Twist and bow is a big quality issue for PCB board fabrication, which IPC-6012 says that maximum PCB board twist and warp is 0.75%.  At Linked Electronics printed circuit boards of SMT values ≤0.75%, non-SMT values ≤1%, buried and blind holes boards values ≤1.25%

Warped PCB boards will be a big problem to populate, especially if they do not have any flexibility to lay flatSo how to prevent bow and twist on PCB boards? See following advise by Linked Electronics Co., Limited engineer.

1.PCB Design

a. A symmetrical build-up of prepregs unless there is impedance control requirements, for example, 6 Layer PCBs, the preg quantity between 1~2 layer and 5~6 layer should be the same.

b. Avoid using mixing suppliers for FR4 and prepreg, the raw material should be the same supplier. Different supplier will result in lamination issue.

c. The distribution of copper should be balanced. For example, for a 6-layer electronic circuit board, the 1 layer is full of copper and 6 layer has very few traces. Linked Electronics advise engineer use some copper thieving to balance the copper from layer to layer.

d. Symmetrical copper thickness, for a 6-layer PCB board, the copper thickness of 1 layer and 6 layer should be the same.

2.Bake the FR4 material at the temperate of 150 degrees for 8~12 hours to remove the moisture inside the FR4 before PCB manufacturing.

3.The way to cool the boards after HASL. Because of the high temperature shock (approximately 250 degrees), the PCB boards should be taken to an even marble or steel plate to cool. But some factories just want to increase the brightness of the surface treatment. After HASL, boards are put Inside cool water for sever seconds, which will cause twist and bow or some other lamination issues.

4. For final quality inspection, printed circuit boards will be test 100% by Linked Electronics. And for the electronic circuit boards of which twist and warp is more than 0.75%, there will be some rework. These PCB boareds will be re-baked for 3~6 hours and placed on flat surface with great pressure. Some electronic circuit boards might require 2~3 times repeat work to repair.

Contact us Linked Electronics today, let's have a discussion about your PCB demand.