PCB Technical - Three major processes of FPC design and production

FPC is also called flexible circuit board. The PCBA assembly and soldering process of FPC is very different from that 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 it leaves 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.

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 thick, some are thinner, and some have reinforced metal plates. Therefore, the junction 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 is required to be 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.

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.