Circuit board warpage has a great impact on the production of printed circuit boards. Warpage is also one of the important problems in the circuit board production process. The board with components on it will bend after welding, making it difficult for the component legs to be neat.

The board cannot be installed into the chassis or the socket inside the machine. Therefore, the warping of the circuit board will affect the normal operation of the entire subsequent process.

At this stage, printed circuit boards have entered the era of surface mounting and chip mounting, and the process requirements for circuit board warpage are getting higher and higher. So we need to find the reason why the circuit board is warping.

1. Engineering design:

Things to note when designing printed circuit boards:

A. The arrangement of prepreg sheets between layers should be symmetrical. For example, for a six-layer board, the thickness between layers 1 to 2 and 5 to 6 should be consistent with the number of prepreg sheets, otherwise it will easily warp after lamination.

B. Multilayer core boards and prepregs should use products from the same supplier.

C. The line pattern areas on the outer layer A and B should be as close as possible. If side A is a large copper surface and side B only has a few lines, this kind of printed board will easily warp after etching. If the line areas on the two sides are too different, you can add some independent grids on the sparse side for balance.

2. Drying plate before unloading:

The purpose of drying the copper-clad laminate (150 degrees Celsius, 8 ± 2 hours) before unloading is to remove the moisture in the board, and at the same time completely solidify the resin in the board, further eliminating the remaining stress in the board, which is helpful in preventing the board from warping. helpful.

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 exceptions for some board factories. Currently, the drying time regulations of various PCB factories are inconsistent, ranging from 4 to 10 hours. It is recommended to decide based on the grade of the printed boards produced and the customer's requirements for warpage.

Both methods are feasible, whether to cut the pieces into panels and then bake them or to bake them as a whole piece and cut them out. It is recommended to cut the materials and then bake them. The inner layer boards should also be baked.

3. Longitude and latitude direction of prepreg:

The warp and weft shrinkage rates of prepregs after lamination are different, and the warp and weft directions must be distinguished when blanking and laminating. Otherwise, it is easy to cause the finished board to warp after lamination, which is difficult to correct even if pressure is applied to bake the board.

Many reasons for the warping of multi-layer boards are caused by not distinguishing the longitude and weft directions of the prepreg sheets during lamination, and they are stacked randomly.

How to distinguish longitude and latitude? The rolling direction of the rolled prepreg is the warp direction, and the width direction is the weft direction; for copper foil boards, the long side is the weft direction, and the short side is the warp direction. If you are not sure, you can check with the manufacturer or supplier.

4. Stress relief after lamination:

After hot and cold pressing, the multilayer board is taken out, the burrs are cut or milled, and then placed flat in an oven at 150 degrees Celsius for 4 hours to gradually release the stress in the board and completely solidify the resin. This step cannot be omitted.

5. Thin sheets need to be straightened during electroplating:

When using 0.4~0.6mm ultra-thin multi-layer boards for surface electroplating and pattern electroplating, special clamp rollers should be made. After clamping the thin plate on the fly bar of the automatic electroplating line, use a round stick to clamp the entire fly bar. The rollers are strung together to straighten all the boards on the rollers so that the plated boards will not deform.

Without this measure, the sheet will bend after plating a copper layer of 20 to 30 microns, and it will be difficult to repair.

6. Cooling of the board after hot air leveling:

During hot air leveling, the printed board is subjected to the high temperature impact of the soldering bath (about 250 degrees Celsius). After being taken out, it should be placed on a flat marble or steel plate to cool naturally, and then sent to the post-processing machine for cleaning. This is very good for preventing the board from warping.

In order to enhance the brightness of the lead-tin surface, some factories put the board into cold water immediately after being leveled with hot air, and then take it out after a few seconds for post-processing. This kind of hot and cold impact may cause warping of some types of boards. Curved, layered or blistered.

In addition, an air flotation bed can be installed on the equipment for cooling.

7. Treatment of warped boards:

In a well-managed factory, 100% flatness inspection of printed boards will be carried out during final inspection. All unqualified boards will be picked out, placed in an oven, baked at 150 degrees Celsius under heavy pressure for 3 to 6 hours, and allowed to cool naturally under heavy pressure.

Then remove the pressure and take out the board, and then check the flatness. This can save part of the board. Some boards need to be baked and pressed two to three times before they can be flattened. If the above-mentioned anti-warping process measures are not implemented, baking and pressing some boards will be useless and they will have to be scrapped.

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