Explain the design skills of FPC's welded golden finger

background

FPC, flexible circuit board. The number of circuits of one FPC can exceed 100, but it only occupies a thickness of 0.2mm. Although more expensive than flat cable, it is widely used in compact intelligent hardware. Especially mobile phones, smart watches and other scenes.

Today we will focus on the design skills of the golden finger of the welded FPC.

The quality of hardware products is good, and the design accounts for more than half!

FPC welding skills

Benefits of welding FPC

FPC welding can save 2 board to board connectors or 1 Zif plug-in connector, saving the cost of 1-2 yuan.

Therefore, welding FPC is adopted for many low-end products. (Manual welding is faster than machine mounting.)

Welding solid FPC, if well designed, is not only cheaper than the connector, but also stronger than the connector: welding must be more solid than the buckle.

The disadvantage is that the welding area is larger than the connector, and the maintenance is more complex.

FPC welding skills

Double sided gold finger+tin hole

Although there is only one layer of bonding pad corresponding to the gold finger, the gold finger of FPC cannot be made into only one layer.

FPC is a non-metallic substrate, and the heat transfer is very slow. If there is only one layer of gold finger, it is difficult to weld because the heat of the soldering iron cannot be transferred during manual welding.

Similarly, the tin hole on the bonding pad is also for heat transfer, and allows the molten solder to flow between the upper and lower layers.

FPC welding skills

This is both sides of the same welded FPC

Double sided wiring+additional via

Early engineers only made double-sided pads. However, FPC is often disconnected during product production.

Later, it was found that the FPC manufacturer was not to blame for the poor quality, but the stress was too high and easy to break, which must be improved by design.

Finally, we invented the way of "double-sided wiring+additional vias" to solve this problem. No matter how you fold it, the FPC will not be damaged.

FPC welding skills

Cause of damage: FPC is bendable, which is based on the overall stretching of copper foil. After FPC welding, the welding surface cannot be stretched. Once it is pulled up, the stress point is concentrated on the edge of the welding surface, and the edge will be pulled off.

After changing to double-sided wiring, the problem of bottom fracture has not been solved, but because there are two layers of wiring, one layer is broken and another layer is broken, the signal will not be broken.