How to prevent the warpage of PCB circuit boards

- Nov 10, 2017-

1. Why is the PCB board required to be very flat?

    In the automated instrumentation line, if the printed board is not flat, it will cause misalignment, the components can not be inserted into the hole of the board and the surface mount pad, and even the automatic inserter will be damaged. The board on which the components are mounted is bent after soldering, and the component legs are difficult to trim. The board can't be installed in the chassis or the socket inside the machine, so it is very troublesome for the circuit board factory to encounter the board. At present, the printed circuit board has entered the era of surface mounting and chip mounting, and the requirements of the circuit board manufacturers for the pcb circuit board board must be stricter.

2. Standards and test methods for warpage

    According to US IPC-6012 (1996 edition) (identification and performance specifications for rigid printed boards), the maximum warpage and distortion allowed for surface mount printed boards is 0.75%, and the other boards allow 1.5%. This is an increase in the requirement for surface mount printed boards than IPC-RB-276 (1992 version). At present, the warpage degree approved by each electronic assembly factory, regardless of double-sided or multi-layer, 1.6mm thickness, is usually 0.70-0.75%, and many SMT, BGA boards, the requirement is 0.5%. Some electronics factories are encouraging the standard of warpage to 0.3%, and the method of testing warpage is in accordance with GB4677.5-84 or IPC-TM-650.2.4.22B. Place the pcb circuit board on the certified platform, insert the test pin into the place with the highest warpage, and test the diameter of the needle. Dividing the length of the curved edge of the printed board, you can calculate the warp of the printed board. The curvature is gone.

Third, the anti-plate warping during the manufacturing process

1. Engineering design: Note on the design of printed boards:

    A. Multilayer core sheets and prepregs should use the same supplier's products.

    B. The arrangement of the inter-layer prepreg should be symmetrical, for example, a six-layer pcb circuit board, and the thickness between the layers 1 to 2 and 5 to 6 and the number of sheets of the prepreg should be the same, otherwise it is easy to warp after lamination.

    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. Drying before baking:

    The purpose of the copper plate pre-feeding baking plate (150 degrees Celsius, time 8±2 hours) is to remove the moisture in the plate, and at the same time completely cure the resin in the plate, further eliminating the residual stress in the plate, which is to prevent the plate from being warped. Help. At present, many double-sided pcb and multi-layer circuit boards still adhere to the step of before or after baking. However, there are some exceptions for plate mills. At present, the time regulations for baking boards of PCB boards 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. After stress removal after lamination:

    The multi-layer pcb circuit board is taken out after hot pressing and cold pressing, and the burrs are cut or milled, and then baked in the oven at 150 ° C for 4 hours to gradually release the stress in the plate and completely cure the resin. This step is not allowed. Omitted.

4. Straightening when thin plate plating:

    0.6~0.8mm thin-plate multi-layer circuit board should be used 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, the clip on the whole flying bar is used with a round stick. The rollers are strung together 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.

5. 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. Some pcb factories are used to enhance the brightness of the lead-tin surface. After the hot air is leveled, the board is put into the cold water. After a few seconds, it is taken out for post-processing. This kind of hot and cold impact is likely to occur for some types of boards. Warped, layered or foamed. In addition, an air floating bed can be installed on the equipment for cooling.

6. The warp and weft of the prepreg:

    After the prepreg is laminated, the warp and weft shrinkage rates are different, and the warp and weft directions must be separated 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 wound in the warp direction and the width direction is the weft direction; for the copper foil sheet, the long side is in the weft direction and the short side is the warp direction. If it is not certain, it can be inquired to the manufacturer or supplier.

7. Treatment of warped board:

    The well-managed 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. If the above-mentioned anti-warping process measures are not implemented, some of the board drying pressure is useless and can only be scrapped.

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