PCBA Tech - What is the process of SMT mounting FPC production?

Flexible Printed Circuit (FPC) is a highly reliable and excellent flexible printed circuit board made of polyimide or polyester film. It has the characteristics of high wiring density, light weight, thin thickness and good bendability.

In the production process, in order to prevent excessive opening and short circuits from causing too low yield or reducing the problem of FPC board scrap and replenishment caused by rough process problems such as drilling, rolling, and cutting, and to evaluate how to select materials to achieve customer use For the best effect of flexible circuit boards, pre-production pretreatment is particularly important.

SMT mount FPC

Pre-natal pretreatment, there are three aspects that need to be dealt with, and all three aspects are completed by engineers. The first is the FPC board engineering evaluation, which is mainly to evaluate whether the customer's FPC board can be produced, whether the company's production capacity can meet the customer's board-making requirements and unit cost; if the engineering evaluation is passed, the next step is to prepare materials immediately to meet each production link Finally, the engineer processes the customer’s CAD structure drawing, gerber line data and other engineering documents to suit the production environment and production specifications of the production equipment, and then delegates the production drawings and MI (engineering process card) to the The production department, document control, purchasing and other departments enter the regular production process.

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 storage time between printing, mounting and soldering after the FPC is fixed on the carrier board is as short as possible. There are two kinds of carrier boards with positioning pins and without positioning pins.

The carrier board without positioning pins needs to be used in conjunction with the positioning template with positioning pins. First put the carrier board on the positioning pins of the template, so that the positioning pins are exposed through the positioning holes on the carrier board, and put the FPC piece by piece. The exposed positioning pins are then fixed with tape, and then the carrier board is separated from the FPC positioning template for printing, patching and welding. The carrier board with positioning pins has been fixed with several spring positioning pins about 1.5mm in length. The FPC can be directly put on the spring positioning pins of the carrier board one by one, and then fixed with tape. In the printing process, the spring positioning pin can be completely pressed into the carrier plate by the steel mesh without affecting the printing effect.

Method one (fixed with single-sided tape): Use thin, high-temperature single-sided tape to fix the four sides of the FPC on the carrier board to prevent the FPC from shifting and warping. The viscosity of the tape should be moderate, and it must be easy to peel off after reflow. There is no residual glue on the surface. If you use an automatic tape machine, you can quickly cut tapes of the same length, which can significantly improve efficiency, save costs, and avoid waste.

Method two (fixed with double-sided tape): first use high-temperature resistant double-sided tape to stick to the carrier board, the effect is the same as the silicone plate, and then paste the FPC to the carrier board, pay special attention to the viscosity of the tape not to be too high, otherwise it will peel off after reflow soldering When, it is easy to cause the FPC to tear. After repeated ovens, the viscosity of the double-sided tape will gradually decrease. If the viscosity is too low to reliably fix the FPC, it must be replaced immediately. This station is the key station to prevent the FPC from getting dirty, and it is necessary to wear finger cots for work. Before the carrier is reused, it needs to be properly cleaned. It can be wiped with a non-woven cloth dipped in detergent, or an anti-static sticky roller can be used to remove surface dust, tin beads and other foreign objects. Don't use too much force when picking up and placing FPC. FPC is fragile and prone to creases and breaks.

2. FPC solder paste printing:

FPC does not have very special requirements for 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. At the same time, FPC is mostly connected board, and the yield of FPC is relatively low. Therefore, it is normal for the whole PNL to contain some bad PCS. This requires the placement machine to have the BAD MARK recognition function, otherwise, in the production of this type of non-integral When PNL is good board, the production efficiency will be greatly 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.

1) Temperature curve test method:

Due to the different heat absorption properties of the carrier board and the different types of components on the FPC, the temperature rises at different speeds after being heated during the reflow soldering process, and the heat absorbed is also different. Therefore, carefully set the temperature curve of the reflow oven to improve the quality of soldering. Great influence. A safer method is to place two FPC-equipped carrier boards before and after the test board according to the carrier board interval during actual production. At the same time, mount the components on the FPC of the test carrier board, and use high-temperature solder wire to test the temperature. The probe is welded on the test point, and the probe wire is fixed on the carrier board with a high-temperature adhesive tape. 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 FPC's temperature uniformity is not good, it is best to use the temperature curve method of heating/heat preservation/reflow, so that the parameters of each temperature zone are easier to control, and the FPC and components are affected by thermal shock. Some. According to experience, it is best to adjust the furnace temperature to the lower limit of the solder paste technical requirements. The wind speed of the reflow furnace is generally the lowest wind speed that the furnace can use. The chain of the reflow furnace should be stable and free of jitter.

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.