Prevent board warpage during circuit board manufacturing
1. Engineering design: matters needing attention in printed board design:
A. The arrangement of prepregs between layers should be symmetrical. For example, for a six-layer board, the thickness between 1-2 and 5-6 layers and the number of prepregs should be the same, otherwise it will be warped after lamination.
B. Multilayer board core board and prepreg should use the products of the same supplier.
C. The circuit pattern area of the outer surface A and B should be as close as possible. If the A side is a large copper side, and the B side only takes a few lines, this printed board is easily warped after etching. If the line area of the two sides is too different, you can add some independent grids on the thin side to balance.
2. Baking board before cutting:
The purpose of pre-baking the copper clad laminate (150 degrees Celsius, time 8 ± 2 hours) is to remove the moisture in the board and at the same time completely cure the resin in the board, further eliminating the remaining stress in the board, which is useful for preventing the board from warping Help. At present, many double-sided and multi-layer boards still adhere to the step of pre-baking or post-baking. However, there are exceptions for some sheet metal factories. At present, the time requirements for baking boards in various PCB factories are also inconsistent, ranging from 4 to 10 hours. It is recommended to decide according to the grade of the printed board produced and the customer's requirements for warpage. Either of the two methods is feasible after cutting into a panel and then baking or cutting the whole block and then cutting. It is recommended to bake the board after cutting. The inner layer board should also be baked.
3. Longitude and latitude of prepreg:
After lamination, the shrinkage rates of warp direction and weft direction are different. When cutting and laminating, the warp direction and weft direction must be distinguished. Otherwise, it is easy to cause warpage of the finished board after lamination, and it is difficult to correct even if the pressure drying board is pressed. Many of the reasons for the warpage of the multilayer board are caused by the lamination of the prepreg in latitude and longitude during lamination.
How to distinguish latitude and longitude? The rolled up prepreg is in the warp direction and the width direction is the weft direction. For copper foil, the long side is the weft direction and the short side is the warp direction. If you are unsure, you can check with the manufacturer or supplier.
4. Stress relief after lamination:
The multilayer board is taken out after hot and cold pressing, cut or milled off the burrs, and then placed in an oven at 150 degrees Celsius for 4 hours to gradually release the stress in the board and completely cure the resin. This step cannot be omitted.
5. Straightening of the thin plate plating:
Special platen rollers should be made for 0.4 ~ 0.6mm ultra-thin multi-layer board for plate surface plating and graphic plating. After the thin plate is clamped on the flybar on the automatic plating line, use a round bar to clamp the entire flybar Rollers are strung together to straighten all the boards on the rollers so that the plated board will not deform. Without this measure, after plating a copper layer of 20-30 microns, the sheet will bend and is difficult to remedy.
6. Cooling of the board after leveling with hot air:
The hot air of the printed board is impacted by the high temperature of the solder bath (about 250 degrees Celsius) when it is leveled. After being taken out, it should be placed on a flat marble or steel plate to cool naturally and sent to the post-processing machine for cleaning. This is very good for the board to prevent warpage. In order to enhance the brightness of the surface of lead and tin in some factories, the hot air of the board is immediately put into cold water after being leveled. After a few seconds, it is taken out for post-processing. This kind of hot and cold impact may cause warpage to some models Song, layering or blistering. In addition, an air floating bed can be installed on the equipment for cooling.
7. Treatment of warped board:
In an orderly managed factory, the printed boards will be inspected for 100% flatness during final inspection. All unqualified boards will be picked out and placed in an oven, baked at 150 degrees Celsius and heavy pressure for 3 to 6 hours, and naturally cooled under heavy pressure. Then remove the pressure and take out the board to check the flatness, which can save some of the board, and some boards need to be baked and pressed two to three times to be leveled. If the above-mentioned anti-warping process measures are not implemented, some of the boards will be useless and can only be scrapped.
