In the printed circuit boards production process, sometimes there are warping phenomenon for PCB boards, one reason is caused by the character of board material, on the other hand, during printed circuit boards production process due to the affect by thermal stress and chemical factors, or incorrect production technology they all can make pcb boards warpping. Below technical requirements listed can avoided or mitigate the PCB warping during production.
1. Engineering design: the arrangement of prepreg between layers should be corresponded; the core board and prepreg for multi-layer PCBs should be chosen from the same supplier, because products from the same supplier are with same characteristics and high matching degree; The area of outer C/S surface should be as same as possible, which can adopt independent grid.
2. Drying boards before material input: generally under 150 degrees and 6 to 10 hours, to remove water vapor in the boards, make the resin further solidifying completely and eliminate the stress in the boards; no matter inner or the two surface of boards, all of them need to be drying before boards cutting.
3. The longitude and weft direction of the prepreg should be noted before lamination of the laminated boards for multi-layer PCBs: the shrink proportion is different in longitude and weft direction, and the direction should be distinguished before prepreg material input and when core boards input. Generally the short direction of prepreg is the warp direction; the long direction of copper clad laminate is the longitude direction.
4. Eliminate stress after lamination, cold pressure after hot pressure, trim the blurs;
5. Drying PCB boards before drilling: under 150 degrees for 4 hours;
6. The thin boards are not to be brushed by machine,it's better to adopt chemical cleaning.Special fixture is used when do electroplating to prevent boards bending and folding.
7. After HASL, cooling the PCB boards to room temperature on the flat marble or steel plate natually, or clean them after cooling by air float bed;
If PCB boards has warping phenomenon, the following methods can be used:
Under 150 degrees or heat pressure 3-6 hours, pressing by flat, smooth steel plates, and baking 2-3 times.
Some big PCB factory use large oven baking sheet. A large stack of copper clad laminates are placed into the oven before production start, and then are baked for a few hours to a dozen hours at the near glass temperature of the boards. The warping and deformation of PCBs produced by the coated copper clad laminate is relatively small, and the quality rate of pcb products is much higher. For some small PCB plants, if they don't have such a large oven, the boards can be cut to small ones and then roasted, but the boards should be pressed when be roasted that to keep the foundation flat in the stress relaxation process. The plate temperature should not be too high, if too high temperature will discolor the boards. Also not too low, if too low temperature it will take a long time to make the boards stress relaxation.