How to prevent PCB board warping and deformation in AMT processing plant?

We all know that the warping of PCB boards has a great impact on the production of printed PCB boards, and warping is one of the important problems in the production process of PCB boards. Bent, component foot is difficult to clean. The PCB board cannot be installed on the socket or socket inside the machine, so the PCB board warping will affect the normal operation of the entire subsequent process.

At present, printed circuit board has entered the era of surface mount and chip mount. AMT mounting process has higher and higher requirements for the warpage degree of PCB circuit board, so it is necessary to find the warpage degree of PCB board in the middle. The reason.

One. Engineering design:

1.Points for attention in PCB design:

(1) The arrangement of prepregs between layers should be symmetrical, such as six-layer PCB, the thickness of 1-2 layers and 5-6 layers should be consistent with the number of prepregs, otherwise the PCB board is easy to warp after lamination;

(2) Core plate and prepreg of multilayer board shall use the same supplier's product;

(3) The outer circuit pattern area of side A and side B should be as close as possible. If side A is large copper and side B has only a few lines, this PCB printed board will easily warp the PCB after etching. If the PCB circuit area difference between the two sides is too large, you can add some separate grid on the thin side to balance.

2. PCB baking board before cutting in AMT patch workshop:

The purpose of pre-baking (150 degrees Celsius for 8±2 hours) is to remove moisture from the PCB, while the resin in the PCB fully solidifies and the stress in the remaining PCB is further removed, thus stopping the PCB. Warping helps a lot.

At present many double-sided, multilayer PCB board still adhere to the step of baking board before or after cutting, but there are also some board factories. The number of hours is recommended to be determined according to the grade of printed board produced and the customer's warping requirements; Cut into a board and bake or bake in one piece. Both methods are feasible. It is recommended to bake the inner plate after cutting the material plate.

3. Warp and weft direction of prepreg:

The meridional and zonal shrinkage of prepreg is different after laminating, so the meridional and zonal shrinkage of prepreg must be distinguished during cutting and laminating. Otherwise, it is easy to cause warping of the finished PCB board after fitting, even if the pressure baking board is difficult to correct; Multilayer PCB warping is caused by the lamination of the warp and weft of prepreg material is not clearly differentiated, disorderly heap. Of.

How can warp and weft be distinguished? The rolling direction of the prepreg is meridional and the width direction is zonal. For copper foil, the long side is zonal and the short side is meridional. Check with the manufacturer or supplier if you are unsure.

4. Stress elimination after fitting:

After hot pressing and cold pressing, PCB multilayer board is removed, cut or ground burrs, and then put in the oven at 150 degrees Celsius for 4 hours, so that the stress in the PCB board is gradually released and the resin is fully released.

5, plate plating needs to be straightened:

0.4 ~ 0.6mm ultra-thin PCB multilayer plate surface plating and graphic plating should use special clamping roller. The clamp rollers are strung together so that all PCB boards on the rollers are straightened so that the PCB board will not be deformed after electroplating.

If this measure is not taken, plating a 20 to 30 micron layer of copper will cause the plate to bend and be difficult to repair.

6. Cooling of PCB board after hot air leveling:

Printed PCB board will be affected by the high temperature of solder groove (about 250 degrees Celsius) in hot air pressure. After removal, it should be placed on the flat marble or steel plate to cool naturally, and then sent to the aftertreatment machine for cleaning. It's good. In order to improve the brightness of the lead tin surface, some PCB factories put the PCB board into cold water immediately after it is levelled with hot air, and take it out after a few seconds for post-treatment. Warping, delamination, or bubbling may occur, and an air float bed may be added for cooling with AMT technology equipment.

7, warped PCB board processing:

For well-managed PCB board manufacturers, printed PCB boards are tested 100% flatness at the time of final inspection. All unqualified PCB boards will be picked out and placed in an oven, baked at 150 degrees Celsius and high pressure for 3 to 6 hours, and cooled naturally under high pressure. Then remove the PCB board, check the flatness, can save part of the PCB board. If the anti-warping process measures of PCB board mentioned above are not in place, some PCB board drying and pressing are useless, AMT chip factory can only scrap this PCB circuit board.