How to prevent PCB printing plate warping?

1 Why is the circuit board required to be very flat?

In the automatic insertion line, if the printing plate is not smooth, it will cause inaccurate positioning. The components cannot be inserted into the hole and surface of the board, and even the automatic insertion machine will be damaged.It is difficult to trim and trim the feet of the components.The board also can not be installed in the machine box or the socket in the machine, so, the assembly plant to meet the plate warping is also very troublesome.At present, the printing plate has entered the era of surface installation and chip installation, and the assembly plant must be more and more strict.

2 The standard and test method of warping degree.

According to the American ipc-6012 (1996) << rigidity printing plate identification and performance specification >>, the maximum warp and distortion of the surface mounting plate is 0.75%, and the other various boards are allowed 1.5%.This is higher than the ipc-rb-276 (1992) requirement for surface mounting plates.At present, the warping degree of each electronic assembly plant, whether double-sided or multi-layer, 1.6mm thick, usually 0.70 ~ 0.75%, many SMT, BGA board, the requirement is 0.5%.Some electronics factories are agitating to raise the standard of warping to 0.3 per cent, and test warping measures in accordance with gb4677.5-84 or ipc-tm-650.2.4.22b.Put PCB JingJian fixed platform, the test needle to warp degree of the largest local, to test the diameter of the needle, divided by the curve length of PCB, warp degree of the printed circuit board can be calculated.

3 Warping during manufacturing.

1. Engineering design:

A. The arrangement of the semi-solidified film between layers should be symmetrical. For example, the thickness between 1 ~ 2 and 5 ~ 6 layers should be the same as that of the semi-solidified film, otherwise it will be easy to warp after laminating.

B. Multilayer board and semi-cured films shall use the same supplier's products.

C. The surface area of the outer A and B should be as close as possible.If A face is A large copper surface, and B is only A few lines, the plate is easy to warp after etching.If the area of the two sides of the line is too different, it can be balanced by adding some independent grids on the thin side.

2. Baking sheet before feeding:

Sheet blanking before baking plate (150 degrees Celsius, 8 + 2 hours) for the purpose is to remove the moisture inside the board, and make the resin cured within plate, further eliminating residual stress in plate, which is helpful to prevent the plate warping.At present, many double-sided, multilayer boards still adhere to the material before or after the process.But there are also some board factory, now the PCB factory baking plate time rules also inconsistent, ranging from 4 to 10 hours, suggested that the class according to the production of printed circuit board and customer demand for warp degrees to decide.After cutting into a split plate after baking or after the whole material is baked, the two methods are feasible.The inner plate should also be baked.

3. The warp and weft direction of semi-cured films:

It is necessary to distinguish between the meridional and latitudinal direction of the substrates and the laminates.Otherwise, it is easy to cause the finished plate warping after laminating, even if it is difficult to correct the pressure plate.Many of the reasons for the warping of the multilayer plate are that the warp and weft of the laminated semi-cured films are undifferentiated and caused by the overlapping.

How to distinguish between warp and weft?The direction of the rolled semi-cured film is the direction of direction, and the width direction is the zonal direction.In the case of copper foil, long edge time, short edge is the direction, if not, can be determined to the manufacturer or supplier.

4. Stress relief after lamination:

Sandwich plate, after fulfilling the hot-pressing cold-press cut or milling burrs, then flat on baking in the oven 150 degrees Celsius for 4 hours, so that the intraplate stress gradually release and make the resin cured, this step is omitted.

5. The plate needs to be straightened during electroplating:

0.4 ~ 0.6 mm ultrathin multilayer panel plating and electroplating graphics should make special clamping roller, the flying of the automatic electroplating line on the clip on sheet, with a round stick the whole clip roll string on the fly, thus all the boards on straightening roll, so the board will not deformation after plating.Without this measure, the sheet will bend and be difficult to remedy after plating the copper layer of 20 or 30 microns.

6. Cooling of board after hot air leveling:

The hot air of the printing plate is affected by the high temperature of the soldering tin trough (about 250 degrees Celsius), and should be placed on the flat marble or steel plate to cool naturally, and then be cleaned by the post-processing machine.This is good for the warping of the board.Some factory to enhance the brightness of the surface of lead, tin, the board into the cold water, immediately after hot air leveling out after a few seconds in the reprocessing, such a fever a cold shock, for certain types of boards is likely to produce warping, layered or blister.In addition, the equipment can be equipped with a gas floating bed for cooling.

7. Processing of warping boards:

In an orderly factory, the printed board will perform a 100% flatness check when the final inspection is conducted.All unqualified boards will be picked out and placed in the oven for 3 to 6 hours at 150 degrees Celsius and under heavy pressure, and naturally cool under pressure.Then unload the pressure and take out the board, which can be used to check the flatness, so that some boards can be saved, and some boards need two to three times of baking pressure to be levelled.If the anti-warping measures mentioned above are not implemented, some boards are useless and can only be scrapped.