During the SMT mounting process, if there is unevenness in the PCB board, it can cause inaccurate positioning, prevent components from aligning with the holes and surface mounting pads of the PCB board, and even damage the automatic insertion machine. The board on which the components are installed is bent after welding, making it difficult to trim the component feet neatly. This will seriously affect the quality of the entire PCBA board, and even affect the subsequent assembly of the entire machine. So, the assembly plant is also very troubled when encountering board warping. At present, printed boards have entered the era of surface installation and chip installation, and assembly plants are bound to have increasingly strict requirements for board warping.

Standards and testing methods for warpage

According to the American IPC-6012 (1996 edition)<<<Qualification and Performance Specification for Rigid Printed Boards>>, the maximum allowable warpage and distortion for surface mounted printed boards is 0.75%, and other types of boards are allowed to be 1.5%. This increases the requirements for surface mounted printed boards compared to IPC-RB-276 (1992 edition). 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 increase the standard for warpage to 0.3%, and the testing method for warpage should follow GB4677.5-84 or IPC-TM-650.24.22B. Place the printed board on a verified platform and insert the test needle into the area with the highest warpage. Divide the diameter of the test needle by the length of the curved edge of the printed board to calculate the warpage of the printed board.

How to effectively prevent PCB board warping

1. Engineering design: Precautions for printed circuit board design:

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

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

C. The area of the outer A-side and B-side line graphics should be as close as possible. If the A-side is a large copper surface and the B-side only runs a few wires, this type of printed board is prone to warping after etching. If the area difference between the two sides of the line is too large, independent grids can be added on the sparse side for balance.

2. Drying board before cutting:

The purpose of baking the copper clad board before cutting (150 degrees Celsius, time 8 ± 2 hours) is to remove moisture from the board, while fully curing the resin inside the board, further eliminating residual stress in the board, which is helpful in preventing board warping. At present, many double-sided and multi-layer boards still adhere to the step of drying the boards before or after cutting. However, there are also exceptions for some board factories. Currently, the drying time regulations for PCB factories are also inconsistent, ranging from 4 to 10 hours. It is recommended to decide based on the grade of the printed boards produced and the customer's requirements for warpage. Both methods are feasible, whether to cut and dry the board after splicing or to cut and dry the whole block. It is recommended to cut and dry the board after cutting. The inner layer board should also be baked.

3. Longitudinal and latitudinal directions of semi cured sheets:

The longitudinal and latitudinal shrinkage rates of semi cured sheets after lamination are different, 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. Many of the reasons for the warping of multi-layer boards are due to the unclear warp and weft directions of the semi cured sheets during lamination, resulting in disorderly stacking.

How to distinguish between longitude and latitude? The direction of the rolled semi cured sheet is in the meridional direction, while the width direction is in the latitudinal direction; For copper foil, the long side is in the latitude direction, while the short side is in the longitude direction. If unsure, please consult the manufacturer or supplier.

4. Stress relief after lamination:

After completing hot pressing and cold pressing, remove the multi-layer board, cut or mill off the burrs, and then lay it 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.

5. Straightening is required for thin plate electroplating:

Special clamping rollers should be made for electroplating and pattern electroplating of 0.4-0.6mm ultra-thin multi-layer 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 roller, so that the electroplated board will not deform. Without this measure, after electroplating a copper layer of 20 to 30 microns, the thin plate will bend and be difficult to remedy.

6. Cooling of the board after hot air leveling:

During the hot air leveling of printed boards, they are subjected to high temperature impact from a soldering bath (approximately 250 degrees Celsius). After removal, they should be placed on a flat marble or steel plate for natural cooling, and then sent to a post-processing machine for cleaning. This is very beneficial for preventing warping of the board. Some factories, in order to enhance the brightness of the lead tin surface, immediately pour the board into cold water after hot air leveling, and take it out for post-treatment after a few seconds. This hot cold impact may cause warping, layering, or foaming of certain types of boards. In addition, an air floating bed can be installed on the equipment for cooling.

7. Treatment of warped boards:

A factory with orderly management will conduct a 100% flatness inspection of printed boards during final inspection. Any unqualified boards will be picked out and placed in an oven. They will be dried at 150 degrees Celsius and under heavy pressure for 3-6 hours, and cooled naturally under heavy pressure. Then release the pressure and remove the board for flatness inspection, which can save some boards. Some boards require two to three times of drying and pressing to be leveled. The pneumatic board warping and straightening machine represented by Shanghai Huabao has achieved very good results in remedying circuit board warping through the use of Shanghai Bell. If the anti warping measures mentioned above are not implemented, some boards will not be used for drying and pressing, and can only be scrapped.