Electronic Design - Beware of warping of PCB printed boards

If the PCB printed board is not flat on the automatic assembly line, it will cause inaccurate positioning, the components cannot be inserted into the holes and surface mount pads of the board, and the automatic insertion machine may even be damaged. The board with the components is bent after welding, and the component feet are difficult to cut neatly. The board cannot be installed on the chassis or the socket inside the machine, so the assembly plant is also very annoyed when the PCB board is warped.

2. Standards and test methods for warpage

The maximum allowable warpage and distortion for surface-mounted printed boards is 0.75%, and 1.5% for other boards. This has improved the requirements for surface-mounted printed boards than IPC-RB-276 (1992 edition). At present, the warpage permitted by various electronic assembly plants, regardless of double-sided or multi-layer, 1.6mm thickness, is usually 0.70-0.75%. For many SMT and BGA boards, the requirement is 0.5%. Some electronics factories are urging to increase the standard of warpage to 0.3%, and the method of testing warpage is in accordance with GB4677.5-84 or IPC-TM-650.2.4.22B. 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

1. Engineering design: Matters needing attention in PCB design:

A. The arrangement of the interlayer prepreg should be symmetrical, for example, for a six-layer 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 core board and prepreg should use the same supplier's products.

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 completely solidify, and further eliminate the remaining stress in the board, which is useful for preventing the board from warping. Helping. At present, many double-sided and multi-layer boards still adhere to the step of baking before or after the blanking. However, there are exceptions for some board 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, and 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 check with the manufacturer or supplier.

4. Stress relief after lamination:

The PCB multilayer 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:

0.4～0.6mm ultra-thin multi-layer boards are used for surface plating and pattern plating. 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:

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 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 for post-processing after a few seconds. 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 to three times before they can be leveled. The pneumatic board warping and straightening machine represented by Shanghai Huabao has a very good effect in remedying PCB warpage through the use of Shanghai Bell.