In the automatic cartridge line, if the printed circuit board is not smooth, it will lead to inaccurate positioning, components can not be inserted into the holes of the board and surface mounting pad, or even crash the automatic cartridge machine. It is difficult to trim the components of the components when they are bent after welding. The boards can not be loaded into the chassis or the sockets in the machine, so the assembly plant is also very troubled by the plate warping. At present, PCB has entered the era of surface mounting and chip mounting, assembly plants must be increasingly stringent requirements for board warping.
二.Standard and test method of warpage
According to IPC-6012 (1996 edition) "Specification for the Identification and Performance of Rigid PCB". The maximum allowable warpage and distortion for surface mounting of PCB are 0.75% and 1.5% for other kinds of PCB. This improves the requirements for surface mounting PCB than the PCB - R - 27 (1992 edition). At present, the warpage allowed by various electronic assembly plants, whether double-sided or multi-layer, 1.6mm thickness, usually 0.70-0.75%, many SMT, BGA board, the requirement is 0.5%.Some electronics factories are urging the warpage standard to be raised to 0.3%, and the method of testing warpage follows GB4677.5-84 or IPC-TM-6126.96.36.199B. Put the PCB on the verified platform, insert the test needle into the place where the warpage is greatest, to test the diameter of the needle, divided by the length of the curved edge of the PCB, you can calculate the warpage of the PCB.
三、Warpage prevention in manufacturing process
1、Engineering design: matters needing attention in PCB Design:
A.The arrangement of interlaminar semi-solidified sheets should be symmetrical, such as six-layer sheets, and the thickness of 1-2 and 5-6 layers should be the same as the number of sheets of semi-solidified sheets, otherwise the sheets will warp easily after lamination.
B.Multilayer board and prepreg shall use the same supplier's products.
C. The line area of the outer A plane and the B plane should be as close as possible. If the A surface is large copper surface and the B surface only takes a few lines, the printed board will easily warp after etching. If the area difference between the two lines is too large, a few independent grids can be added on the thin side to balance.
2、Drying before blanking:
The purpose of drying the sheet (150 degrees Celsius, 8 +2 hours) before cutting is to remove moisture from the sheet and to completely solidify the resin in the sheet, thus further eliminating the residual stress in the sheet, which is helpful to prevent the sheet from warping. At present, many double-sided and multi-storey boards still insist on the step before or after drying. However, there are some exceptions in some PCB factories. At present, the drying time of PCB factories is not the same, from 4 to 10 hours. It is suggested that the drying time should be decided according to the grade of printed boards produced and the warpage requirements of customers. It is feasible to cut it into pieces or bake it after the whole material is dried. The two methods are feasible. The inner layer should also be baked.
3、The warp and weft direction of prepreg:
The warp and weft shrinkage of semi-cured laminates are different after lamination. The warp and weft must be distinguished when blanking and laminating. Otherwise, after laminating, it is easy to warp the finished products, even if the pressure is dried, it is hard to correct. Many of the reasons for the warpage of multi-layer laminates are caused by the indistinguishable warp and weft directions of semi-cured laminates.
How to distinguish warp and weft? The rolling direction of the rolled semi-cured sheet is longitude, while the width direction is latitude. For copper foil, the long side is longitude and the short side is longitude. If you are not sure, you can consult the manufacturer or supplier.
After hot-pressing and cold-pressing, the laminates are taken out, cut or milled off, and then laid flat in the oven at 150 degrees Celsius for 4 hours to gradually release the stress in the laminates and completely solidify the resin. This step can not be omitted.
5、Thin plates need to be straightened when plating.
Special clamping rollers should be made when 0.4-0.6 mm ultra-thin multi-layer plate is used for plate surface electroplating and graphic electroplating. After clamping the sheet on the flying bar on the automatic electroplating line, the clamping rollers on the whole flying bar are strung together with a round stick so as to straighten all the plates on the rollers so that the plate after electroplating will not deform. Without this measure, after plating a twenty or thirty micron copper layer, the sheet will be bent and difficult to remedy.
6、Cooling after hot air leveling:
PCB hot air leveling through the solder trough (about 250 degrees Celsius) high temperature impact, after removal should be placed on a flat marble or steel plate natural cooling, sent to the after-treatment machine for cleaning. This is very good for preventing warpage. Some factories to enhance the brightness of the lead and tin surface, the board immediately into cold water after hot air leveling, a few seconds after removal in the post-processing, such a hot and cold impact, on certain types of board is likely to produce warping, stratification or foaming. In addition, air floatation beds can be installed on the equipment for cooling.
In order to manage the factory, PCB will make 100% evenness check when final inspection. All unqualified boards will be picked out, placed in the oven, baked at 150 degrees Celsius and under heavy pressure for 3 to 6 hours, and natural cooling under heavy pressure. Then take the board out of the pressure, in order to make a flatness check, so that part of the board can be saved, and some boards need to do two to three times before the drying pressure can be leveled. The pneumatic plate warping and straightening machine of Shanghai Huabao agency has been used by Shanghai Bell to remedy the warping of PCB. If the above warpage prevention measures are not implemented, some boards will not be used for baking and pressing.
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