How to Prevent PCB Warping
PCB warping is one of the important issues which has a huge effect on board fabrication. Suppose board with installed components is bent after soldering, component feet tend to be disorder, and this PCB board cannot be installed into the chassis or machine socket, therefore PCB board warping will affect the operation of the entire process. To prevent PCB warping, you should pay more attention to the following aspects in the process of PCB fabrication.
When design your print circuit board, you should consider the following points:
A. The arrangement of interlayer prepregs should be symmetrical, take 6-layer board as example, the thickness and prepreg sheets number should be the same between layer 1 to 2 and 5 to 6, otherwise, it is easy to warp after lamination.
B. Multilayer board and prepreg should be supplied from the same supplier.
C. Line graphic area on outer layer A side and B side should be as close as possible. If the A side is a large copper surface, and B side only take a few lines, printed circuit board like this is easy to warp after etching. If the line area between the two sides is too large, you can add some independent grid on the side with few lines to keep balance.
The purpose of the pre-baking of the CCL (150℃ for 8±2 hours) is to remove the moisture in the plate and to completely solid the resin in the sheet to further eliminate the remaining stresses in the sheet, which is helpful for preventing board warpage. At present, many double-sided or multi-layer boards still adhere to the pre-press or post-baking step. Baking time from 4h to 10h is different in different PCB manufacturers, time is determined according to printed circuit board grade and customer requirements for PCB warpage.
The Latitude and Longitude of Prepreg
The shrinkage of latitude and longitude of laminated prepreg is not the same, so you must distinguish latitude and longitude of the cutting and lamination. Otherwise, it is likely to cause board warping after lamination; even put press on the board is also difficult to solve the warpage. How to distinguish between latitude and longitude? The direction of the rolled prepreg is longitude, while the width direction is latitude. For the copper foil, the long side is the longitude direction and the short side is latitude. If you’re unsure about that, you can ask your manufacturer directly.