source:Other information release time:2023-08-03 Hits: Popular:PCB Assembly company
1. Engineering design: When designing printed boards, precautions should be taken: A. The arrangement of interlayer semi cured sheets should be symmetrical, for example, for six layer boards, the thickness and number of semi cured sheets between layers 1-2 and 5-6 should be different, otherwise they may easily warp after lamination.
B. Multi layer core boards and semi cured sheets should use products from the same supplier.
C. The area of the outer A-side and B-side line graphics should be as close as possible. If the A-side is a large copper surface and the B-side only runs a few wires, this type of printed board is prone to warping after etching. If the area difference between the two sides of the line is too large, independent grids can be added on the sparse side for balance.
2. Drying the board before cutting: The purpose of drying the board before cutting the copper clad board (150 degrees Celsius, time 8 ± 2 hours) is to remove moisture from the board, while allowing the resin inside the board to fully cure, further eliminating the remaining stress in the board, which is helpful in avoiding board warping. At present, many double-sided and multi-layer boards still adhere to the step of drying the boards before or after cutting. However, there are also some exceptions for local board factories. Currently, the time rules for drying boards in various PCB factories are not different, ranging from 4-10 hours. It is recommended to make decisions based on the level of printed boards consumed and the customer's request for warpage. Both methods are feasible: cutting and baking after splicing, or cutting and baking after cutting a large piece of material. It is recommended to cut and bake the board after cutting. The inner layer board should also be baked.
3. The warp and weft directions of the semi cured sheet:
The longitudinal and latitudinal shrinkage rates of semi cured sheets are different after lamination, and it is necessary to distinguish between them during cutting and lamination. Otherwise, it is easy to form scrap board warping after lamination, and even with pressure drying, it is difficult to correct it. Many of the reasons for the warping of multi-layer boards are due to the unclear longitudinal and latitudinal directions of the semi cured sheets during lamination, resulting in disorderly stacking.
4. Stress relief after lamination: After completing hot pressing and cold pressing, remove the multi-layer board, cut or mill off the burrs, and then lay it flat in an oven at 150 degrees Celsius for 4 hours to gradually release the stress inside the board and allow the resin to fully cure. This step cannot be omitted.
5. When electroplating thin plates, straightening is required:
Special clamping rollers should be manufactured for electroplating and pattern electroplating of 0.4-0.6mm ultra-thin multi-layer boards. After clamping the thin plate on the flying bar on the automatic electroplating line, a round rod should be used to string the clamping rollers on the entire flying bar, thereby straightening all the boards on the roller, so that the electroplated board will not deform. Without this measure, after electroplating a copper layer of 20 to 30 microns, the thin plate will bend and be difficult to compensate for.
6. Cooling of the board after hot air leveling:
Printed board hot air leveling is often subjected to high temperature impact from a soldering bath (about 250 degrees Celsius). After removal, it should be placed on a flat marble or steel plate for natural cooling, and then sent to a post-processing machine for cleaning. This is very beneficial for preventing warping of the board. Some factories, in order to enhance the brightness of the surface of lead and tin, immediately pour the board into cold water after hot air leveling, and then take it out after a few seconds before stopping for disposal. This hot cold impact may cause warping, layering, or foaming of certain types of boards. In addition, an air floating bed can be installed on the equipment to stop cooling.
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