Board-to-board flexible circuit hot-pressing tin melting connection structure and manufacturing method thereof

Abstract

The invention discloses a board-to-board flexible circuit hot-pressing tin melting connection structure and a manufacturing method thereof, and belongs to the technical field of board-to-board electronic welding interconnection. The connection structure comprises a PCB and an FPC, the PCB is provided with first bonding pads, and each first bonding pad is provided with a solder paste layer in advance; the end part of the FPC is provided with pressure welding finger areas, and each pressure welding finger area is provided with a second bonding pad area and a tin through hole array; the tin through hole array penetrates through the base material layer of the FPC, and tin through holes are formed in the non-functional area of the second bonding pad area or the edge of the second bonding pad area; the first bonding pad of the PCB and the second bonding pad area of the FPC are electrically and mechanically connected in a pulse hot pressing mode. The method comprises the steps of forming a tin through hole array on the FPC, printing solder paste on the PCB, performing reflow soldering and pre-fixing, performing hot-pressing alignment, performing hot-pressing welding and detecting. According to the invention, the problems of tin overflow, insufficient tin, continuous tin, pseudo soldering and the like in the prior art are effectively solved, and the welding consistency and the product yield are improved.

Description

Technical Field

[0001] This invention relates to the technical field of board-to-board electronic soldering interconnection, and in particular to a board-to-board flexible circuit hot-press soldering connection structure and manufacturing method, which is especially suitable for high-density, thin and light interconnection scenarios in consumer electronics products, such as smartwatches, AR / VR headsets, foldable screen phones, medical wearable devices, etc. Background Technology

[0002] As electronic devices become lighter, thinner, shorter, and smaller, flexible printed circuit boards (FPCs) are widely used in consumer electronics, communication equipment, smart wearables, and medical devices due to their advantages such as high flexibility, high wiring density, light weight, and thinness. In the assembly process of FPCs, soldering is one of the key steps, and its quality directly affects the performance and reliability of electronic devices.

[0003] In recent years, hot-barbonding technology has been widely used due to its advantages of requiring no additional flux, occupying little space, and being suitable for automated production. This technology primarily uses heat and pressure to transfer molten solder directly from one surface to another, achieving a reliable connection. Despite its many advantages, hot-barbonding still presents some challenges in practical applications, such as solder overflow and bridging. When there is too much solder paste or the pad design is inadequate, molten solder can easily spread laterally, leading to short circuits between adjacent solder joints. Additionally, insufficient solder and cold solder joints can occur due to significant differences in pad area (e.g., the pads of an NFC module are much longer than those of an FPC), which can cause molten solder paste to be carried away, resulting in insufficient solder or incomplete soldering in certain areas. Furthermore, issues such as solder joint misalignment and insufficient flatness also frequently occur, usually due to uneven pressure from the hot press head or slight warping of the FPC leading to poor contact. These problems not only affect the appearance of the solder joint but can also negatively impact the overall performance and reliability of electronic devices, especially in applications requiring precise and stable connections.

[0004] Existing solutions, such as adding flux and adjusting the stencil thickness, have limited effectiveness and may introduce new contamination risks. Therefore, a novel technical solution that synergistically optimizes structure and process is urgently needed to fundamentally solve the above problems. Summary of the Invention

[0005] The purpose of this invention is to provide a board-to-board flexible circuit thermo-pressed solder connection structure and manufacturing method with controllable process and high reliability, overcoming problems such as solder overflow, insufficient solder, solder bridging, and cold solder joints in the prior art, and improving welding quality and product yield.

[0006] To achieve the above objectives, the solution of the present invention is: Board-to-board flexible circuit thermo-pressed solder joint structure A board-to-board flexible circuit thermo-pressed solder joint structure, comprising: The PCB has multiple first pads, each of which has a pre-applied solder paste layer. The FPC has a solder finger area at its end corresponding to each first pad of the PCB. Each solder finger area has a second pad area and an array of through holes. The array of through holes penetrates the substrate layer of the FPC and the through holes are located in the non-functional area of ​​the second pad area or at the edge of the second pad area. The first pad of the PCB and the second pad area of ​​the FPC are electrically and mechanically connected by pulsed thermoforming.

[0007] Furthermore, the through-hole is circular, elliptical, or polygonal, and the inner wall of the through-hole is metallized to form a conductive path, which is used as an auxiliary heat dissipation or grounding path.

[0008] Furthermore, the through holes are distributed in a stepped manner along the extension direction of the FPC pads, and the density of through holes near the high solder flow risk end is greater than that at the far end, forming a gradient solder discharge channel.

[0009] Furthermore, the board-to-board flexible circuit hot-press solder connection structure also includes a heat insulation pad, which is disposed between the FPC and the PCB, and the heat insulation pad has an opening at the position corresponding to the first solder pad.

[0010] The present invention can also employ the following solutions: A method for manufacturing a board-to-board flexible circuit thermo-pressed solder connection structure includes the following steps: S1. Create an array of through-holes in the solder finger area of ​​the FPC; S2. Solder paste is applied to the first pad of the PCB using stencil printing, and the opening size of the stencil is smaller than the size of the first pad; S3. Send the PCB into the reflow oven for pre-soldering, so that the solder paste partially solidifies and adheres to the surface of the first pad. S4. Align the second pad area of ​​the FPC with the first pad of the PCB and align it using an optical alignment system. S5. Apply high temperature and high pressure using a pulse hot press head to remelt the solder paste and connect it to the FPC pads; S6. After cooling, perform X-ray inspection and ICT online testing to confirm the absence of defects such as bridging, insufficient solder, and cold solder joints.

[0011] Furthermore, in step S1, the steps for creating the through-hole array of the FPC include: A1. Design through holes during the FPC CAM design stage. The through holes should avoid FPC traces and functional areas. Design the through holes in non-functional areas or at the edge of the second pad area. A2. Prepare FPC substrate; A3. Apply a protective film to the FPC substrate; A4. Perform laser drilling; Complete the fabrication of the through-hole array.

[0012] Furthermore, in step A1, determining the location of the through-hole includes the following steps: A11. Obtain the Gerber files for the FPC and PCB; A12. Identify the layout and size differences between the first pad of the FPC and the PCB solder finger area; A13. Determine if the dimensions of the first pad on the PCB and the second pad area on the FPC match; if they match, proceed to the next step; otherwise, omit or add a few holes. A14. Plan the initial position and diameter of the through-hole; A15. Mark the location of through-holes in CAM software, avoiding traces and functional areas; A16. Output Laser Data for use by laser drilling equipment.

[0013] Furthermore, the steps for creating the through-hole array of the FPC also include: A5. Hole wall metallization: Chemical copper plating is used to deposit a layer of copper on the hole wall of the through-hole to connect to the ground network, which enhances the stability of the FPC and PCB soldering structure and can also serve as an EMI shielding path to improve the product's anti-interference capability.

[0014] Furthermore, the steps for creating the through-hole array of the FPC also include: A6. Pre-filling solder: Use screen printing or dispensing to pre-fill the through-hole with solder paste smaller than the through-hole.

[0015] Furthermore, in step S5, the hot press head of the pulse hot press welding covers the entire soldering area, and the heating method is resistance heating or ceramic heating; a multi-stage temperature control strategy is adopted during the hot press process: first, the temperature is rapidly increased to 220°C at a fixed speed and maintained for a fixed time to allow the solder paste to be wetted at a constant temperature, then the temperature is slowly increased to 260°C and held, and the temperature is naturally cooled after the preset holding time is reached; a dynamic pressure adjustment strategy is adopted during the hot press welding process: the initial pressure is 0.8~1.0MPa, and after the solder paste begins to melt, it is slowly increased to 1.5~2.0MPa to achieve a staged pressing mode of "first bonding, then compaction".

[0016] The staged pressing method avoids the problems of FPC deformation or pad misalignment caused by cold high pressure. The initial low pressure ensures smooth FPC bonding, and the pressure is increased after the temperature rises and the solder paste softens to promote full wetting and metallurgical bonding. This method can reduce the cold solder joint rate by more than 60%, and is especially suitable for precision connections of thin FPCs (thickness <0.1mm).

[0017] By adopting the above solution, the present invention, a board-to-board flexible circuit thermoforming solder connection structure and manufacturing method, proposes for the first time to set up a "solder penetration array" on the FPC to regulate solder paste flow, fundamentally solving the contradiction between solder overflow and insufficient solder. The solder penetration array is set at the edge of the non-functional area or the second pad area of ​​the FPC's soldering finger area. During the thermoforming process, after the solder paste melts, excess solder paste can flow into multiple solder penetration arrays, thereby suppressing solder paste overflow. The solder penetration array can effectively absorb excess solder, significantly reducing the probability of solder bridging after thermoforming the FPC and PCB. The solder paste is concentrated in the first and second pad areas, improving the fullness of the thermoforming. In addition, after excess solder paste flows into the solder penetrations, the residual solder paste in the penetrations helps to enhance mechanical anchoring and improve vibration resistance. This invention is compatible with various substrates and product forms and has been mass-produced in projects such as NFC antennas for smartwatches, camera modules for laptops, and backlight strips for displays. Calculations show that adopting this invention increases daily production capacity by 18% and reduces rework costs by 40%, resulting in significant economic benefits. Detailed Implementation

[0018] To further explain the technical solution of the present invention, the present invention will be described in detail below through specific embodiments.

[0019] This invention discloses a board-to-board flexible circuit thermo-pressed solder connection structure, comprising: PCB and FPC.

[0020] The PCB is a rigid board with multiple regularly arranged first pads on its surface. The material can be FR-4, BT resin or glass substrate (such as the substrate where the BOE / OLED panel driver IC is located). Each first pad has a pre-applied solder paste layer.

[0021] FPCP (Flexible Printed Circuit) is a flexible printed circuit board composed of a polyimide (PI) film substrate layer and a copper foil layer. The ends of the FPC have solder finger areas corresponding to each first pad on the PCB. Each solder finger area has a second pad area and an array of through-holes. The through-hole array penetrates the substrate layer of the FPC, and the through-holes are located in the non-functional areas of the second pad area or at the edge of the second pad area. The through-holes are micron-sized vias with a diameter of 0.2-0.6 mm, allowing excess molten solder to flow into the holes, thereby suppressing lateral overflow. The through-holes can be blind vias or through-holes, preferably through-holes for scheduling and profit release. FPCs typically support functional units such as NFC antennas, camera modules, and backlight strips. Experiments show that products without through-holes exhibit significant solder overflow exceeding 18% after soldering; while introducing circular through-holes with a diameter of 0.4 mm and a spacing of 0.8 mm reduces the solder overflow rate to below 2%.

[0022] Preferably, the through-hole can be circular, elliptical, or polygonal, and the inner wall of the through-hole is metallized to form a conductive path, which can be used as an auxiliary heat dissipation or grounding path.

[0023] Electrical and mechanical connections are achieved between the first pad of the PCB and the second pad of the FPC via pulsed thermoforming. A pulsed thermoforming head simultaneously heats multiple solder joints from above, with the temperature set between 230 and 280°C for approximately 5 seconds, and the pressure maintained at 1.0 to 1.5 MPa. Under these conditions, the solder paste completely melts, forming a strong IMC (intermetallic compound) interface between the second pad of the FPC and the first pad of the PCB, with a shear force reaching 8 to 12 gf / pad.

[0024] The through holes are distributed in a stepped manner along the extension direction of the first pad of the FPC, and the density of through holes near the high solder flow risk end is greater than that at the far end, forming a gradient solder discharge channel.

[0025] In scenarios where mismatched pad lengths cause solder paste to tend to concentrate and drag towards one end (such as NFC modules), the through-holes are designed with a non-uniform density layout—more small holes are placed at the front end of the solder flow direction (i.e., the high-risk area for solder overflow), for example, two holes per millimeter, and reduced to one hole per millimeter at the rear end, forming a gradient structure of "dense at the front and sparse at the back." This achieves orderly guidance and graded absorption of molten solder, avoiding local overload that could cause interconnection between holes or surface bulging, and improving solder discharge efficiency by more than 35%; it is particularly suitable for long strip-shaped grounding pads or power pad areas.

[0026] The board-to-board flexible circuit hot-press solder connection structure of the present invention also includes a heat insulation pad, which is disposed between the FPC and the PCB, and the heat insulation pad has an opening at the position corresponding to the first solder pad.

[0027] A composite thermal pad with a thickness of 20~50μm is pre-placed between the PCB and FPC. The material can be silicone-based with ceramic particles. Laser-cut windows are made at the corresponding pad positions to allow direct contact of solder paste, achieving efficient heat conduction in the soldering area and thermal isolation in the non-soldering area to prevent heat diffusion from damaging surrounding components. At the same time, it plays a stress buffering role, alleviates warping problems caused by thermal expansion coefficient mismatch, improves temperature field uniformity, and enhances overall soldering consistency. The hot-pressing method of setting thermal pads is particularly suitable for heat-sensitive substrates such as glass substrates or COF.

[0028] This invention also discloses a method for manufacturing a board-to-board flexible circuit thermo-pressed solder connection structure, which includes the following steps: S1. Create an array of through-holes in the solder finger area of ​​the FPC; Designing and fabricating a through-hole array in the finger-pinning area of ​​an FPC can improve the reliability of thermoforming soldering. By rationally arranging the through-holes, the flow behavior of molten solder paste can be effectively controlled, resolving common defects such as solder overflow, insufficient solder, and solder bridging. The steps for creating the through-hole array in an FPC include: A1. Design through-holes during the FPC CAM design phase. The through-holes should avoid FPC traces and functional areas, and be designed at the edge of non-functional areas or the second pad area. Determining the processing location of the through-holes includes the following steps: A11. Obtain the Gerber files for the FPC and PCB; A12. Identify the layout and size differences between the first pad of the FPC and the PCB solder finger area; A13. Determine if the dimensions of the first pad on the PCB and the second pad area on the FPC match; if they match, proceed to the next step; otherwise, omit or add a few holes. A14. Plan the initial position and diameter of the through-hole; A15. Mark the location of through-holes in CAM software, avoiding traces and functional areas; A16. Output Laser Data for use by laser drilling equipment.

[0029] A2. Prepare FPC substrate; A3. Apply a protective film to the FPC substrate; A4. Perform laser drilling. Based on the laser data file entered in step A16, perform laser drilling on the FPC to form an array of through-holes. Laser drilling is highly adaptable and precise, suitable for the diverse needs of electronic products such as smart wearables. Taking a smartwatch as an example, UV laser is used to process 0.4mm diameter circular through-holes, which are distributed on both sides of the finger bonding area of ​​the FPC, with 3 through-holes on each side.

[0030] A5. Hole wall metallization: A layer of copper of about 0.3~0.5μm is deposited on the hole wall of the through-tin hole using chemical copper plating, and then electroplated to thicken it to ≥20um to form a conductive anchor. It can be connected to the ground network. Hole wall metallization can enhance the stability of the FPC and PCB soldering structure, and can also serve as an EMI shielding path to improve the product's anti-interference ability.

[0031] A6. Pre-filling solder treatment: Using screen printing or dispensing, solder paste smaller than the through hole is pre-filled into the through hole. By pre-filling solder paste into the through hole, an additional solder source can be provided to compensate for the solder loss during the soldering process. It is especially suitable for large pads to small pads, such as NFC connection.

[0032] The above steps complete the fabrication of the through-hole array.

[0033] Designing and fabricating through-hole arrays in the finger-pin bonding area of ​​FPC combines structural innovation with advanced technology. This not only solves the core pain point in hot-bar soldering but also provides a feasible path for future higher-density, smaller-pitch flexible electronic interconnects, demonstrating significant industrial application value.

[0034] S2. Solder paste is applied to the first pad of the PCB using stencil printing. The stencil opening size is smaller than the first pad size. A laser-cut stencil is used, with a preferred thickness of 100-120 μm. The opening size is 0.2 mm smaller than the pad (i.e., an "under-opening" design) to prevent excessive solder paste spread. The printing angle is controlled at 45°-60°, and the squeegee speed is 30-50 mm / s to ensure consistent solder paste formation. Compared to traditional "equal opening" or "over-opening" designs, this invention uses an "under-opening" design, reducing the solder paste volume variation coefficient to <8%, thus reducing local buildup. The stencil openings in this invention are trapezoidal or inverted conical (wider at the top and narrower at the bottom), which facilitates demolding, improves solder paste contour consistency, and reduces bridging.

[0035] S3. Send the PCB into the reflow oven for pre-soldering, so that the solder paste partially solidifies and adheres to the surface of the first pad. The temperature profile of the reflow oven is set as follows: preheating zone (100~150℃) → constant temperature zone (180~200℃) → reflow peak (235~245℃) → cooling. The goal is to make the solder paste initially melt and adhere to the pad, but not completely wet the FPC. Pre-fixing in the reflow oven can prevent the solder paste from shifting during subsequent handling and improve the alignment accuracy.

[0036] S4. Align the second pad area of ​​the FPC onto the first pad of the PCB and align it using an optical alignment system; use a visual alignment system (CCD camera + image recognition algorithm) to automatically calibrate the position of the FPC and the PCB; the alignment accuracy can reach within ±20μm; it can be used with a vacuum adsorption platform to fix the FPC to prevent slippage and greatly reduce the risk of soldering misalignment, especially suitable for fine-pitch connections below 0.3mm pitch.

[0037] S5. High temperature and pressure are applied using a pulse hot press head to remelt the solder paste and connect it to the FPC pads. The hot press head covers the entire solder area, and the heating method is resistance heating or ceramic heating. A multi-stage temperature control strategy is adopted during the hot press process to balance efficiency and soldering quality: first, the temperature is rapidly increased to 220°C at a rate of 2°C / s, held for 10s to allow the solder paste to be wetted at a constant temperature, then slowly increased to 260°C and held for 5s before naturally cooling down. The segmented heating avoids thermal shock, and the holding time is sufficient to complete the full wetting. The average peel force test value reaches 10.5gf.

[0038] A dynamic pressure adjustment strategy is employed during the hot-press welding process: the initial pressure is 0.8~1.0 MPa, and after the solder paste begins to melt, it is slowly increased to 1.5~2.0 MPa, achieving a staged pressing mode of "first bonding, then compaction". This staged pressing mode avoids the problems of FPC deformation or pad misalignment caused by cold high pressure. The initial low pressure ensures stable FPC bonding, and the pressure is increased after the temperature rises and the solder paste softens to promote full wetting and metallurgical bonding. This method can reduce the cold solder joint rate by more than 60%, and is especially suitable for precision connections of thin FPCs (thickness <0.1 mm).

[0039] S6. After cooling, perform X-ray inspection and ICT online testing to confirm the absence of defects such as bridging, insufficient solder, and cold solder joints. X-ray inspection is used to observe whether there are voids, bridging, or insufficient solder inside; ICT testing is used to verify the conductivity of all signal channels. This invention, through collaborative innovation in structural design and process, successfully solves the long-standing reliability bottleneck in hot-bar welding. It is suitable for the widespread application of next-generation wearable devices, miniaturized modules, and high-density interconnect systems, and has broad market prospects.

[0040] The above embodiments are not intended to limit the product form and style of the present invention. Any appropriate changes or modifications made by those skilled in the art should be considered as not departing from the patent scope of the present invention.

Claims

1. A board-to-board flexible circuit thermo-pressed solder connection structure, characterized in that, include: A PCB having at least one first pad, each first pad having a pre-applied layer of solder paste. The FPC has a solder finger area at its end corresponding to each first pad of the PCB. Each solder finger area has a second pad area and an array of through holes. The array of through holes penetrates the substrate layer of the FPC and the through holes are located in the non-functional area of ​​the second pad area or at the edge of the second pad area. The first pad of the PCB and the second pad area of ​​the FPC are electrically and mechanically connected by pulsed thermoforming.

2. The board-to-board flexible circuit thermoforming solder connection structure as described in claim 1, characterized in that: The through-hole is circular, elliptical, or polygonal, and the inner wall of the through-hole is metallized to form a conductive path, which is used as an auxiliary heat dissipation or grounding path.

3. The board-to-board flexible circuit thermo-pressed solder connection structure as described in claim 1, characterized in that: The through holes are distributed in a stepped manner along the extension direction of the FPC pads, and the density of through holes near the high solder overflow risk area is greater than that at the far end, forming a gradient solder discharge channel.

4. The board-to-board flexible circuit thermoforming solder connection structure as described in claim 1, characterized in that: It also includes a heat insulation pad, which is placed between the FPC and the PCB, and the heat insulation pad has an opening at the position corresponding to the first pad.

5. A method for manufacturing a board-to-board flexible circuit thermo-pressed solder connection structure as described in claim 1, characterized in that, Includes the following steps: S1. Create an array of through-holes in the solder finger area of ​​the FPC; S2. Solder paste is applied to the first pad of the PCB using stencil printing, and the opening size of the stencil is smaller than the size of the first pad; S3. Send the PCB into the reflow oven for pre-soldering, so that the solder paste partially solidifies and adheres to the surface of the first pad. S4. Align the second pad area of ​​the FPC with the first pad of the PCB and align it using an optical alignment system. S5. Using pulse thermoforming, a hot press head is used to apply high temperature and pressure, causing the solder paste on the PCB to remelt and connect with the second pad area of ​​the FPC. S6. After cooling, perform X-ray inspection and ICT online testing to confirm the absence of defects such as bridging, insufficient solder, and cold solder joints.

6. The manufacturing method of a board-to-board flexible circuit thermo-pressed solder connection structure as described in claim 5, characterized in that: In step S1, the steps for creating the through-hole array of the FPC include: A1. Design through holes during the FPC CAM design stage. The through holes should avoid FPC traces and functional areas. Design the through holes in non-functional areas or at the edge of the second pad area. A2. Prepare FPC substrate; A3. Apply a protective film to the FPC substrate; A4. Perform laser drilling; Complete the fabrication of the through-hole array.

7. The manufacturing method of a board-to-board flexible circuit thermo-pressed solder connection structure as described in claim 6, characterized in that: Step A1, determining the location of the through-hole machining includes the following steps: A11. Obtain the Gerber files for the FPC and PCB; A12. Identify the layout and size differences between the first pad of the FPC and the PCB solder finger area; A13. Determine if the dimensions of the first pad on the PCB and the second pad area on the FPC match; if they match, proceed to the next step; otherwise, omit or add a few holes. A14. Plan the initial position and diameter of the through-hole; A15. Mark the location of through-holes in CAM software, avoiding traces and functional areas; A16. Output Laser Data for use by laser drilling equipment.

8. The manufacturing method of a board-to-board flexible circuit thermo-pressed solder connection structure as described in claim 4, characterized in that, The steps for creating the through-hole array of the FPC also include: A5. Hole wall metallization: Chemical copper plating is used to deposit a layer of copper on the hole wall of the through-hole to connect to the ground network, which enhances the stability of the FPC and PCB soldering structure and can also serve as an EMI shielding path to improve the product's anti-interference capability.

9. A method for manufacturing a board-to-board flexible circuit thermo-pressed solder joint structure as described in claim 4 or 5, characterized in that, The steps for creating the through-hole array of the FPC also include: A6. Pre-filling solder: Use screen printing or dispensing to pre-fill the through-hole with solder paste smaller than the through-hole.

10. The manufacturing method of a board-to-board flexible circuit thermo-pressed solder connection structure as described in claim 5, characterized in that: In step S5, the hot press head of the pulse hot press welding covers the entire soldering area, and the heating method is resistance heating or ceramic heating. During the hot press process, a multi-stage temperature control strategy is adopted: first, the temperature is rapidly increased to 220°C at a fixed speed and kept at a fixed time for the solder paste to be constantly wetted, then the temperature is slowly increased to 260°C and kept at that temperature, and the temperature is naturally cooled after the holding time reaches the preset time. A dynamic pressure adjustment strategy is adopted in the hot-press welding process: the initial pressure is 0.8~1.0MPa, and after the solder paste begins to melt, it is slowly increased to 1.5~2.0MPa to achieve a staged pressing mode of "first bonding, then compaction".