First of all, why should PCB flat?
If the car on an assembly line of circuit boards, may result in inaccurate positioning, or components cannot be inserted into the hole, can not be installed to the solder, even damage the machine.If the PCB assembly after bending, is very difficult to cut component pins and tidy.Assembled PCBA board cannot be packed in boxes, so very troublesome plate bending problem.So far, the printed circuit board is installed on the surface and chip time, there will be more stricter PCB assembly plate flatness requirement.
Second, flatness standard and test instruction
According to IPC6012 rigid printing plate qualification and performance specifications, the largest warp and distortion of the surface mounting plate is 0.75%, other 1.5% is allowed.Compared with IPC276 (V1992), it has increased the surface of the mounting plate flatness standard.And now, most of the assembly manufacturers allow flatness is 0.7% – 0.75%, about double PCB or multi-layer PCB plate with 1.6 mm thickness.Many SMT and BGA board need 0.5%.Even some manufacturers want to control in 0.3%.According to GB4677.5-84 or IPCTM6188.8.131.52 B, plate flatness test method is to put the PCB on the proven platform, will test the needle on the buckling position.The length of the test pin diameter divided by the board in the edge of the flatness is printed circuit board.
Third, how to avoid PCB warp in the manufacturing process
Here are seven points for your reference
A) PCB design: we should pay attention to PCB design
* Presoak material should be balanced in the layer, for example,6 layer board, the thickness of the PP and quantity should be in 1 to 2 l and 5-6 l is the same, otherwise easy to cause bending.
* use the same manufacturer’s core laminate and laminated Presoak.
* circuit design area should be close to the top side and bottom side.If the top side with large copper area, and has only a few at the bottom of the circuit, it may cause warping after etching.If the circuit between the top and bottom are large, we can add some independent grid to balance it.
B) baking laminate before production
Baking laminate, bake at 150 ℃ for 8 + 2 hours to remove excess water, and can be cured resin to remove excess stress.This helps to avoid PCB warp.Now, a lot of double deck plate and sandwich plate is still baking laminate before production.We suggest baking laminate after cutting, and the inner laminate should be baked.
C) the latitude and longitude direction presoak
Latitude and longitude of the PP shrinkage rate after laminated is different, so it must be kept in the same layer pressure direction.Otherwise easy to cause plate warping, and difficult to correct.The cause of the multi-layer plate buckling is mainly due to the PP mixed latitude and longitude.
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