Prevention of warpage deformation of PCB

- Aug 01, 2018-

The influence of circuit board warpage on the production of printed circuit boards is very large. Warpage is also one of the important problems in the manufacturing process of circuit boards. The boards on which components are mounted are bent after soldering, and the components are difficult to tidy. The board can't be installed in the chassis or the socket inside the machine, so the board warpage will affect the normal operation of the entire subsequent process. At this stage, the printed circuit board has entered the era of surface mounting and chip mounting, and the process requirements for circuit board warpage are getting higher and higher. So we have to find the reason for the halfway to help warp.

1. Engineering design: Precautions for printed board design: A. The arrangement of inter-layer prepreg should be symmetrical, for example, six-layer board, the thickness between 1~2 and 5~6 layers and the number of prepregs should be the same, otherwise the layer It is easy to warp after pressing. B. Multilayer core sheets and prepregs should use the same supplier's products. C. The area of the line pattern of the A and B sides of the outer layer should be as close as possible. If the A side is a large copper surface and the B side is only a few lines, the printed board is easily warped after etching. If the line areas on both sides are too different, add a separate grid on the thin side to balance.

2. Pre-feeding baking sheet: The copper-clad board pre-feeding baking sheet (150 degrees Celsius, time 8±2 hours) aims to remove the moisture in the board and at the same time completely cure the resin in the board to further eliminate the residual stress in the board. This is helpful in preventing plate warpage. At present, many double-sided, multi-layer boards still adhere to the step of before or after baking. However, there are some exceptions for sheet metal. At present, the time regulations for baking sheets of PCB factories are also inconsistent, from 4-10 hours. It is recommended to decide according to the grade of the printed board produced by the customer and the customer's requirements for warpage. After cutting into a jigsaw and drying it, the whole piece of material is baked and then unloaded. Both methods are feasible. It is recommended to dry the plate after cutting. The inner panel should also be baked.

3. Warp and weft of prepreg: The warp and weft shrinkage rates of the prepreg are different after lamination, and the warp and weft directions must be distinguished when cutting and laminating. Otherwise, it is easy to cause warpage of the finished board after lamination, even if it is pressed, it is difficult to correct. The reason for the warpage of the multi-layer board is that the latitude and longitude of the prepreg is not separated when it is laminated, and it is caused by the disorder. How to distinguish the warp and latitude? The rolled prepreg is rolled in the warp direction and the width direction is the weft direction; for the copper foil board, the long side is the weft direction, and the short side is the warp direction.

4. After stress removal after lamination: the multi-layer board is taken out after hot pressing and cold pressing, cut or milled, and then baked in an oven at 150 ° C for 4 hours to gradually release the stress in the board and complete the resin. Curing, this step cannot be omitted.

5. Straightening of thin plate requires straightening: 0.4~0.6mm ultra-thin multi-layer board for plate surface plating and pattern plating. Special nip rolls should be made. After the thin plate is clamped on the flying plate of the automatic plating line, use a circle. The sticks string the nip rolls on the entire flying bar to straighten all the plates on the roll so that the plate after plating does not deform. Without this measure, after plating a copper layer of twenty or thirty microns, the sheet would bend and be difficult to remedy.

6. Cooling of the board after hot air leveling: When the hot plate of the printed board is leveled, it will be impacted by the high temperature of the solder bath (about 250 degrees Celsius). After being taken out, it should be placed on the flat marble or steel plate for natural cooling, and sent to the post-processing machine for cleaning. . This is good for the board to prevent warpage. In some factories, in order to enhance the brightness of the lead-tin surface, the hot air of the board is put into the cold water immediately after being leveled. After a few seconds, it is taken out for post-treatment. This kind of hot and cold impact is likely to cause warping on some types of boards. Curved, layered or blistering. In addition, an air floating bed can be installed on the equipment for cooling.

7. Treatment of warped plates: Manage the orderly factory, the printed board will be 100% flatness inspection during the final inspection. Any unqualified boards will be picked out, placed in an oven, baked at 150 ° C and under heavy pressure for 3 to 6 hours, and naturally cooled under heavy pressure. Then, the plate is taken out by pressure relief, and the flatness is checked. This saves some of the boards, and some of the boards need to be pressed two to three times to level.

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