What are the methods to prevent PCB board warpage?

by:A-TECH      2021-03-15
1. Why is the circuit board required to be very flat on the automated assembly line, if the printed board is not flat, it will cause inaccurate positioning, components cannot be inserted into the board holes and surface mount pads, or even crash the automatic Inserting machine. The board on which the components are installed is bent after soldering, 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 on board warping. PCB will introduce you to methods to prevent PCB board warping. 2. Standards and test methods for warpage According to the US IPC-6012 (1996 edition) <>, the maximum allowable warpage and distortion for surface-mounted printed boards is 0.75% , 1.5% is allowed for other various boards. This is higher than IPC-RB-276 (1992 edition) to the requirement of surface mount printed board. 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 when designing printed boards:    A. The arrangement of interlayer prepregs should be symmetrical, such as six-layer boards, the thickness and the thickness between 1~2 and 5~6 layers 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 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. Baking before blanking:    Bake before blanking of copper clad laminate (150 degrees Celsius, time 8±2 hours) The purpose 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 pre- or post-baking. However, there are some exceptions to the board factories. The current PCB drying time regulations of various PCB factories are not consistent, 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. Relieve stress after lamination: take out the multi-layer board after hot pressing and cold pressing, cut or mill off the burrs, and then put it flat in an oven at 150 degrees Celsius for 4 hours to gradually release the stress in the board and make the resin complete Curing, this step cannot be omitted. 5. The thin plate needs to be straightened during electroplating:   0.4~0.6mm ultra-thin multilayer plate should be made of special nip rollers for surface electroplating and pattern electroplating. 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: 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 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 inspected 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 to 6 hours, and cooled naturally under heavy pressure. Then remove the pressure and 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. Here to enter the pcb pricing page https://www./
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