mm; and by using a flexible circuit board process, the density of the wires is greatly improved, and the smallest wire width and the wire spacing are 0.1 mm. According to the advantages, high-density welding of the flexible flat cable can be achieved, and the problems of connection of different carrier plates in a packaging structure, leading-out of a product connecting end after packaging and the like in the prior art are solved.

Description

Technical Field

[0001] The invention relates to the technical field of electronic packaging, and in particular to a method for processing and assembling a flux-free FPC flexible flat cable for high-density assembly. Background Art

[0002] In the field of electronic packaging, especially in high-reliability ceramic packaging structures, there are many ways to achieve electrical performance connections between different substrates, such as bonding, manual wire bonding, transition piece welding, etc. These connection methods have specific usage scenarios, and the assembly spacing, occupied space, and assembly in three-dimensional directions are all limited. In the field of packaging, multiple substrates are often parallel and packaged in special shapes. The way to connect between boards is to use connectors, which takes up a lot of space, has a low connection density, and has a fixed relative relationship between boards, which is contrary to the trend of miniaturization and flexible assembly of packages.

[0003] At present, the common electronic packaging methods are as follows:

[0004] (1) Wire bonding: Based on the wire bonding process, the substrates are interconnected by means of gold wire, aluminum wire, copper wire, copper tape, etc. The advantage of wire bonding is that the solder joints take up less space, but the substrates need to be fixed in position in advance, and the assembly position is relatively fixed, and needs to be in the same plane or parallel. The height difference and spacing between the substrates cannot exceed the bondable size range.

[0005] (2) Connectors: Based on the welding and clamping process, the connectors are pre-welded on the two sides of the carrier board, and then the circuits are interconnected through the connectors. Connectors are widely used and have high structural stability, but they occupy a large space. The main space includes the insulation size of the lead wire and the size of the clamping fixing part.

[0006] (3) Transition piece: Based on the welding process, reflow soldering and laser welding are used to complete the welding connection between the two carriers. This method requires high relative positioning accuracy between the carriers, requires reflow soldering, and adds auxiliary fixtures. After welding is completed, the stress is mainly borne by the solder joints.

[0007] (4) Others: A hybrid form of the above processes, such as using a transition carrier to achieve interconnection through wire bonding on both sides. Summary of the invention

[0008] The object of the present invention is to provide a flux-free FPC flexible flat cable processing and assembly method for high-density assembly, which can solve the deficiencies in the prior art, solve the problems of connecting different carrier boards in the packaging structure and leading out the connection ends of the packaged products, and realize high-density welding of the flexible flat cable.

[0009] To achieve the above object, the present invention adopts the following technical solutions:

[0010] A method for processing and assembling a flux-free FPC flexible flat cable for high-density assembly, the method comprising the following steps:

[0011] S1. Design of substrate pads

[0012] Make pads on the carrier board and perform solder mask treatment on the non-soldering area of ​​the pads.

[0013] S2, FPC soft cable design

[0014] A flexible flat cable designed for FPC includes a top insulating layer, a middle copper layer and a bottom insulating layer arranged in sequence; a local window opening process is performed on the top insulating layer and the bottom insulating layer to expose the middle copper layer, and the window opening area is used for welding; a through hole is made on the middle copper layer, and the through hole serves as a welding hole for welding positioning and a welding channel for solder on a carrier board.

[0015] S3, Ball Planting

[0016] A first solder ball is prefabricated on the solder pad.

[0017] S4, Soft Cable Positioning

[0018] Place the flexible flat cable on the soldering pad so that the soldering hole on the flexible flat cable is aligned with the first solder ball to achieve positioning of the flexible flat cable.

[0019] S5, Soft Cable Welding

[0020] A ball is planted on the upper surface of the flexible flat cable to obtain a second solder ball, and the flexible flat cable is soldered. When the second solder ball is completely melted, it is soldered together with the first solder ball prefabricated on the solder pad. Between the two solder balls is the middle copper layer with a window area on the flexible flat cable.

[0021] S6. Reinforcement

[0022] Use epoxy glue to fix the flexible flat cable on the carrier board to reinforce the soldering points of the flexible flat cable and complete the connection on one side of the flexible flat cable.

[0023] Preferably according to the present invention, the pad is a circular pad, and the minimum diameter of the pad is 0.2 mm; except for the lead-out terminal on the pad, the pad is surrounded by a solder resist treatment.

[0024] Preferably according to the present invention, the intermediate copper layer adopts a single-layer structure or a double-layer structure, the single-layer structure is a layer of copper foil located between the top insulating layer and the bottom insulating layer, and the double-layer structure includes two layers of copper foil located between the top insulating layer and the bottom insulating layer, and an intermediate insulating layer is provided between the two layers of copper foil.

[0025] Preferably according to the present invention, the size of the through hole is the same as the size of the pad; and the diameter of the through hole is 0.15-0.3 mm.

[0026] Preferably according to the present invention, the size of the solder ball is the same as the size of the solder pad.

[0027] Preferably according to the present invention, the method further comprises the following steps:

[0028] S7. Place a solder ball on the solder pad of the carrier board on the other side of the flexible flat cable, align the solder hole on the flexible flat cable with the solder ball, heat and melt the solder ball by welding, and connect the flexible flat cable to the carrier board on the other side by welding.

[0029] Preferably, according to the present invention, in step S7, a solder ball is implanted on the pad of the carrier board on the other side of the flexible flat cable by using a ball implantation method. After the solder ball melts, it is welded together with the solder in the through hole of the flexible flat cable to form a nail-like structure, thereby completing the welding of the other side of the flexible flat cable.

[0030] Compared with the prior art, the advantages of the present invention are:

[0031] (1) The present invention combines the ball planting process with the FPC flexible flat cable process. The ball planting process is used to greatly reduce the welding size, with the minimum pad size of 0.2mm. At the same time, the flexible circuit board process is used to greatly increase the wire density, with the thinnest line width and line spacing of 0.1mm. Combining the above advantages, the present invention can achieve high-density welding of flexible flat cables, solving the problems of connecting different carriers in the packaging structure and leading out the connection end of the packaged product in the prior art.

[0032] (2) The present invention is a high-density assembly process for SIP packaging products and hybrid integrated circuit products. Compared with the connector and transition parts method, the present invention can greatly reduce the assembly area and significantly improve the assembly density; compared with the wire bonding method, the present invention increases the scope of application, and greatly improves the reliability of the relative position and fixing method of the substrate; the present invention can realize the parallel assembly, face-to-face assembly, vertical assembly and other methods of the carrier, which significantly improves the flexibility.

[0033] (3) The present invention mainly solves the problem of connection between different carriers. The existing wire bonding, manual wire welding, and transition piece connection all have defects. The proposed solution is helpful to achieve the connection between different carriers. The method is simple and easy to operate. The present invention is aimed at ceramic packaging in high-end application fields, and has high requirements for reliability, operability, and reworkability. Ball implantation technology is often considered to be cold welding in the industry, which requires the addition of flux and reflow. The present invention innovatively uses prefabricated solder balls to change the original solder ball-substrate welding mode to a solder ball-solder welding mode. Compared with the original mode, the welding result is more reliable, and the addition of flux is eliminated, simplifying the process. Conventional FPC substrates are mainly used for flexible substrates. In the present invention, the FPC substrate is optimized to achieve the densest assembly, which greatly reduces the size required for connection between boards and increases the routing density. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is a method flow chart of the method for processing and assembling the flux-free FPC flexible flat cable for high-density assembly in the present invention;

[0035] Figure 2 This is a schematic diagram of the flexible flat cable in the present invention;

[0036] Figure 3 It is a schematic diagram of the welding state in the present invention.

[0037] in:

[0038] 1. Solder pad, 2. Carrier board, 3. Flexible cable, 4. Glue reinforcement point, 5. Soldering position, 6. Glue position. DETAILED DESCRIPTION

[0039] The present invention will be further described below in conjunction with the accompanying drawings:

[0040] like Figure 1 A method for processing and assembling a flux-free FPC flexible flat cable for high-density assembly is shown, the method comprising the following steps:

[0041] S1. Design of substrate pads

[0042] A pad 1 is made on a carrier board 2, and a solder resist process is performed on a non-welding area of ​​the pad 1. The carrier board 2 is a ceramic substrate.

[0043] S2, FPC soft cable design

[0044] A flexible flat cable 3 is designed for FPC, and the flexible flat cable 3 includes a top insulating layer, a middle copper layer and a bottom insulating layer arranged in sequence; the top insulating layer and the bottom insulating layer are partially opened to expose the middle copper layer, and the window area is used for connecting other components or welding; through holes are made on the middle copper layer, and the through holes serve as welding holes for welding positioning and welding channels for solder on a carrier.

[0045] S3, Ball Planting

[0046] A first solder ball is prefabricated on the solder pad 1 .

[0047] S4, Soft Cable Positioning

[0048] like Figure 2 As shown, the flexible flat cable 3 is placed on the soldering pad 1 , and the soldering hole on the flexible flat cable 3 is aligned with the first solder ball to achieve positioning of the flexible flat cable.

[0049] In step S4, ensure that the welding hole on the flexible flat cable 3 is completely aligned with the first solder ball on the carrier board 2 to avoid short circuit or other electrical problems. Before welding, a clamp or adhesive can be used to temporarily fix the position of the flexible flat cable to ensure that it will not move during the welding process. Select a suitable welding temperature and time to ensure a good welding effect. Either reflow soldering or manual soldering can be used, depending on the specific situation. After completing the welding, check the quality of each solder joint to ensure that there are no problems such as cold soldering, bridging or insufficient solder.

[0050] S5, Soft Cable Welding

[0051] The second solder ball is planted on the upper surface of the flexible flat cable 3 to obtain the second solder ball, and the flexible flat cable is welded. When the second solder ball is completely melted, it is welded together with the first solder ball prefabricated on the pad. The middle copper layer with the window area on the flexible flat cable is between the two solder balls. The solder ball is heated and melted by welding, so that the flexible flat cable and the carrier are connected by welding through the melted solder ball. Figure 2 and Figure 3 shown.

[0052] S6. Reinforcement

[0053] Use epoxy glue to fix the flexible flat cable on the carrier board to reinforce the solder joints of the flexible flat cable and complete the connection on one side of the flexible flat cable. Figure 2 As shown, the glue reinforcement point 4 is as follows Figure 3 shown.

[0054] In step S6, in order to prevent the soldering point from failing due to pulling, the soldering point needs to be reinforced. Specifically, the flexible flat cable is fixed to the carrier board using epoxy glue to complete the connection on one side.

[0055] Preferably according to the present invention, the pad is a circular pad, and the minimum diameter of the pad is 0.2 mm; except for the lead-out terminal on the pad, the pad is surrounded by a solder resist treatment.

[0056] Preferably according to the present invention, the intermediate copper layer adopts a single-layer structure or a double-layer structure, the single-layer structure is a layer of copper foil located between the top insulating layer and the bottom insulating layer, and the double-layer structure includes two layers of copper foil located between the top insulating layer and the bottom insulating layer, and an intermediate insulating layer is provided between the two layers of copper foil.

[0057] Preferably according to the present invention, the size of the through hole is equal to the size of the pad; and the diameter of the through hole is 0.15-0.3 mm.

[0058] Preferably, according to the present invention, the size of the solder ball matches the size of the pad. The material of the solder ball is tin-lead alloy (Sn-Pb), lead-free alloy (such as Sn-Ag-Cu) or other suitable alloys. Ensure that the diameter of the solder ball matches the size of the pad to obtain good electrical connection and mechanical stability. In this embodiment, the size of the solder ball is the same as the size of the carrier pad.

[0059] Preferably according to the present invention, the method further comprises the following steps:

[0060] S7. Place a solder ball on the solder pad of the carrier board on the other side of the flexible flat cable, align the solder hole on the flexible flat cable with the solder ball, heat and melt the solder ball by welding, and connect the flexible flat cable to the carrier board on the other side by welding.

[0061] In summary, the flux-free FPC flexible flat cable processing and assembly method for high-density assembly described in the present invention has two innovative points. One is that the ball planting process is used to greatly reduce the welding size, and the minimum pad size is 0.2mm. The second is that the flexible circuit board process is used to greatly improve the wire density, and the thinnest line width and line spacing are 0.1mm. Combining the above advantages, the present invention can achieve high-density welding of flexible flat cables. The present invention uses the ball planting process and the flexible circuit board (FPC) process to achieve high-density interconnection between different carriers through process design and product design. By using the ball planting process, the welding size is greatly reduced, and the minimum pad size is 0.2mm; by using the flexible circuit board process, the wire density is greatly improved, and the thinnest line width and line spacing are 0.1mm. Combining the above advantages, the present invention can achieve high-density welding of flexible flat cables, and solve the problems existing in the prior art of connecting different carriers in the packaging structure and leading out the connection ends of the packaged products.

[0062] The above-described embodiments are merely descriptions of preferred implementations of the present invention and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the scope of protection determined by the claims of the present invention.

Claims

1. A method for processing and assembling a flux-free FPC flexible flat cable for high-density assembly, characterized in that: The method comprises the following steps: S1. Design of substrate pads Make pads on the carrier board and perform solder mask treatment on the non-soldering area of ​​the pads; S2, FPC soft cable design A flexible flat cable designed for FPC, the flexible flat cable comprising a top insulating layer, a middle copper layer and a bottom insulating layer arranged in sequence; a local window opening process is performed on the top insulating layer and the bottom insulating layer to expose the middle copper layer, and the window opening area is used for welding; a through hole is made on the middle copper layer, and the through hole is used as a welding hole for welding positioning and a welding channel for solder on a carrier board; S3, Ball Planting prefabricate a first solder ball on the solder pad; S4, Soft Cable Positioning Placing the flexible flat cable on the soldering pad so that the soldering hole on the flexible flat cable is aligned with the first solder ball to achieve positioning of the flexible flat cable; S5. Soft cable welding Planting a ball on the upper surface of the flexible flat cable to obtain a second solder ball, and soldering the flexible flat cable. When the second solder ball is completely melted, it is soldered together with the first solder ball prefabricated on the solder pad, and between the two solder balls is an intermediate copper layer with a window area on the flexible flat cable; S6. Reinforcement Use epoxy glue to fix the flexible flat cable on the carrier board to reinforce the soldering points of the flexible flat cable and complete the connection on one side of the flexible flat cable.

2. The method for processing and assembling a flux-free FPC flexible flat cable for high-density assembly according to claim 1, characterized in that: The pad is a circular pad, and the minimum diameter of the pad is 0.2 mm. Except for the lead-out terminal on the pad, the pad is surrounded by a solder resist treatment.

3. The method for processing and assembling a flux-free FPC flexible flat cable for high-density assembly according to claim 1, characterized in that: The middle copper layer adopts a single-layer structure or a double-layer structure. The single-layer structure is a layer of copper foil located between the top insulating layer and the bottom insulating layer. The double-layer structure includes two layers of copper foil located between the top insulating layer and the bottom insulating layer, and an middle insulating layer is provided between the two layers of copper foil.

4. The method for processing and assembling a flux-free FPC flexible flat cable for high-density assembly according to claim 1, characterized in that: The size of the through hole is the same as that of the pad; the diameter of the through hole is 0.15-0.3 mm.

5. The method for processing and assembling a flux-free FPC flexible flat cable for high-density assembly according to claim 1, characterized in that: The size of the solder ball is the same as that of the solder pad.

6. The method for processing and assembling a flux-free FPC flexible flat cable for high-density assembly according to claim 1, characterized in that: The method further comprises the following steps: S7. First, place a solder ball on the solder pad of the carrier board on the other side of the flexible flat cable, align the solder hole on the flexible flat cable with the solder ball, heat and melt the solder ball by welding, and connect the flexible flat cable to the carrier board on the other side by welding.

7. The method for processing and assembling a flux-free FPC flexible flat cable for high-density assembly according to claim 6, characterized in that: In step S7, a solder ball is placed on the pad of the carrier board on the other side of the flexible flat cable by ball planting. After the solder ball melts, it is welded with the solder in the through hole of the flexible flat cable to form a nail-like structure, thereby completing the welding of the other side of the flexible flat cable.

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