1. Why is the circuit board requirement very flat
In the automatic insertion line, if the printing circuit board is not flat, it will cause the location to be inaccurate, the components can’t be inserted into the hole and surface of the plate, and even the automatic plug loader.When the pcb board which assembled component is welded after soldering,and the foot of the element is difficult to be cut neatly.The PCB board also can’t be installed in the machine box or the socket in the machine, so, the pcb assembly factory meeting plate warped is also very troublesome.At present, the printing circuit board has entered the era of surface installation and chip installation, and the printed circuit board assembly plant must be more and more strict with the requirement of plate warped.
2. Standard and test methods for warp
According to the United States IPC-6012 (1996 edition) << rigid printed circuit board identification and performance specifications >>, the allowable maximum warping and distortion of the surface mounting plate is 0.75%, and all other pcb boards are allowed 1.5%.This has increased the requirements for the surface mounting plate board of the ipc-rb-276 (1992 version).At present, the warp degree of the license of each electronic pcb assembly plant, no matter double or multi-layer pcb, 1.6mm thickness, usually 0.70 ~ 0.75%, many SMT, BGA plate, the requirement is 0.5%.Some electronics factories are agitating for a 0.3 percent increase in warping standards, and the test warping measures follow gb4677.5-84 or ipc-tm-650.2.4.22b.Put PCB board on a verified platform, the test needle to warp degree of the largest local, to test the diameter of the needle, divided by the curve length of PCB board, warp degree of the printed circuit board can be calculated.
3. Warping during manufacturing process
1. Engineering design: the design of the printing circuit board shall be noted:
A. The arrangement of the prepreg between the layers should be symmetrical, such as the six laminates PCB Board, the thickness of 1 ~ 2 and 5 ~ 6 layers should be consistent with the number of the semi-solidified pieces, otherwise the layer pressure will be easy to warp.
B. Multi-laminated core pcb and semi-cured tablets shall be used in the same supplier’s products.
C. The area of the outer A and B lines should be as close as possible.If A face is A large copper surface, and B is only A few lines, the plate is easy to warp after etching.If the two sides of the line area difference is too large, you can add some indifferent grid on the lean side, in order to balance.
2, Baking board before cutting:
CCL baking pcb board before cutting (150 degrees,8±2 hours) for the purpose is to remove the moisture inside the board, and make the resin cured within plate, further eliminating residual stress in plate, which is helpful to prevent the board warping.At present,many double-sided pcb, multi-layer pcb boards still adhere to the pre-blanking or post-baking step.But there are also some pcb board manufacturing factory, now the PCB circuit board factory baking board time rules also inconsistent, ranging from 4 to 10 hours, suggested that the class according to the production of printed circuit board and customer demand for warp degrees to decide.Cut into pieces or after the whole piece of bake bake oven cut the material, the two methods are feasible, it is recommended cutting board after cutting. The inner board should also be drying board.
3. The warp and weft of prepreg:
After the prepreg lamination, the shrinkage in the warp and weft directions is different, and the warp and weft directions must be distinguished when blanking and stacking.Otherwise, it is easy to warp the finished plate after laminating, even if it is hard to correct.Multi-layer pcb warping reasons, many of the lamination of the prepreg when the warp and weft did not distinguish between, indiscriminate duplication caused by.
How to distinguish between latitude and longitude? Roll prepreg rolled up direction is the warp, and the width direction is the weft; copper foil board for the long side of the latitudinal, short side is warp, if not sure to check with the manufacturer or supplier.
4. Stress after laminating:
multilayer pcb board, after fulfilling the hot-pressing cold-press cut or milling burrs, then flat on baking in the oven 150 degrees Celsius for 4 hours, so that the intraplate stress gradually release and make the resin cured, this step is omitted.
5. Need to straighten the thin plate while plating:
0.4 ~ 0.6mm thin multi-layer pcb board for the circuit board plating and graphic plating should be made of special nip roll, automatic plating line on the Feiba clip on the sheet, with a round bar to the entire clip on the Fiba The strings are strung together to straighten all the printed circuit boards on the roller so that the plated plates will not deform. Without this measure, after plating twenty or thirty microns of copper layer, the sheet will be bent, and difficult to remedy.
6. Board cooling after hot air leveling:
The hot air of the printed circuit board is affected by the high temperature of the solder trough (about 250 degrees Celsius). After it is removed, it should be put into the flat marble or steel plate to cool naturally, and then the post-processing machine is cleaned.This is good for the warping of boards.Some factory to enhance the brightness of the surface of lead, tin, the board into the cold water, immediately after hot air leveling out after a few seconds in the reprocessing, such a fever a cold shock, for certain types of boards is likely to produce warping, layered or blister.In addition, the air floating bed can be added to cool the equipment.
7. warping board processing:
In a well-managed factory, the board will have a 100% flatness check on the final inspection. All unacceptable pcb boards will be picked out, placed in an oven, baked at 150 degrees Celsius and heavy pressure for 3 to 6 hours, and under the pressure of natural cooling. Then unload the plate and remove the pcb board, in the flatness check, so that part of the board can be saved, and some printed circuit boards need to be two to three times the bake in order to level.If the above-mentioned anti-warping process measures are not implemented, some board baking is useless, only scrapped.