How to prevent the bow and twist of PCB?

                                                                       How to prevent the bow and twist of PCB ?

    Serious bow and twist of PCB will cause inaccurate positioning of components. When SMT and THT are done, the components pins are not tidy, which will bring great difficulties to assembly and installation, and the PCB after assembly has great potential quality problems.

    The IPC-6012 standard specifies that the maximum bow and twist of the PCB with SMT is ≤0.75%. Some PCBs with higher requirements such as those with BGA design, required ≤0.5% even ≤0.3% , and the bow and twist of PCB without SMT is ≤1.5%. Calculation method of PCB warpage = warpage height / curved edge length.

Methods to prevent PCB warpage:

1. Engineering design: 

A. The inter-layer prepreg arrangement should be correspond, that is, the model and thickness of the PP should be symmetrical. 

B. The core and prepreg of multi-layer PCB should use the same supplier. 

C. The area of the copper foil is as close as possible or equivalent，for example: 8 layers PCB    L1 = L8, L2 = L7, L3 = L6, L4 = L5

2. Drying the material before cutting:

Generally drying the material before cutting is 150 degrees for 6 - 10 hours, eliminate moisture inside the material, further making the resin cured completely and eliminating the internal stress of the material.

3. Pay attention to the warp and weft directions of PP and CCL before cutting and lamination:

The shrinkage ratios in the warp and weft directions are different. PP and CCL should pay attention to keep the warp and weft in the same direction when cutting and lamination. Generally, the direction of the PP coil is the warp direction, and the long direction of the CCL is the warp direction.

4, Stress relief after lamination：

After lamination, cold pressing and trimming the burrs;

5. Drying the material before drilling: 150 degrees for 4 hours;

6. Thin PCB is best not mechanically brushed, it is recommended to use chemical cleaning. Special fixtures should be used during plating to prevent PCB bending and folding.

7. After HASL or LF-HASL surface finished, it is naturally cooled to room temperature on a smooth marble or steel plate, or cleaned after cooling on an air floating bed;