IPC-6012, SMB--SMT circuit board maximum warpage or distorted 0.75%, other board warpage is generally not more than 1.5%; electronic assembly plant allows the warping degree (double sided / multilayer) is usually 0.70---0.75%, (1.6mm thickness) actually many boards such as SMB, BGA board for warpage is less than 0.5%; part of the plant even less than 0.3%.
The calculation method of the warping degree = the height / length of the curved side
Prevention of warp of circuit board:
1 Engineering design:
The arrangement of the interlayer semi - curing sheets should correspond to the;
The same supplier products should be used in the multi layer plate core plate and semi - curing plate;
C/S surface area of the outer surface is as close as possible to the surface, can be used as an independent grid.
2 pre baking plate:
Generally 150 degrees 6--10 hours, excluding water vapor in the plate, further make the resin curing completely, eliminating the stress in the plate; opening the pre baked board, regardless of the inner layer or the need for both sides!
3 the direction of the warp and weft of the plate curing sheet should be paid attention to in the front of the multilayer board laminated sheet:
The warp and weft direction is not the same as the shrinkage, and the semi - curing sheet should pay attention to the direction of longitude and latitude; the core board should also pay attention to the direction of the warp and weft.
4 drill before baking plate: 150 degrees 4 hours.
5 laminated thick cold stress eliminating pressure plate, cutting edges.
6 sheet is best not to pass through the mechanical grinding brush, it is recommended to use chemical cleaning; electroplating, the use of special fixture to prevent the plate bending fold.
7 natural cooling to room temperature or air flotation after cooling in the flat marble or steel plate.
Warping board processing: 150 degrees or hot pressing 3--6 hours, using the smooth and smooth weight of the steel plate, 2-3 baking.