1. Why is the circuit board required to be very flat
In the automatic insertion line, if the printed board is not smooth, it will cause inaccurate positioning, and the components can not be inserted into the holes of the board and the surface mount pad, and even break the automatic insertion machine. The board on which the components are installed is bent after welding, and the component feet are difficult to cut neatly. The board can not be installed in the chassis or the socket in the machine, so the assembly plant encountered the board is also very troublesome. At present, printed boards have entered the era of surface installation and chip installation, and the requirements of assembly plants for board warping must be more and more stringent.
2. Standard and test method of warpage degree
According to the United States IPC-6012 (1996 edition) (Identification and Performance Specification for Rigid Printed boards), the maximum allowable warping and distortion for surface-mounted printed boards is 0.75%, and 1.5% for various other boards. This increases the requirements for surface-mounted printed boards over IPC-RB-276 (1992 version). At present, the warping degree of each electronic assembly plant is permitted, regardless of double-sided or multi-layer, 1.6mm thickness, usually 0.70 ~ 0.75%, and many SMT and BGA boards require 0.5%. Some electronics factories are pushing to raise the warpage standard to 0.3%, and the warpage test method is GB46677.5-84 or IPC-TM-650.2.4.22B. Put the printed board on the checked platform, insert the test needle to the place with the greatest warpage, divide the diameter of the test needle by the length of the curved edge of the printed board, and you can calculate the warpage of the printed board.
3. anti-plate warping in the manufacturing process
3.1. engineering design: printed board design should be noted:
a. The arrangement of interlayer semi-cured sheets should be symmetrical, such as six-layer plates, and the thickness between 1 ~ 2 and 5 ~ 6 layers and the number of semi-cured sheets should be consistent, otherwise it is easy to warp after lamination.
b. Multi-layer core board and semi-cured sheet should be made of the same supplier.
c. The line graphic area of the outer A side and B side should be as close as possible. If the A side is a large copper side, and the B side is only a few lines, the printed board is easy to warp after etching. If the line area difference between the two sides is too large, some independent grids can be added on the thin side to balance.
3.2. Drying plate before cutting:
The purpose of drying the copper clad plate before cutting the material (150 degrees Celsius, time 8±2 hours) is to remove the moisture in the plate, while the resin in the plate is completely cured, further eliminating the remaining stress in the plate, which is helpful to prevent the plate warping. At present, many double-sided, multi-layer boards still adhere to the step of drying the board before or after loading. However, there are some board factory exceptions, the current PCB factory drying board time provisions are also inconsistent, from 4-10 hours, it is recommended to decide according to the production of printed board grade and customer requirements for warpage degree. Cut into pieces after drying or whole aniseed after drying, the two methods are feasible, it is recommended to cut the material after drying board. The inner board should also be baked.
3.3. Warp and weft direction of semi-cured film:
The warp and weft shrinkage of the semi-cured sheet after laminating are different, and the warp and weft must be distinguished when cutting and laminating. Otherwise, it is easy to cause warping of the finished plate after lamination, and it is difficult to correct even if the pressure is added to the drying plate. Many of the reasons for the warping of multilayer plates are caused by the confusion of the warp and weft of the semi-cured sheet during lamination.
How to distinguish longitude and latitude? The direction of the rolled semi-cured sheet is meridional, and the width direction is zonal. For copper foil plate, the long side is weft direction, and the short side is warp direction. If you are not sure, you can inquire with the manufacturer or supplier.
3.4. Stress removal after lamination:
The multi-layer plate is taken out after hot pressing and cold pressing, cut or mill off the raw edge, and then placed flat in the oven at 150 degrees Celsius for 4 hours, so that the stress in the plate is gradually released and the resin is completely cured, this step cannot be omitted.
3.5. the plate needs to be straightened:
0.6 ~ 0.8mm thin plate multi-layer circuit board for plate plating and graphic plating should be made of special clamping roller, in the automatic plating line of the flying bar on the plate, with a round stick to the whole flying bar of the clamping roller string, so as to straighten all the boards on the roller, so that the plate will not be deformed after plating. Without this measure, after electroplating a copper layer of 20 or 30 microns, the sheet will bend and be difficult to remedy.
3.6. Cooling of the board after hot air leveling:
The hot air of the printed board is usually affected by the high temperature of the solder tank (about 250 degrees Celsius), and it should be placed on the flat marble or steel plate to cool naturally after it is sent to the processor for cleaning. It's good for warping the board. Some factories in order to enhance the brightness of the lead tin surface, the board immediately after the hot air leveling into cold water, a few seconds later out for post-processing, this hot and cold impact, some models of the board is likely to produce warping, layering or foaming. In addition, the equipment can be equipped with an air floating bed for cooling.
3.7. Processing of warping board:
In a well-managed factory, the printed board will be checked for 100% flatness in the final inspection. Any unqualified boards will be picked out, put in the oven, baked at 150 degrees Celsius and under pressure for 3 to 6 hours, and cooled naturally under pressure. Then the pressure relief to take out the board, in the flatness check, so that part of the board can be saved, and some boards need to be made two to three times of baking to smooth. The pneumatic plate warping and straightening machine used by Shanghai Bell has a very good effect in repairing the warping of the circuit board. If the above anti-warping process measures are not implemented, some of the boards are useless and can only be scrapped.
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