Effective Ways to Prevent PCB Thick Copper Plate from Warping (Part One)

1. Why does the PCB thick copper need to be flat? 　　

On the automated assembly line, if the printed circuit board is not flat, it will cause inaccurate positioning, components cannot be inserted into the holes and surface mount pads of the board, and even the automatic insertion machine will be damaged. The board with the components is bent after welding, and the component feet are difficult to cut neatly. If this is the case, the PCB board cannot be installed on the chassis or the socket inside the machine, so it is also very annoying for the assembly plant to encounter the board warping. At present, printed circuit boards have entered the era of surface mounting and chip mounting, and assembly plants must have stricter and stricter requirements for board warping.

2. Standards and test methods for warpage

According to (Appraisal and Performance Specification for Rigid Printed Boards), the maximum allowable warpage and distortion for surface-mounted printed boards are 0.75% and 1.5% for other boards. At present, the warpage permitted by various electronic assembly factories, regardless of a double-sided circuit board or multilayer circuit board, PCB thick copper plate, 1.6mm thickness, usually 0.70～0.75%, and for many SMT and BGA boards, the requirement is 0.5%. Some electronics factories are advocating to raise the standard of warpage to 0.3%. Put the printed board on the verified platform, insert the test pin to the place where the degree of warpage is the greatest, and divide the diameter of the test pin by the length of the curved edge of the printed board to calculate the warpage of the printed board. The curvature is gone.

3. Anti-board warping during the manufacturing process of PCB thick copper plate

1.) Engineering design:  Matters needing attention when designing printed boards:

• The arrangement of the interlayer prepreg should be symmetrical, such as a six-layer PCB board, the thickness between 1-2 and 5-6 layers, and the number of prepregs should be the same, otherwise, it is easy to warp after lamination.
• Multilayer circuit board core board and prepreg should use the same supplier’s products.
• The area of ​​the circuit pattern of the A side and 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, this kind of printed board will easily warp after etching. If the area of ​​the lines on the two sides is too different, you can add some independent grids on the thin side for balance.

2.) Baking board before blanking:

The purpose of drying the board before cutting the PCB thick copper plate (150 degrees Celsius, time 8±2 hours) is to remove the moisture in the board and at the same time make the resin in the board completely solidify, and further eliminate the remaining stress in the board, which is effective to prevent the board from warping. helpful. At present, many double-sided, multi-layer PCBs still adhere to the step of baking before or after blanking. However, there are exceptions for some plate factories. The current PCB drying time regulations are also inconsistent, ranging from 4-10 hours. It is recommended to decide according to the grade of the printed board produced and the customer’s requirements for warpage. The two methods are both feasible after cutting into a piece of panel and then baking or the whole piece of bulk material is blanked after baking. It is recommended to bake the panel after cutting. The inner board should also be baked.