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FPC is also called flexible circuit board. The PCBA assembly and welding process of FPC is very different from that of rigid circuit board. Because FPC board is not hard enough and is relatively soft, if a special carrier is not used, it cannot be fixed and transmitted, and it is also impossible to complete basic SMT processes such as printing, patching, and furnace passing. 1. Pretreatment of FPC FPC board is relatively soft and is generally not vacuum packed when leaving the factory. It is easy to absorb moisture in the air during transportation and storage. It needs to be pre-baked before SMT casting to slowly and forcibly discharge the moisture. Otherwise, under the high temperature impact of reflow soldering, the moisture absorbed by FPC will quickly gasify into water vapor and protrude from FPC, which is easy to cause FPC delamination, blistering and other defects.
The pre-baking conditions are generally 80-100℃ for 4-8 hours. In special cases, the temperature can be raised to above 125℃, but the baking time needs to be shortened accordingly. Before baking, a small sample test must be made to determine whether the FPC can withstand the set baking temperature. You can also consult the FPC manufacturer for appropriate baking conditions. When baking, the FPC cannot be stacked too much. 10-20PNL is more appropriate. Some FPC manufacturers will put a piece of paper between each PNL for isolation. It is necessary to confirm whether this piece of paper for isolation can withstand the set baking temperature. If not, the isolation paper needs to be removed before baking. After baking, the FPC should not have obvious discoloration, deformation, warping and other defects. It needs to be inspected by IPQC before it can be put into production. 2. Production of special carrier board According to the CAD file of the circuit board, read the hole positioning data of the FPC to manufacture high-precision FPC positioning templates and special carrier boards, so that the diameter of the positioning pins on the positioning template matches the aperture of the positioning holes on the carrier board and the positioning holes on the FPC. Many FPCs are not of the same thickness because they need to protect some circuits or for design reasons. Some places are thicker and some places are thinner. Some have reinforced metal plates. Therefore, the joints between the carrier board and the FPC need to be processed, polished and grooved according to the actual situation. The function is to ensure that the FPC is flat during printing and mounting. The material of the carrier board is required to be light, thin, high-strength, less heat absorption, fast heat dissipation, and small warping deformation after multiple thermal shocks. Commonly used carrier materials include synthetic stone, aluminum plate, silicone plate, special high temperature resistant magnetized steel plate, etc. 3. Production process. Here we take ordinary carrier as an example to elaborate on the key points of FPC SMT. When using silicone plate or magnetic fixture, the fixation of FPC is much more convenient and does not require the use of tape. The key points of printing, patch, welding and other processes are the same. 1. Fixation of FPC: Before SMT, the FPC needs to be accurately fixed on the carrier. It is particularly important to note that the shorter the storage time from the time the FPC is fixed on the carrier to the time of printing, mounting and welding, the better. There are two types of carriers: with positioning pins and without positioning pins. The carrier without positioning pins needs to be used in conjunction with the positioning template with positioning pins. First, put the carrier on the positioning pins of the template so that the positioning pins are exposed through the positioning holes on the carrier. Put the FPC on the exposed positioning pins one by one, and then fix it with tape. Then separate the carrier from the FPC positioning template for printing, mounting and welding. The carrier with positioning pins has been fixed with several spring positioning pins about 1.5mm long. The FPC can be directly put on the spring positioning pins of the carrier one by one, and then fixed with tape. In the printing process, the spring positioning pins can be completely pressed into the carrier by the steel mesh, which will not affect the printing effect. Method 1 (single-sided tape fixation): Use a thin, high-temperature resistant single-sided tape to fix the four sides of the FPC on the carrier board to prevent the FPC from deflecting or warping. The tape viscosity should be moderate, and it must be easy to peel off after reflow soldering, and there should be no residual adhesive on the FPC. If an automatic tape machine is used, it can quickly cut the tape of the same length, which can significantly improve efficiency, save costs, and avoid waste. Method 2 (double-sided tape fixation): First use a high-temperature resistant double-sided tape to stick on the carrier board, the effect is the same as the silicone plate, and then stick the FPC to the carrier board. Pay special attention to the viscosity of the tape not being too high, otherwise it is easy to cause the FPC to tear when peeling off after reflow soldering. After repeated furnace passes, the viscosity of the double-sided tape will gradually decrease. When the viscosity is too low to reliably fix the FPC, it must be replaced immediately. This station is a key station to prevent the FPC from getting dirty, and finger cots are required. Before reusing the carrier, it needs to be properly cleaned. You can use a non-woven cloth dipped in a detergent to scrub it, or you can use an anti-static dust-sticking roller to remove surface dust, tin beads and other foreign matter. When taking and placing the FPC, be careful not to use too much force, as the FPC is fragile and prone to creases and breaks. 2. FPC solder paste printing: FPC has no special requirements on the composition of solder paste. The size of the solder ball particles and the metal content are based on whether there is a fine-pitch IC on the FPC. However, FPC has higher requirements on the printing performance of solder paste. The solder paste should have excellent thixotropy, be easy to print and demould, and be able to firmly adhere to the FPC surface, without poor demoulding, blocking the steel mesh leaks, or collapse after printing. Because the FPC is mounted on the carrier board, there is a high temperature resistant tape for positioning on the FPC, which makes its plane inconsistent. Therefore, the printed surface of the FPC cannot be as flat and have the same thickness and hardness as the PCB. Therefore, it is not suitable to use a metal scraper, but a polyurethane scraper with a hardness of 80-90 degrees. It is best for the solder paste printer to have an optical positioning system, otherwise it will have a greater impact on the printing quality. Although the FPC is fixed on the carrier board,However, there will always be some tiny gaps between FPC and carrier board, which is the biggest difference from PCB hard board, so the setting of equipment parameters will also have a great impact on the printing effect. The printing station is also a key station to prevent FPC from getting dirty. It is necessary to wear finger gloves when working. At the same time, the station should be kept clean and the steel mesh should be wiped frequently to prevent solder paste from contaminating the gold fingers and gold-plated buttons of FPC. 3. FPC patch: According to the characteristics of the product, the number of components and the patch efficiency, medium and high-speed patch machines can be used for placement. Since each FPC has an optical MARK mark for positioning, there is little difference between SMD mounting on FPC and mounting on PCB. It should be noted that although the FPC is fixed on the carrier, its surface cannot be as flat as the PCB hard board. There must be a local gap between the FPC and the carrier. Therefore, the nozzle descent height, blowing pressure, etc. need to be accurately set, and the nozzle movement speed needs to be reduced. At the same time, most FPCs are connected boards, and the yield rate of FPC is relatively low. Therefore, it is normal for some bad PCS to be contained in the whole PNL. This requires the placement machine to have a BAD MARK recognition function. Otherwise, when producing such non-whole PNLs that are all good boards, the production efficiency will be greatly reduced. 4. Reflow of FPC: A forced hot air convection infrared reflow oven should be used, so that the temperature on the FPC can change more evenly and reduce the occurrence of poor welding. If single-sided tape is used, because only the four sides of the FPC can be fixed, the middle part will be deformed due to the hot air state, the pad is easy to tilt, and the molten tin (liquid tin at high temperature) will flow to produce empty soldering, continuous soldering, and tin beads, which will increase the process defect rate. 1) Temperature curve test method: Due to the different heat absorption of the substrate and the different types of components on the FPC, the speed of temperature rise after being heated during the reflow soldering process is different, and the amount of heat absorbed is also different. Therefore, carefully setting the temperature curve of the reflow soldering furnace has a great impact on the welding quality. A more reliable method is to place two substrates with FPCs in front and behind the test board according to the actual substrate spacing during production, and at the same time mount components on the FPC of the test substrate, and use high-temperature solder wire to solder the test temperature probe to the test point, and use high-temperature resistant tape to fix the probe wire to the substrate. Note that the high-temperature resistant tape cannot cover the test point. The test points should be selected near the solder joints and QFP pins on each side of the carrier board, so that the test results can better reflect the real situation. 2) Setting of temperature curve: In the furnace temperature debugging, because the temperature uniformity of FPC is not good, it is best to use the temperature curve of heating/insulation/reflow, so that the parameters of each temperature zone are easier to control, and the FPC and components are less affected by thermal shock. According to experience, it is best to adjust the furnace temperature to the lower limit of the technical requirements of the solder paste. The wind speed of the reflow furnace generally adopts the lowest wind speed that the furnace can adopt. The chain stability of the reflow furnace should be good and there should be no shaking. 5.Inspection, testing and sub-boarding of FPC: Since the carrier absorbs heat in the furnace, especially the aluminum carrier, the temperature is high when it comes out of the furnace, so it is best to add a forced cooling fan at the furnace outlet to help cool down quickly. At the same time, the operator needs to wear heat-insulating gloves to avoid being burned by the high-temperature carrier. When taking the welded FPC from the carrier, the force should be even, and brute force should not be used to avoid the FPC being torn or creased. The removed FPC is placed under a magnifying glass of more than 5 times for visual inspection, focusing on the problems of residual glue on the surface, discoloration, gold finger tinning, tin beads, empty soldering of IC pins, and continuous soldering. Since the surface of FPC cannot be very flat, the misjudgment rate of AOI is very high, so FPC is generally not suitable for AOI inspection, but with the help of special test fixtures, FPC can complete ICT and FCT tests. Since FPC is mostly connected, it may be necessary to separate the boards before ICT and FCT tests. Although the separation operation can be completed using tools such as blades and scissors, the operation efficiency and quality are low, and the scrap rate is high. If it is a large-scale production of special-shaped FPC, it is recommended to make a special FPC stamping separation mold for stamping and segmentation, which can greatly improve the operation efficiency. At the same time, the edges of the punched FPC are neat and beautiful, and the internal stress generated during stamping and cutting is very low, which can effectively avoid solder cracking. In the assembly and welding process of PCBA flexible electronics, the precise positioning and fixation of FPC is the key point. The key to good fixation is to make a suitable carrier board. The second is the pre-baking, printing, patch and reflow soldering of FPC. Obviously, the SMT process of FPC is much more difficult than that of PCB hard board, so it is necessary to accurately set the process parameters. At the same time, strict production process management is equally important. It is necessary to ensure that the operators strictly implement every regulation in the SOP. The line engineers and IPQC should strengthen the inspection, timely discover the abnormal situation of the production line, analyze the reasons and take necessary measures to control the defective rate of the FPC SMT production line within dozens of PPM. In the PCBA production process, it is necessary to rely on a lot of machines and equipment to assemble a board. Often the quality level of a factory's machines and equipment directly determines the manufacturing capacity. The basic equipment required for PCBA production includes solder paste printer, SMT machine, reflow soldering, AOI detector, component foot trimmer, wave soldering, tin furnace, board washer, ICT test fixture, FCT test fixture, aging test rack, etc. PCBA processing plants of different sizes will be equipped with different equipment.
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提升卡
变色卡
千斤顶
京公网安备 11010802033920号
