PCB Technical - How to prevent warping of multilayer circuit boards

How to prevent warping of multilayer circuit boards

1. Why is the multilayer circuit board required to be very flat?

In the automatic assembly line, if the printed multilayer circuit board is not flat, it will cause inaccuracies, components cannot be inserted into the holes and surface mount pads of the board, and even the automatic insertion machine will be damaged. The multi-layer circuit board with the components is bent after soldering, and the component feet are difficult to cut neatly. The multi-layer circuit board cannot be installed on the chassis or the socket in the machine, so it is also very annoying for the assembly plant to encounter the board warping. At present, printed 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 the US IPC-6012 (Edition 1996) (Identification and Performance Specification for Rigid Printed Circuit Boards), the maximum allowable warpage and distortion for surface-mounted printed circuit boards is 0.75%, and other boards allow 1.5%. This is higher than IPC-RB-276 (1992 edition) to the requirement of surface mount printed circuit board. At present, the warpage permitted by various electronic assembly plants, whether double-sided circuit boards or multilayer circuit boards, is 1.6mm thick, usually 0.70-0.75%, and for many SMT and BGA boards, the requirement is 0.5%. Some electronic factories are urging to increase the standard of warpage to 0.3%, and the method of testing warpage complies with GB4677.5-84 or IPC-TM-650.2.4.22B. Put the printed circuit board on the verified platform, insert the test pin to the place with the greatest degree of warpage, and divide the diameter of the test pin by the length of the curved edge of the printed circuit board to calculate the printed circuit The warpage of the board has gone.

3. Anti-board warping during the manufacturing process

1. Engineering design: Matters needing attention when designing printed boards:
A.Multilayer circuit board core board and prepreg should use the same supplier's products.
B. The arrangement of interlayer prepregs should be symmetrical, for example, for six-layer boards, 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.
C. The area of the circuit pattern on side A and side B 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 baking the board before cutting the copper clad laminate (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 of the multilayer circuit board factory completely solidify, and further eliminate the remaining stress in the board. It is helpful to prevent the board from warping. At present, many double-sided circuit boards and multilayer circuit boards still adhere to the step of baking before or after blanking. However, there are exceptions to some plate factories. The current PCB drying time regulations of various PCB factories are also inconsistent, ranging from 4 to 10 hours. It is recommended to decide according to the grade of the printed board produced and the customer's requirements for warpage. Bake after cutting into a jigsaw or blanking after the whole block is baked. Both methods are feasible. It is recommended to bake the board 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 during blanking and lamination. 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 distinguished in the warp and weft directions during lamination, and they are stacked randomly.

How to distinguish the latitude and longitude? The rolling direction of the rolled prepreg is the warp direction, while the width direction is the weft direction; for the copper foil board, the long side is the weft direction, and the short side is the warp direction. If you are not sure, you can ask the multi-layer PCB manufacturer or supplier. Inquire.

4. Stress relief after lamination:
The multi-layer circuit 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. The thin plate needs to be straightened during electroplating:
Special nip rollers should be made when the 0.6～0.8mm thin multi-layer circuit board is used for surface electroplating and pattern electroplating. After the thin plate is clamped on the flybus on the automatic electroplating line, use a round rod 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:
When the printed board is leveled by hot air, it is impacted by the high temperature of the solder bath (about 250 degrees Celsius). After being taken out, it 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 good for preventing warpage of the board. In some multi-layer PCB factories, in order to enhance the brightness of the lead-tin surface, the multi-layer circuit boards are immediately put into cold water after being leveled by hot air, and taken out after a few seconds for post-processing. This kind of hot and cold impact will affect some The board of the model is likely to warp, delamination or blistering. In addition, an air flotation bed can be installed on the equipment for cooling.

7. Treatment of warped board:
In an orderly managed multi-layer PCB factory, the printed circuit boards 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 circuit boards need to be bake and pressed two to three times before they can be leveled.