One Why is PCB board required to be very flat

On the automated insertion line, if the printed board is not flat, it can cause inaccurate positioning, prevent components from being inserted into the holes and surface mounting pads of the board, and even damage the automatic insertion machine. The board on which the components are installed bends after welding, making it difficult to cut the component legs flat and neat. The board cannot be installed on the chassis or socket inside 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 mount and chip mount, and circuit board manufacturers are bound to have increasingly strict requirements for PCB board warping.  

II Standards and testing methods for warpage

According to the US IPC-6012 (1996 edition) (Identification and Performance Specification for Rigid Printed Boards), the maximum allowable warpage and distortion for surface mounted printed boards is 0.75%, while for other types of boards it is 1.5%. This has raised the requirements for surface mount printed boards compared to IPC-RB-276 (1992 version). At present, the allowable warpage for electronic assembly plants, whether double-sided or multi-layer, with a thickness of 1.6mm, is usually 0.70-0.75%. For many SMT and BGA boards, the requirement is 0.5%. Some electronic factories are advocating to raise the standard for warpage to 0.3%, and the testing method for warpage follows GB4677.5-84 or IPC-TM-650.2.4.22B. Place the PCB circuit board on the verified platform, insert the test needle into the area with the highest warpage, and divide the diameter of the test needle by the length of the curved edge of the PCB to calculate the warpage of the PCB.

3、 Preventing board warping during the manufacturing process

1. Engineering design: Precautions to be taken when designing printed circuit boards:

A. Multi layer core boards and semi cured sheets should use products from the same supplier.  

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

C. The area of the circuit pattern on the outer A and B sides should be as close as possible. If the A side is a large copper surface and the B side only has a few wires, this printed circuit board is easily warped after etching. If the area difference between the two sides of the line is too large, some independent grids can be added to the sparse side for balance.  

2. Drying the board before cutting:

The purpose of baking the copper-clad board before cutting (150 degrees Celsius, time 8 ± 2 hours) is to remove the moisture inside the board and completely cure the resin inside the board, further eliminating the remaining stress in the board, which is helpful in preventing board warping. At present, many double-sided PCBs and multi-layer circuit boards still adhere to the step of baking the board before or after cutting. But there are also exceptions for some board factories. Currently, the drying time for PCB boards varies among different PCB factories, ranging from 4-10 hours. It is recommended to decide based on the grade of printed boards produced and the customer's requirements for warpage. Whether to cut and dry the assembled board or to dry the whole piece of material before cutting, both methods are feasible. It is recommended to cut and dry the board. The inner layer board should also be baked.  

3. Stress relief after lamination:

After completing hot pressing and cold pressing, the multi-layer PCB circuit board is taken out, trimmed or milled off, and then placed flat in an oven at 150 degrees Celsius for 4 hours to gradually release the stress inside the board and fully cure the resin. This step cannot be omitted.

4. When electroplating thin plates, straightening is required:

Special clamping rollers should be made for surface electroplating and graphic electroplating of 0.6-0.8mm thin multi-layer circuit boards. After clamping the thin plate on the flying bar on the automatic electroplating line, a round rod should be used to string the clamping rollers on the entire flying bar, thereby straightening all the boards on the rollers. This way, the electroplated boards will not deform. Without this measure, after electroplating a copper layer of 20-30 microns, the thin plate will bend and be difficult to remedy.

5. Cooling of the board after hot air leveling:

When the printed circuit board is leveled by hot air, it is subjected to high temperature impact from the soldering tank (about 250 degrees Celsius). After removal, it should be placed on a flat marble or steel plate for natural cooling, and then sent to the post-processing machine for cleaning. This is very beneficial for preventing warping of the board. Some PCB factories, in order to enhance the brightness of the lead tin surface, immediately put the board into cold water after hot air leveling, and take it out for post-processing after a few seconds. This one hot and one cold impact may cause warping, delamination, or blistering on certain types of boards. In addition, an air flotation bed can be installed on the equipment for cooling.  

6. Longitudinal and latitudinal directions of the semi cured sheet:

The shrinkage rates in the warp and weft directions are different after lamination of semi cured films, and it is necessary to distinguish between them during cutting and lamination. Otherwise, it is easy to cause warping of the finished board after lamination, and even if pressure is applied to dry the board, it is difficult to correct it. The reason for the warping of multi-layer boards is often due to the improper differentiation of the warp and weft directions of the semi cured sheets during lamination, resulting in disorderly stacking.  

How to distinguish longitude and latitude? The direction in which the semi-finished sheet is rolled up is in the warp direction, while the width direction is in the weft direction; For copper foil board, the long side is in the warp direction, and the short side is in the weft direction. If you cannot determine, you can inquire with the manufacturer or supplier.  

7. Treatment of warped boards:

A well managed factory will conduct a 100% flatness check on printed boards during final inspection. Any unqualified boards will be picked out and placed in an oven. They will be baked at 150 degrees Celsius and under heavy pressure for 3-6 hours, and then naturally cooled under heavy pressure. Then release the pressure and take out the board for flatness inspection, which can save some boards. Some boards need to be baked and pressed two to three times before leveling. If the anti warping process measures mentioned above are not implemented, some boards cannot be baked or pressed, and can only be scrapped.