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How to prevent PCB warpage
1. Why the circuit board is required to be very flat
In the automated assembly line, if the printed 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. The 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 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 (1996 edition) <
3. Anti-board warping during the manufacturing process
1. Engineering design: Matters needing attention when designing printed boards:
A. Multilayer core board and prepreg should use the same supplier's products.
B. 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.
C. The area of the circuit pattern on the A side and the 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 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 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. After cutting into a piece of panel, bake it or unload the whole block after baking. Both methods are feasible. 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 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 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 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:
The well-managed PCB factory, the printed board will be 100% flatness inspection 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.
