Welding process for flexible printed circuit (FPC) and printed circuit board (PCB)

Abstract

The invention discloses a welding process for a flexible printed circuit (FPC) and a printed circuit board (PCB). According to the method, a systematic solution is provided for solving the problems that in an existing Hotbar welding technology, technological parameters depend on experience setting, and a pre-tinning and point Flux technology is lack of optimization; according to the invention, the reliability and stability of the welding of the FPC soft board and the PCB hard board are improved, the production cost is reduced, and the method has good application prospect and analysis.

Description

Technical Field

[0001] The invention belongs to the technical field of hotbar welding, and in particular relates to a welding process for a FPC soft board and a PCB hard board. Background Art

[0002] As electronic devices develop towards miniaturization, thinness and high performance, the connection technology between flexible circuit boards (FPCs) and printed circuit boards (PCBs) becomes increasingly important. FPCs are widely used in smart phones, tablet computers, wearable devices and other fields due to their excellent bendability and thinness. As a support and interconnection carrier for electronic components, the reliability and stability of PCBs are crucial to the overall performance of electronic devices.

[0003] At present, there are many ways to connect FPC and PCB, including connector connection, anisotropic conductive adhesive (ACF) connection, and welding. Although connector connection is easy to install and disassemble, it is large in size and difficult to meet the needs of miniaturization of electronic equipment. It is also prone to poor contact after long-term use. The ACF connection process is relatively complex, the cost is high, and the connection strength is not as good as welding. As a traditional connection method, welding has the advantages of high connection strength and good reliability, and is still a commonly used connection technology in electronic equipment.

[0004] Among welding technologies, surface mount technology (SMT) is the most widely used one. SMT technology solders electronic components to PCBs through a reflow soldering process, and has the advantages of high automation and high production efficiency. However, traditional SMT technology is mainly aimed at the welding between rigid components and PCBs, and it is difficult to meet the welding requirements between FPC soft boards and PCB hard boards. This is mainly because the substrate of FPC soft boards is usually thinner and has poor heat resistance. It is easy to warp and deform in the high temperature environment of reflow soldering, resulting in welding defects and affecting the reliability of the product. In addition, the traditional SMT reflow soldering process is difficult to accurately control the amount of solder and the welding area. For the welding of fine-pitch solder joints between FPC soft boards and PCB hard boards, it is easy to have problems with too much or too little solder, which in turn leads to defects such as bridging and cold soldering.

[0005] In response to the above problems, hot bar welding (Hotbar) technology has gradually been applied to the connection between FPC soft board and PCB hard board. Hotbar welding technology melts the solder and connects the pads of FPC soft board and PCB hard board by local heating and pressurization. Compared with the traditional reflow soldering process, the heat of Hotbar welding technology is more concentrated, with less thermal impact on surrounding components, and is more suitable for welding FPC soft boards with poor heat resistance.

[0006] There is still much room for improvement in the existing welding technology of FPC soft boards and PCB hard boards, especially how to use Hotbar welding technology to improve welding quality. Summary of the invention

[0007] The purpose of the present invention is to provide a welding process for a FPC soft board and a PCB hard board to solve the problems raised in the above background technology.

[0008] To achieve the above object, the present invention provides the following technical solution: a welding process of a FPC soft board and a PCB hard board; comprising the following steps: The method comprises the following steps: S1. Adjusting the steel mesh opening: testing and selecting appropriate steel mesh opening parameters according to the types of PCB hard board and FPC soft board to be welded, and determining the steel mesh opening parameters according to the test results; S2. Setting the steel mesh according to the steel mesh opening parameters determined in step S1; S3. Adjust solder paste printing parameters: Test and select appropriate solder paste printing parameters according to the types of PCB hard board and FPC soft board to be soldered, and determine the solder paste printing parameters according to the test results; S4. Adjust the point Flux parameters: Test and select appropriate point Flux parameters according to the types of PCB hard board and FPC soft board to be welded, and determine the point Flux parameters according to the test results; S5. Adjust the welding parameters of the Hotbar device: Test and select appropriate Hotbar device welding parameters according to the types of PCB hard boards and FPC soft boards to be welded, and determine the welding parameters based on the test results; S6. In the Hotbar welding area of ​​the PCB hard board, pre-tinning is performed by SMT solder paste printing process using the steel mesh determined in step S2 and the solder paste printing parameters determined in step S3; S7. In the pre-tinned PCB hard board Hotbar welding area, perform point Flux processing according to the point Flux parameters determined in step S4; S8. Use the welding parameters determined in step S5 to set the Hotbar device and solder the FPC soft board to the Hotbar welding area of ​​the PCB hard board.

[0009] As a preferred implementation, adjusting the steel mesh opening parameters in step S1 includes the following steps: S11. Select at least three steel meshes with different opening parameters according to the product type; S12. Printing solder paste on the Hotbar soldering area of ​​the PCB hard board using the steel mesh; S13. The solder paste is printed on the PCB hard board for welding quality inspection, the welding quality inspection includes the solder paste printing integrity, solder joint shape and solder joint strength; S14. Select the steel mesh opening parameters with the best welding quality as the final steel mesh opening parameters.

[0010] As a preferred implementation, adjusting the solder paste printing parameters in step S3 includes the following steps: S31. Select at least three different solder paste printing parameter groups according to the product type, each group of parameters including printing speed, scraper pressure, demolding speed and demolding length; S32. Printing solder paste in the Hotbar soldering area of ​​the PCB hard board using the solder paste printing parameter group; S33 solder paste printing quality test of the PCB hard board after solder paste printing, the solder paste printing quality test includes solder paste thickness, solder paste area and solder paste shape; S34. Select the solder paste printing parameter group with the best solder paste printing quality as the final solder paste printing parameters.

[0011] As a preferred implementation, the solder paste printing parameters determined in step S3 include: a printing speed of 35-45 mm / s, a scraper pressure of 4.0-5.0 kg, a demolding speed of 1.0-2.0 mm / s, and a demolding length of 3 mm.

[0012] As a preferred implementation, adjusting the point Flux parameter in step S4 includes the following steps: S41. Select at least three different point Flux weights according to product type; S42. Performing a point Flux process on the Hotbar soldering area of ​​the PCB hard board using the point Flux weight; S43. The appearance of the PCB hard board after the point Flux is inspected, and the appearance inspection includes the Flux coverage area and Flux thickness; S44. Select the point Flux weight with the best appearance as the final point Flux parameter.

[0013] As a preferred embodiment, adjusting the welding parameters of the Hotbar device in step S5 includes the following steps: S51. Select at least three different Hotbar equipment welding parameter groups according to the product type, each group of parameters including Hotbar pressure, a heating time, a constant temperature time, a second heating time, a second welding time and a welding temperature; S52. Using the Hotbar device welding parameter group to solder the FPC soft board to the Hotbar welding area of ​​the PCB hard board; S53 after welding PCB hard board and FPC soft board welding quality inspection, the welding quality inspection includes solder joint appearance, solder joint strength and temperature curve; S54. Select the Hotbar equipment welding parameter group with the best welding quality as the final Hotbar equipment welding parameters.

[0014] As a preferred embodiment, the welding parameters of the Hotbar device determined in step S5 include: Hotbar pressure is 1.4kg, first heating time is 2s, first constant temperature time is 3s, second heating time is 2s, second welding time is 5s, low temperature pad temperature is 345-245°C, and high temperature pad temperature is lower than 315°C.

[0015] As a preferred implementation, the steel mesh openings determined in step S1 are circular, and the diameter of the steel mesh openings is 0.30 mm.

[0016] As a preferred implementation, the point Flux parameter determined in step S4 is: the weight of the point Flux is 0.15-0.25 mg.

[0017] Compared with the prior art, the present invention has the following beneficial effects: The present invention optimizes the steel mesh opening parameters, solder paste printing parameters, spot Flux parameters and Hotbar equipment welding parameters before Hotbar welding through a systematic testing method, ensuring that each parameter matches the type of PCB hard board and FPC soft board to be welded, thereby significantly improving welding quality and reliability, and effectively reducing solder joint defects such as bridging, cold solder joints, insufficient solder joint strength and other problems. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a schematic diagram of the process flow of the present invention; Figure 2 It is a reference diagram for the steel mesh parameter test verification of the present invention; Figure 3 This is a reference diagram for testing and verifying solder paste printing parameters of the present invention; Figure 4 It is a reference diagram for the parameter test and verification of the point flux device of the present invention; Figure 5 Hotbar parameter test verification reference for the present invention Figure 1 ; Figure 6 Hotbar parameter test verification reference for the present invention Figure 2 ; Figure 7 Hotbar parameter test verification reference for the present invention Figure 3 ; Figure 8 Hotbar parameter test verification reference for the present invention Figure 4 ; DETAILED DESCRIPTION

[0019] Example 1 See also Figure 1-8 The present invention provides a welding process for a FPC soft board and a PCB hard board, comprising the following steps: S1. Adjust the steel mesh opening: Test and select appropriate steel mesh opening parameters according to the types of PCB hard board and FPC soft board to be welded. The specific steps are as follows: S11. Make several groups of steel meshes with different opening parameters, the steel meshes are made of stainless steel. The openings of each group of steel meshes are circular, but the opening diameters are different. In this embodiment, they are: Steel mesh 1: The opening diameter of the steel mesh of A1-A17, B1-B16, and C1-C17 is 0.28mm; Steel mesh 2: The opening diameter of the steel meshes of A1-A17, B1-B16, and C1-C17 is 0.30 mm; Steel mesh 3: The opening diameter of the steel meshes of A1-A17, B1-B16, and C1-C17 is 0.32mm; S12. Use the above steel meshes to print solder paste on the hotbar soldering area of ​​the same PCB hard board. The solder paste uses SAC305 (Sn96.5Ag3.0Cu0.5) lead-free solder paste.

[0020] S13. Perform soldering quality inspection on the PCB hard board after solder paste printing. The inspection items include: Solder paste printing integrity: Use a magnifying glass to observe whether the solder paste printing pattern is complete and whether there are defects such as defects such as defects, bridges, etc.

[0021] Solder joint shape: Use a microscope to observe the shape of the solder joint after Hotbar welding to see if it is full and round, without defects such as cold solder joints and false solder joints.

[0022] Solder joint strength: Use a tensile tester to test the tensile value of the solder joint and evaluate the solder joint strength.

[0023] S14. According to the test results, select the steel mesh opening parameters with the best welding quality. In this embodiment, the steel mesh opening diameters of A1-A17, B1-B16, and C1-C17 of steel mesh 2 are all 0.30 mm, which has the best welding quality. Therefore, this parameter is selected as the final steel mesh opening parameter; (such as Figure 2 ) S2. Setting the steel mesh according to the steel mesh opening parameters determined in step S1; S3. Adjust solder paste printing parameters: S31, setting several groups of different solder paste printing parameter groups, each group of parameters including printing speed, scraper pressure, demoulding speed and demoulding length, testing and selecting appropriate solder paste printing parameters, and determining the solder paste printing parameters according to the test results; S32. Use the above-mentioned several groups of solder paste printing parameter groups to print solder paste in the Hotbar welding area of ​​the same PCB hard board.

[0024] S33. Perform solder paste printing quality inspection on the PCB hard board after solder paste printing. The inspection items include: Solder Paste Thickness: Use a solder paste thickness gauge to measure the thickness of the solder paste.

[0025] Paste Area: Use image analysis software to measure the area of ​​solder paste.

[0026] Solder paste shape: Use a microscope to observe the shape of the solder paste to see if it is full, without collapse, deviation and other defects.

[0027] S34. According to the test results, select the solder paste printing parameter group with the best solder paste printing quality. In this embodiment, the parameter control group with a printing speed of 35-45 mm / s, a scraper pressure of 4.0-5.0 kg, a demoulding speed of 1.0-2.0 mm / s, and a demoulding length of 3 mm has the best solder paste printing quality, so this parameter group is selected as the final solder paste printing parameter (such as Figure 3 ).

[0028] S4. Adjustment point Flux parameters: S41. Set several groups of different point Flux weights. In this embodiment, different point Flux weights are set as examples, which are 0.10mg, 0.15mg, 0.20mg, and 0.25mg respectively.

[0029] S42. Use a dispensing machine to perform a Flux treatment on the Hotbar welding area of ​​the same PCB hard board according to the above-mentioned groups of different Flux weights. The treated PCB hard board pads are as follows Figure 4 shown.

[0030] S43. Perform appearance inspection on the PCB hard board after Flux. Inspection items include: Flux coverage area: Use a magnifying glass to observe whether the Flux coverage area is uniform and whether it completely covers the pad.

[0031] Flux thickness: Use a microscope to observe whether the thickness of the Flux is uniform and appropriate.

[0032] S44. According to the detection results, select the point Flux weight with the best appearance, such as Figure 4 In this embodiment, when the point Flux weight is 0.15-0.25 mg, the coverage area and thickness of the Flux are most ideal, so this weight range is selected as the final point Flux parameter.

[0033] S5. Adjust the welding parameters of the Hotbar equipment: S51. Set several different Hotbar equipment welding parameter groups, each group of parameters includes Hotbar pressure, a heating time, a constant temperature time, a second heating time, a second welding time and welding temperature. Figure 5-8 ; S52. Using the above-mentioned several groups of Hotbar equipment welding parameter groups, the FPC soft board is welded to the Hotbar welding area of ​​the PCB hard board.

[0034] S53. Perform welding quality inspection on the welded PCB hard board and FPC soft board. The inspection items include: Appearance of solder joints: Use a microscope to observe the appearance of solder joints to check whether they are full and round, and whether there are defects such as false solder joints, cold solder joints, etc.

[0035] Solder joint strength: Use a tensile tester to test the tensile strength of the solder joint.

[0036] Temperature curve: Use thermocouples to measure the temperature curve during welding to ensure that the temperature curve meets the welding process requirements.

[0037] S54. According to the detection results, select the Hotbar equipment welding parameter group with the best welding quality. In this embodiment, the parameter group Hotbar pressure is 1.4kg, the first heating time is 2s, the first constant temperature time is 3s, the second heating time is 2s, the second welding time is 5s, the low temperature pad temperature is 235-245°C, and the high temperature pad temperature is lower than 315°C. The welding quality is the best, so this parameter group is selected as the final Hotbar equipment welding parameter.

[0038] S6. In the Hotbar welding area of ​​the PCB hard board, pre-tinning is performed by SMT solder paste printing process using the steel mesh determined in step S2 and the solder paste printing parameters determined in step S3; S7. In the pre-tinned PCB hard board Hotbar welding area, perform point Flux processing according to the point Flux parameters determined in step S4; S8. Use the welding parameters determined in step S5 to set the Hotbar device and solder the FPC soft board to the Hotbar welding area of ​​the PCB hard board.

[0039] Summarize: Adjust the steel mesh opening parameters (S1 and S11-S14, Figure 2 ): Beneficial effect: By testing several groups of different steel mesh opening parameters ( Figure 2 ), and select according to the welding quality test results, ensuring that the steel mesh opening parameters match the PCB hard board and FPC soft board to be welded, thereby improving the integrity of solder paste printing and the forming quality of solder joints, and avoiding welding defects caused by inappropriate steel mesh opening parameters, such as too much or too little solder paste, poor solder joint shape, etc.

[0040] The hole diameter of the steel mesh with circular openings is 0.30mm. This parameter is the optimal parameter verified by testing, which can ensure good solder paste printing quality and solder joint forming quality, and improve the reliability and consistency of welding.

[0041] Adjust solder paste printing parameters (S3 and S31-S34): Beneficial effects: By testing several different groups of solder paste printing parameter groups and selecting them according to the solder paste printing quality test results, it is ensured that the solder paste printing parameters match the PCB hard board, FPC soft board and steel mesh to be soldered, thereby improving the quality of solder paste printing and avoiding problems such as solder paste thickness, area, shape, etc. caused by inappropriate solder paste printing parameters.

[0042] Beneficial effects of specific solder paste printing parameters: The solder paste printing parameters of printing speed of 35-45mm / s, scraper pressure of 4.0-5.0kg, demoulding speed of 1.0-2.0mm / s, and demoulding length of 3mm are used. This parameter is the best parameter verified by testing, which can ensure good solder paste printing quality, avoid problems such as solder paste collapse, offset, and uneven thickness, and provide a good foundation for subsequent welding.

[0043] Beneficial effects of adjusting point Flux parameters (S4 and S41-S44): By testing several groups of different point Flux weights and selecting according to the appearance inspection results, the point Flux weight is ensured to be moderate, which can not only ensure the wetting effect of the Flux, but also avoid problems such as solder joint contamination or bridging caused by excessive Flux.

[0044] Beneficial effects of specific point Flux parameters: The point Flux weight is 0.15-0.25mg, which is the best parameter verified by testing. It can ensure the wetting effect of Flux, help the solder to better wet the pad, and improve the reliability of the solder joint.

[0045] Beneficial effects of adjusting the Hotbar equipment welding parameters (S5 and S51-S54): By testing several different groups of Hotbar equipment welding parameter groups and selecting them according to the welding quality test results, it is ensured that the Hotbar equipment welding parameters match the PCB hard board and FPC soft board to be welded, thereby improving the welding quality of the solder joints and avoiding problems such as poor solder joint appearance and insufficient solder joint strength caused by inappropriate welding parameters.

[0046] Beneficial effects of specific Hotbar equipment welding parameters: Hotbar pressure of 1.4kg, first heating time of 2s, first constant temperature time of 3s, second heating time of 2s, second welding time of 5s, low temperature pad temperature of 235-245°C, high temperature pad temperature below 315°C. This parameter is the best parameter verified by testing, which can ensure the strength and reliability of the solder joint, and avoid damage to the PCB hard board and FPC soft board caused by high temperature.

[0047] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solution of the present invention, which should be included in the scope of the claims of the present invention.

Claims

1. A welding process for FPC soft board and PCB hard board, characterized in that: The following steps are involved: S1. Adjust the steel mesh opening: Test and select appropriate steel mesh opening parameters according to the types of PCB hard board and FPC soft board to be welded, and determine the steel mesh opening parameters according to the test results; S2. Setting the steel mesh according to the steel mesh opening parameters determined in step S1; S3. Adjust solder paste printing parameters: Test and select appropriate solder paste printing parameters according to the types of PCB hard board and FPC soft board to be soldered, and determine the solder paste printing parameters according to the test results; S4. Adjust the point Flux parameters: Test and select appropriate point Flux parameters according to the types of PCB hard board and FPC soft board to be welded, and determine the point Flux parameters according to the test results; S5. Adjust the welding parameters of the Hotbar device: Test and select appropriate Hotbar device welding parameters according to the types of PCB hard boards and FPC soft boards to be welded, and determine the welding parameters based on the test results; S6. In the Hotbar welding area of ​​the PCB hard board, pre-tinning is performed by SMT solder paste printing process using the steel mesh determined in step S2 and the solder paste printing parameters determined in step S3; S7. Perform point Flux processing in the pre-tinned PCB hard board Hotbar welding area according to the point Flux parameters determined in step S4; S8. Use the welding parameters determined in step S5 to set the Hotbar device and solder the FPC soft board to the Hotbar welding area of ​​the PCB hard board.

2. The welding process according to claim 1, characterized in that: The step S1 of adjusting the steel mesh opening parameters comprises the following steps: S11. Select at least three steel meshes with different opening parameters according to the product type; S12. Using the steel mesh to print solder paste in the Hotbar welding area of ​​the PCB hard board; S13. After solder paste printing, the PCB hard board is subjected to soldering quality inspection, wherein the solder paste printing integrity, solder joint shape and solder joint strength are inspected; S14. Select the steel mesh opening parameters with the best welding quality as the final steel mesh opening parameters.

3. The welding process according to claim 1, characterized in that: Adjusting the solder paste printing parameters in step S3 includes the following steps: S31. Select at least three different solder paste printing parameter groups according to the product type, each group of parameters including printing speed, scraper pressure, demolding speed and demolding length; S32. Printing solder paste on the Hotbar soldering area of ​​the PCB hard board using the solder paste printing parameter group; S33 solder paste printing quality test of the PCB hard board after solder paste printing, the solder paste printing quality test includes solder paste thickness, solder paste area and solder paste shape; S34. Selecting a solder paste printing parameter group with the best solder paste printing quality as the final solder paste printing parameters.

4. The welding process according to claim 3, characterized in that: The solder paste printing parameters determined in step S3 include: a printing speed of 35-45 mm / s, a scraper pressure of 4.0-5.0 kg, a demoulding speed of 1.0-2.0 mm / s, and a demoulding length of 3 mm.

5. The welding process according to claim 1, characterized in that: The step S4 in which the point Flux parameter is adjusted includes the following steps: S41. Select at least three different point Flux weights according to product type; S42. Performing a point Flux process on the Hotbar soldering area of ​​the PCB hard board using the point Flux weight; S43. The appearance of the PCB hard board after the point Flux is inspected, and the appearance inspection includes the Flux coverage area and Flux thickness; S44. Select the point Flux weight with the best appearance as the final point Flux parameter.

6. The welding process according to claim 1, characterized in that: Adjusting the welding parameters of the Hotbar device in step S5 includes the following steps: S51. Select at least three different Hotbar equipment welding parameter groups according to the product type, each group of parameters including Hotbar pressure, a heating time, a constant temperature time, a second heating time, a second welding time and a welding temperature; S52. Using the Hotbar device welding parameter group to solder the FPC soft board to the Hotbar welding area of ​​the PCB hard board; S53 after welding PCB hard board and FPC soft board welding quality inspection, the welding quality inspection includes solder joint appearance, solder joint strength and temperature curve; S54. Select the Hotbar equipment welding parameter group with the best welding quality as the final Hotbar equipment welding parameters.

7. The welding process according to claim 6, characterized in that: The welding parameters of the Hotbar device determined in step S5 include: Hotbar pressure of 1.4 kg, first heating time of 2 s, first constant temperature time of 3 s, second heating time of 2 s, second welding time of 5 s, low temperature pad temperature of 345-245°C, and high temperature pad temperature below 315°C.

8. The welding process according to claim 1, characterized in that: The steel mesh openings determined in step S1 are circular, and the diameter of the steel mesh openings is 0.30 mm.

9. The welding process according to claim 1, characterized in that: The point Flux parameter determined in step S4 is: the weight of the point Flux is 0.15-0.25 mg.

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