PCB Tech - Prevent warpage of PCB thick copper plate

Prevent warpage of PCB thick copper plate

1. Why is the PCB thick copper plate required to be very flat?

In 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 is 0.75%, and 1.5% for other boards. At present, the warpage permitted by various electronic assembly factories, regardless of 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 circuit boards:

A. The arrangement of the interlayer prepreg should be symmetrical, such as 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.

B. Multilayer circuit board core board and prepreg should use the same supplier's products.

The area of the circuit pattern of C, 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 plate before cutting:

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 in preventing 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 large material is blanked after baking. It is recommended to bake the panel after cutting. The inner board should also be baked.

3. The latitude and longitude of the prepreg:

After the prepreg is laminated, the warp and weft shrinkage rates are different, and the warp and weft directions must be distinguished when blanking and laminating. Otherwise, it is easy to cause the finished board to warp after lamination, and it is difficult to correct it even if the pressure is applied to the baking board. Many reasons for the warpage of the multilayer board are that the prepregs are not clearly distinguished in the warp and weft directions during lamination, and they are stacked randomly.

How to distinguish the warp and weft directions? The rolled up direction of the prepreg is the warp direction, and the width direction is the weft direction; for copper foil boards, the long side is the weft direction and the short side is the warp direction. Supplier inquiries.

4. Relieve stress after lamination of PCB thick copper plate:

The multi-layer PCB board is taken out after hot pressing and cold pressing, cut or milled off the burrs, and then placed flat 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 .

5. It needs to be straightened during plating:

When the 0.4～0.6mm ultra-thin multilayer board is used for surface electroplating and pattern electroplating, special clamping rollers should be made. After the thin plate is clamped on the flybus on the automatic electroplating line, a round stick is used to clamp the entire flybus. The rollers are strung together to straighten all the plates on the rollers so that the plates after plating will not be deformed. Without this measure, after electroplating a copper layer of 20 to 30 microns, the sheet will bend and it is difficult to remedy it.

6. Cooling of the board after hot air leveling:

PCB thick copper plates are subjected to the high temperature impact of the solder bath (about 250 degrees Celsius) during hot air leveling. After being taken out, 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 good for preventing warpage of the board. In some factories, in order to enhance the brightness of the lead-tin surface, the boards are put into cold water immediately after the hot air is leveled, and then taken out after a few seconds for post-processing. This kind of hot and cold impact may cause warping on certain types of boards. Twisted, layered or blistered. In addition, an air flotation bed can be installed on the equipment for cooling.

7. Treatment of warped board:

In an orderly managed PCB factory, the printed board will be 100% flatness checked during the final inspection. All unqualified boards will be picked out, put in an oven, baked at 150 degrees Celsius under heavy pressure for 3-6 hours, and cooled naturally under heavy pressure. Then relieve the pressure to take out the board, and check the flatness, so that part of the board can be saved, and some boards need to be bake and pressed two or three times before they can be leveled. If the above-mentioned anti-warping process measures are not implemented, some of the boards will be useless and can only be scrapped.