Why the circuit board is required to be very flat

1、 Why the circuit board is required to be very flat
If the printed circuit boards are not leveled on the automatic assembly line, they will cause inaccuracy. The components can not be inserted into the holes and surface mounts of the boards, or even crash the automatic cartridge loader. The circuit board with components is bent after welding, so it is difficult to cut the component feet in order. The circuit board can not be installed in the chassis or the socket of the machine, so it is also very annoying for the board factory to encounter the warping. At present, PCB has entered the era of surface mounting and chip installation, and the requirements of PCB for plate warping must be stricter.
2、 Standard and test method of warpage
According to ipc-6012 (1996 edition) (identification and Performance Specification for rigid printed circuit boards), the maximum allowable warpage and twist for surface mounted printed circuit boards is 0.75%, and for other boards is 1.5%. Compared with ipc-rb-276 (1992 edition), the requirements for surface mounted printed circuit boards are improved. At present, the allowable warpage of each electronic assembly plant, no matter the double-sided circuit board or multi-layer circuit board, is 1.6mm thick, usually 0.70-0.75%. Many SMT and BGA boards require 0.5%. Some electronic factories are agitating to raise the standard of warpage to 0.3%, and the test method of warpage shall comply with gb4677.5-84 or ipc-tm-650.2.4.22b. Put the printed circuit board on the verified platform, insert the test needle to the place with the largest warpage, divide the diameter of the test needle by the length of the curved edge of the printed circuit board, and then calculate the warpage of the printed circuit board.
3、 Warpage of anti plate during manufacturing
1. Engineering design: precautions for PCB Design:
A. Multi layer PCB core board and semi curing sheet shall use products of the same supplier.
B. The arrangement of interlaminar prepreg should be symmetrical, for example, the thickness of 1-2 and 5-6 interlaminar should be the same as the number of prepreg sheets, otherwise it is easy to warp after lamination.
C. The line graphic area of outer layer a and B shall be as close as possible. If a side is a large copper side, and B side is only a few lines, this printed board is easy to warp after etching. If the line area between the two sides is too large, some independent grids can be added to the dilute side for balance.
2. Drying plate before blanking:
Before the copper clad plate is blanking, the drying plate (150 degrees Celsius, time 8 + 2 hours) is designed to remove the moisture in the board, and the resin in the board is completely solidified, so as to further eliminate the residual stress in the board, which is helpful for preventing the warpage of the board. At present, many double-sided PCB and Multilayer PCB still insist on the step before or after bake. However, there are exceptions in some board factories. At present, the baking time of PCB factories is not the same, from 4 to 10 hours. It is recommended to determine according to the grade of printed boards produced and the requirements of customers for warpage. The two methods are feasible. It is suggested to dry the panel after cutting. The inner layer should also be baked.
3. The longitudinal and latitudinal directions of the prepreg:
After laminating, the shrinkage of the semi-cured sheet is different in warp and weft directions. It is necessary to distinguish the warp and weft directions 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 the multi-layer board are caused by the unclear longitude and latitude direction of the semi curing sheet during lamination and the disorderly stacking.
How to distinguish longitude and latitude? The curled prepreg rolls in a direction of meridional direction, while the width direction is zonal; for copper foil, the zonal and short edges of the long edges are meridional, and can not be querying to PCB producers or suppliers if they are unsure.
4. Stress removal after lamination:
After the hot pressing and cold pressing, the multilayer circuit board is taken out, the burr is cut or milled off, and then it is dried in an oven at 150 ℃ for 4 hours, so as to gradually release the stress in the board and completely cure the resin. This step cannot be omitted.
5. The thin plate needs to be straightened during electroplating:
A special roller should be made when the 0.6 ~ 0.8mm thin plate multi-layer circuit board is used for plate plating and graphic plating. After the thin plate is attached to the flying pad on the automatic plating line, a round bar is used to string the whole roller on the flying bar so as to straighten all the boards on the roller so that the plates 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 of the plate after hot air leveling:
During the hot air leveling of PCB, it will be impacted by the high temperature of solder bath (about 250 ℃), and it shall be placed on the flat marble or steel plate for natural cooling after being taken out, and then sent to the post processor for cleaning. This is good for anti warping board. In some PCB factories, in order to enhance the brightness of the lead tin surface, the board is put into cold water immediately after hot air leveling, and taken out after a few seconds for post-treatment. This kind of hot cold impact is likely to cause warpage, stratification or blistering on some types of boards. In addition, air floating bed can be installed on the equipment for cooling.
7. Treatment of warping board:
In an orderly PCB factory, PCB will be inspected for 100% evenness at final inspection. All unqualified plates will be picked out and put into the oven. Bake at 150 ℃ and under heavy pressure for 3-6 hours, and cool naturally under heavy pressure. Then take out the board by pressure relief, and check the flatness. In this way, some boards can be saved. Some boards can only be leveled after two or three times of baking. Shenzhen City Lutong Technology Co., Ltd. in the use of remedial circuit board warpage has a very good effect. If the above mentioned warpage prevention measures are not implemented, some PCB boards will not be used for baking and pressing.