PCB Tech - Can FPC be repaired and how to repair it

Can flexible PCB be repaired?

The flexible circuit board can be repaired. We will use the repair of the keyboard FPC flexible circuit board to do a detailed description.

1. Remove 21 screws and uncover the old bottom of the keyboard, revealing a piece of white flexible circuit board, and there is a small green main control circuit board in the upper left corner. It is said that the flexible circuit board FPC (Flexible PCB) of the keyboard is made of polyester film (PET, a polymer plastic film). A total of three layers, the upper and lower layers are printed with silver paste or silver foil, and the middle layer is used for insulation (separating the upper and lower layers to prevent short circuits. The upper hole can make the two layers of printed circuit boards instantaneous when the keyboard is hit. Short-circuit to input the corresponding character or command.

2. Only a few keys fail, indicating that the main control board of the keyboard is normal. I took a look at the bottom layer of the flexible circuit board of the keyboard. The failed keys were distributed on the same circuit. Use the diode buzzer of the multimeter, take the FPC line, and go to the connection pin with the main control board (as shown in Figure 3), the multimeter does not ring. Remove the main control board and found that this pin has been burned a few millimeters (other pins are also black, but not so serious), and it is already open.

3. If it is an ordinary PCB board, the circuit breaker is easy to handle, just use a soldering iron to fly a wire. But this is a flexible PCB, a plastic film, and soldering is not feasible. Baidu, it is said that you can draw a few times with a pencil on the broken circuit, but after a short time, it is best to use conductive silver paint to repair the keyboard flexible circuit board. Baidu Encyclopedia says:

Conductive silver paint can make the non-solderable points flow smoothly. It is widely used in repairing open circuit (broken copper circuit) bottom plate, car wind screen heating wire, TV remote control, capacity switch, audio and video tape, venting electrostatic ions, and reducing high-voltage contacts. Resistance value, sealed on the plastic case, shielded to block high-frequency magnetic fields, etc. It can also be used to repair various plastic circuit board flexible circuits such as desktop computers and notebook keyboards.

Conductive silver paint generally has strong adhesion, can be attached to plastics, wood, inner walls, etc., dry quickly (about 3 to 24 hours), low resistance and strong conductivity, can adhere to any plastic fiberboard and metal, etc. . Before use, it must be mixed thoroughly. After mixing it, apply it with a hard-pointed brush or inject it with a syringe. After it is dry, it can be sealed with Electrolube bottom plate film or contact protector. After use, the bottle cap must be tightly closed. Prevent dryness.

Conductive silver lacquer needle, which looks like thermal grease for CPU to dissipate heat

Baidu knows that a friend said, pay attention to the silver paint will spread slowly, so pay attention to the amount of application not too much (less is fine). The domestic conductive silver paint takes effect in about 12-24 hours (the time is determined according to the temperature and air humidity, and the imported one also needs 3-12 hours), and a hot air gun or a hair dryer (temperature control at 40-50 degrees) can also be used to accelerate the conductive silver The effective time of the paint. Conductive silver paint has penetrating power, so it is best to put a piece of paper on the back of the printed film inside the keyboard. The silver paint can be removed after it dries. Another point is that conductive silver paint is usually non-conductive, and does not conduct electricity until it is completely dried.

4. Conductive silver paint seems to be more expensive, 0.3 ml (3 g) is about twenty or thirty yuan. I bought it specifically to fix a broken keyboard. It feels worthless, so I have to find another way. I tried to draw with a pencil, it can be connected, but the connection effect is not good.