PCB board distortion is a problem that may occur in PCBA mass production. It will have a considerable impact on patch and testing, so this problem should be avoided in production. Generally speaking, the reasons for PCB distortion are as follows:
1. The raw materials of the PCB are improperly selected. For example, the T of the PCB is low, especially the paper-based PCB. The processing temperature is too high, and the PCB becomes bent.
2. Unreasonable PCB design and uneven distribution of components will cause excessive thermal stress on the PCB. Larger connectors and sockets will also affect the expansion and contraction of the PCB, resulting in permanent distortion.
3. PCB design problems, such as double-sided PCB, if the copper foil on one side is too large, such as the ground wire, and the copper foil on the other side is too small, it will also cause uneven shrinkage on both sides and deformation.
4. Improper use of the fixture or the distance between the fixture is too small. For example, the middle finger claw in wave soldering is too tight, and the PCB will expand and deform due to the soldering temperature.
5. Excessive temperature during reflow soldering will also cause distortion of the PCB.
In view of the above reasons, engineers have proposed corresponding solutions:
1. When the price and space allow, choose a PCB with a high Tg or increase the thickness of the PCB to obtain the best aspect ratio.
2. Reasonable design of the PCB, the area of the steel foil on both sides should be balanced, and the copper layer should be covered where there is no circuit, and appear in the form of a grid to increase the rigidity of the PCB.
3. Pre-bake the PCB before patching, the condition is 125℃/4h.
4. Adjust the fixture or clamping distance to ensure the space for PCB thermal expansion.
5. The temperature of the welding process should be lowered as much as possible; there has been slight distortion, which can be placed in the positioning fixture, heated and reset to release the stress, and generally satisfactory results will be obtained.
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