1. Why is the circuit board required to be very flat?
In the automatic insertion line, if the printed circuit board is not flat, it will cause inaccuracy, the components cannot be inserted into the holes of the board and the surface mount pads, and even the automatic insertion machine will be damaged. The board on which the components are mounted is bent after welding, and it is difficult to cut the component feet neatly. The board also cannot be installed on the chassis or the socket in the machine, so it is also very troublesome for the circuit board factory to encounter board warping. At present, printed circuit boards have entered the era of surface mounting and chip mounting, and the requirements of circuit board manufacturers on board warpage must be more and more strict.
2. Warpage standard and test method
According to the US IPC-6012 (1996 edition) (Identification and Performance Specification for Rigid Printed Wiring Boards), the maximum allowable warpage and twist for surface-mounted printed circuit boards is 0.75%, and 1.5% for other boards. This raises the requirements for surface mount printed circuit boards than IPC-RB-276 (1992 edition). At present, the warpage permitted by each electronic assembly factory, regardless of the double-sided circuit board or the thickness of 1.6mm, is usually 0.70 to 0.75%, and many SMT and BGA boards are required to be 0.5%. Some electronics factories are agitating to raise 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. Put the printed circuit board on the certified platform, insert the test pin into the place with the largest warpage, and divide the diameter of the test pin by the length of the curved edge of the printed circuit board to calculate the warpage. degree.
3. Anti-board warping during manufacturing
a. Engineering design: Matters needing attention when designing printed circuit boards:
(1). The core board and prepreg of the multi-layer circuit board should use the same supplier's products.
(2). The arrangement of the interlayer prepreg should be symmetrical, for example, for a six-layer board, the thickness between layers 1-2 and 5-6 and the number of sheets of the prepreg should be the same, otherwise it is easy to warp after lamination.
(3). The area of the circuit pattern on the A side and the B side of the outer layer should be as close as possible. If the A side is a large copper surface, and the B side only has a few lines, the printed board will be easily warped after etching. If the line area on the two sides is too different, you can add some independent grids on the thin side for balance.
b. Baking board before cutting:
The purpose of baking the board (150 degrees Celsius, time 8±2 hours) before blanking the copper clad laminate is to remove the moisture in the board, and at the same time, the resin in the board is completely cured, and the remaining stress in the board is further eliminated, which is useful for preventing board warping. helpful. At present, many double-sided circuit boards and multi-layer circuit boards still insist on the step of baking the board before or after cutting. However, there are some exceptions for board factories. At present, the baking time regulations of various PCB factories are also inconsistent, ranging from 4 to 10 hours. It is recommended to decide according to the grade of the printed board produced and the customer's requirements for warpage. Baking after cutting into a jigsaw or drying the whole piece of aniseed before unloading, both methods are feasible, it is recommended to bake the board after cutting. The inner board should also be baked.
c. The warp and weft directions of the prepreg:
The shrinkage rate of the prepreg in the warp direction and the weft direction after lamination is different, and the warp direction and the weft direction must be distinguished when cutting and laminating. Otherwise, it is easy to cause warpage of the finished board after lamination, which is difficult to correct even if the board is baked under pressure. Many of the reasons for the warpage of the multi-layer board are that the warp and weft directions of the prepreg are not clearly distinguished during lamination, and they are stacked randomly.
How to distinguish warp and weft? The rolling direction of the rolled prepreg is 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. If you are not sure, you can check with the PCB manufacturer or supplier.
d. Stress removal after lamination:
The multilayer circuit board is taken out after hot pressing and cold pressing, cut or milled off the burrs, and then placed in an oven at 150 degrees Celsius for 4 hours, so that the stress in the board is gradually released and the resin is completely cured. This step cannot be omitted.
e. The sheet needs to be straightened when electroplating:
0.6～0.8mm thin-plate multilayer circuit board should be made of special clamping rollers for surface electroplating and pattern electroplating. After clamping the thin plate on the flybar on the automatic electroplating line, use a round stick to clamp the entire flybar. The rolls are strung together, thereby straightening all the boards on the rolls so that the plated boards will not be deformed. Without this measure, after electroplating a copper layer of 20 to 30 microns, the sheet will be bent, and it is difficult to repair.
f. Cooling of the board after hot air leveling:
When the printed board is flattened by hot air, it is impacted by the high temperature of the solder bath (about 250 degrees Celsius). After taking it 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 the board to prevent warping. In some PCB factories, in order to enhance the brightness of the lead and tin surface, the board is immediately put into cold water after being leveled by hot air, and taken out after a few seconds for post-processing. Warped, delaminated or blistered. In addition, an air-floating bed can be installed on the equipment for cooling.
g. Treatment of warped boards:
In a well-managed PCB factory, the printed circuit board will be checked for 100% flatness during final inspection. All unqualified boards will be picked out, placed in an oven, baked at 150 degrees Celsius and under heavy pressure for 3 to 6 hours, and cooled naturally under heavy pressure. Then remove the pressure and take out the board, and check the flatness, which can save part of the board. PCSPCB has a very good effect in remedying the warpage of circuit boards. If the above-mentioned anti-warping process measures are not implemented, some circuit boards will be useless and can only be scrapped.
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