The warpage of PCB long board/strip board has a great influence on the production of printed circuit board, and warpage is also one of the important problems in the production process of PCB long board/strip board. The long board is bent after welding, and the component feet are difficult to align. The PCB long board/strip board cannot be installed on the chassis or the socket in the machine, so the warpage of the circuit board will affect the normal operation of the entire subsequent process. At this stage, the printed circuit board has entered the era of surface mounting and chip mounting, and the requirements for the warpage of the circuit board are getting higher and higher. So we have to find the reason for the warping of the halfway gang.
1. Engineering design: Matters needing attention in the design of PCB long board/strip board: A. The arrangement of the prepreg between layers should be symmetrical, such as a six-layer circuit board, the thickness between 1-2 and 5-6 layers and the number of sheets of prepreg It should be consistent, otherwise it will warp easily after lamination. B. Multilayer circuit board core board and prepreg should use the same supplier's products. 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 circuit board is easy to warp after etching. If the line area on the two sides is too different, you can add some independent grids on the thin side for balance.
2. Baking the board before blanking: The purpose of baking the board before blanking the copper clad laminate (150 degrees Celsius, time 8±2 hours) is to remove the moisture in the board, and at the same time, the resin in the board is completely cured, and the remaining stress in the board is further eliminated. This is helpful in preventing board warping. At present, many double-sided and multi-layer boards still insist on the step of baking the board before or after cutting. However, there are some exceptions for board factories. At present, the time requirements for PCB long board/strip board baking are also inconsistent, ranging from 4 to 10 hours. Decide. Baking after cutting into a jigsaw or drying the whole piece of aniseed before unloading, both methods are feasible, it is recommended to bake the board after cutting. The inner board should also be baked.
3. The warp and weft directions 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 cutting and laminating. Otherwise, it is easy to cause warpage of the finished board after lamination, which is difficult to correct even if the board is baked under pressure. Many of the reasons for the warping of multi-layer circuit boards are that the warp and weft directions of the prepregs are not clearly distinguished during PCB lamination, and they are stacked randomly. How to distinguish warp and weft? The rolling direction of the rolled 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. If you are not sure, you can check with the manufacturer or supplier.
4. Stress relief after lamination: After the multi-layer circuit board is taken out after hot pressing and cold pressing, the burrs are cut or milled, and then placed in an oven at 150 degrees Celsius for 4 hours, so that the stress in the board is gradually released and the resin Complete curing, this step cannot be omitted.
5. It is necessary to straighten the thin plate during electroplating: 0.4~0.6mm ultra-thin multilayer circuit board should be made of special clamping rollers for surface electroplating and graphic electroplating. After clamping the thin plate on the fly bar on the automatic electroplating line, use a The round sticks string together the clamping rollers on the whole flybar, thereby straightening all the PCB circuit boards on the rollers, so that the circuit boards after electroplating will not be deformed. Without this measure, after electroplating a copper layer of 20 to 30 microns, the sheet will be bent, and it is difficult to repair.
6. Cooling of PCB circuit board after hot air leveling: When the printed board is leveled by hot air, it will be impacted by high temperature in the solder bath (about 250 degrees Celsius). for cleaning. This is very good for the anti-warping of the PCB long board / long strip board. In some circuit board factories, in order to enhance the brightness of the lead-tin surface, the circuit board is put into cold water immediately after it is leveled by hot air, and taken out after a few seconds for post-processing. Longboards/slats are likely to warp, delaminate or blister. In addition, an air-floating bed can be installed on the equipment for cooling.
7. Processing of warped PCB long board/strip board: In a factory with orderly management, the printed board will be checked for 100% flatness during the final inspection. All unqualified boards will be picked out, placed in an oven, baked at 150 degrees Celsius and under heavy pressure for 3 to 6 hours, and cooled naturally under heavy pressure. Then remove the pressure and take out the PCB long board/strip board, and check the flatness, which can save part of the board. If the above-mentioned anti-warping process measures are not implemented, some boards will be useless for drying and pressing, and can only be scrapped.
Shenzhen pinban Technology Co., Ltd. is a manufacturer specializing in PCB boards, FPC boards, and membrane switch panels. The main products are: PCB circuit board, FPC flexible circuit board, membrane switch, membrane panel, acrylic panel and other products, the company has a vigorous, professional and experienced technical, production and management team, with advanced printing The special production and testing equipment for boards has a standardized operation system, which makes the internal operation more accurate and efficient, and the external service is more systematic and timely, creating greater value for customers.
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