PCB Fabrication

Prevention of circuit board warpage

by:A-TECH      2021-03-20
Circuit board warpage has a great influence on the production of printed circuit boards. Warpage is also one of the important problems in the production process of circuit boards. The board with components is bent after soldering, and the component feet are difficult to be neat. The board cannot be installed on the chassis or the socket inside 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 process requirements for the warpage of the circuit board can be said to be getting higher and higher. Therefore, we have to find the reason for the warpage of the halfway help. In the process of making the circuit board, we must prevent the warpage of the circuit board from the following aspects. 1. Engineering design: Matters needing attention in printed board design: A. 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 the layers It is easy to warp after pressing. B. Multi-layer core board and prepreg should use the same supplier's products. C. The area of u200bu200bthe 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 u200bu200bthe lines on the two sides is too different, you can add some independent grids on the thin side for balance. 2. Drying the board before cutting: The purpose of drying 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. This is helpful to prevent the board from warping. 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 warp and weft direction 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 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 to gradually release the stress in the board and complete the resin Curing, this step cannot be omitted. 5. The thin plate needs to be straightened when electroplating: 0.4~0.6mm ultra-thin multi-layer plates should be made of special nip rollers for surface electroplating and pattern electroplating. After the thin plate is clamped on the fly bus on the automatic electroplating line, a circle is used. The rod strung the nip rollers on the entire fly bus, thereby straightening all the plates on the rollers, so that the plates after plating would 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: The printed board is impacted by the high temperature of the solder bath (about 250 degrees Celsius) when the printed board is leveled by hot air. After taking it out, it should be placed on a flat marble or steel plate for natural cooling, and then sent to the 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 boards: In a well-managed factory, the printed boards will be checked for 100% flatness 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. Shanghai Huabao's pneumatic board warping and straightening machine has been used by Shanghai Bell to have a very good effect in remedying the warpage of the circuit board. If the above-mentioned anti-warping process measures are not implemented, some of the boards will be useless and can only be scrapped.
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