PCB Tech - PCBA processing assembly welding process of FPC soft board

FPC is also called flexible circuit board. The PCBA processing and assembly welding process of FPC is very different from the assembly of rigid circuit board. Because the hardness of FPC board is not enough, it is relatively soft. If you do not use a dedicated carrier board, it will not be able to complete the fixing and transmission. Basic SMT processes such as printing, placement, and furnace cannot be completed.

one. FPC pretreatment

The FPC board is relatively soft, and it 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 is put into the line to force the moisture out slowly. Otherwise, under the high temperature impact of reflow soldering, the moisture absorbed by the FPC will quickly vaporize and become water vapor to protrude from the FPC, which will easily cause defects such as FPC delamination and blistering.

About the PCBA processing assembly welding process of the FPC soft board

The pre-baking conditions are generally at a temperature of 80-100°C and a time of 4-8 hours. Under special circumstances, the temperature can be adjusted to above 125°C, but the baking time needs to be shortened accordingly. Before baking, a small sample test must be done to determine whether the FPC can withstand the set baking temperature. You can also consult the FPC manufacturer for suitable baking conditions. When baking, FPC should not be stacked too much. 10-20PNL is more suitable. 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 it is not necessary to remove the release paper, then bake it. The baked FPC should have no obvious discoloration, deformation, warping and other defects, and it can be put into the line after being qualified by IPQC sampling.

two. Production of dedicated carrier board

According to the CAD file of the circuit board, read the hole positioning data of the FPC to manufacture the high-precision FPC positioning template and the special carrier board, so that the diameter of the positioning pin on the positioning template and the positioning hole on the carrier board and the aperture of the positioning hole on the FPC are the same. match. Many FPCs are not of the same thickness because they want to protect part of the circuit or for design reasons. Some places are thicker, some places are thinner, and some have reinforced metal plates. Therefore, the joint of the carrier board and the FPC needs to be The actual situation is to process, polish and dig grooves, and the function is to ensure that the FPC is flat during printing and placement. The material of the carrier board requires light and thin, high strength, low heat absorption, fast heat dissipation, and small warpage deformation after multiple thermal shocks. Commonly used carrier materials include synthetic stone, aluminum plate, silica gel plate, special high-temperature resistant magnetized steel plate, etc.

three. production process.

Here we take a common carrier board as an example to detail the SMT points of FPC. When using silicone plates or magnetic jigs, the fixing of FPC is much more convenient, without the use of tape, and the process points of printing, patching, welding and other processes are the same.

1. Fixing of FPC:

Before SMT, the FPC needs to be accurately fixed on the carrier board. In particular, it should be noted that the shorter the storage time between printing, mounting and soldering after the FPC is fixed on the carrier board, the better.

2. FPC solder paste printing:

FPC does not have very special requirements on the composition of solder paste. The size and metal content of the solder ball particles are subject to whether there are fine-pitch ICs on the FPC. However, FPC has higher requirements for the printing performance of solder paste, and the solder paste should have excellent Thixotropy, the solder paste should be easy to print and release and firmly adhere to the surface of the FPC, and there will be no defects such as poor release, blocking the leakage of the stencil or collapse after printing.

3. FPC patch:

According to the characteristics of the product, the number of components and the placement efficiency, medium and high-speed placement machines can be used for placement. Since there is an optical MARK mark for positioning on each FPC, there is little difference between SMD mounting on the FPC and mounting on the PCB. It should be noted that although the FPC is fixed on the carrier board, its surface cannot be as flat as a PCB hard board. There will definitely be a partial gap between the FPC and the carrier board. Therefore, the suction nozzle drop height, blowing pressure, etc. It needs to be set accurately, and the moving speed of the suction nozzle needs to be reduced.

4. Reflow soldering of FPC:

The forced hot air convection infrared reflow oven should be used, so that the temperature on the FPC can be changed more uniformly, and the occurrence of poor soldering can be reduced. If you use single-sided tape, because you can only fix the four sides of the FPC, the middle part is deformed under hot air, the pad is easily inclined, and the molten tin (liquid tin at high temperature) will flow, resulting in empty soldering, continuous soldering, Tin beads make the process defect rate higher.

5. FPC inspection, testing and sub-board:

Since the carrier plate absorbs heat in the furnace, especially the aluminum carrier plate, the temperature is higher when it is 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, operators need to wear heat-insulating gloves to avoid being burned by the high-temperature carrier. When taking the soldered FPC from the carrier board, the force should be even, and brute force should not be used to prevent the FPC from being torn or creases.

The removed FPC is visually inspected under a magnifying glass of more than 5 times, focusing on the inspection of residual glue on the surface, discoloration, gold finger staining, tin bead, IC pin empty welding, continuous welding and other problems. Since the surface of FPC cannot be very smooth, which makes the AOI misjudgment rate high, FPC is generally not suitable for AOI inspection, but by using special test fixtures, FPC can complete ICT and FCT tests.

In the assembly and welding process of PCBA flexible electronics, the precise positioning and fixing of FPC are the key points. The key to fixing is to make a suitable carrier board. Followed by FPC pre-baking, printing, placement and reflow soldering. Obviously, the SMT process of FPC is much more difficult than PCB hard board, so it is necessary to accurately set process parameters. At the same time, strict production process management is also important. It is necessary to ensure that operators strictly implement every regulation on SOP and follow the line. Engineers and IPQC should strengthen inspections, discover the abnormal conditions of the production line in time, analyze the causes and take necessary measures to control the defect rate of the FPC SMT production line within dozens of PPM.