Monday, December 27, 2010

【 Weak current College 】 prevent PCB Warp method 】

In automation plug-in line, if not flat printed circuit boards, positioning, components cannot cartridge to the Board of the hole and surface mount pads, can even automatically insert machine crashed. Fitted with components of the Board after bending, welding components difficult to cut flat feet. The Board cannot be mounted to the chassis or the tank outlet, so Assembly plant into warp is very annoying. At present, the PCB has entered into the chip surface mounting and installation times, assembly plant on Warp must increasingly strict requirements.
I. warpage standards and test methods
According to the United States IPC-6012 (1996 Edition) <<刚性印制板的鉴定与性能规范>>, allowing for surface-mounted PCB for maximum warping and distorting 0.75%, other Board allows 1.5%. This ratio IPC-RB-276 (1992 edition) improves on surface mount printed circuit board. At present, the electronics assembly plant license of warpage, whether double or more layers, 1.6mm thickness, usually 0.70 ~ 0.75%, not less SMT, BGA's Board, the request is 0.5%. Some electronic factory is instigating the warpage of standards to 0.3%, the test method in compliance with veneered GB4677.5-84 or IPC-TM-650.2.4.22B. The printed circuit board into the verification platform, the testing pin plug to warp the greatest place to test pin diameter, divided by the PCB curved length is calculated from the printed board of warpage.
II. the manufacturing process-MLB
1. engineering design: PCB design considerations:
A. interlayer prepreg permutations should be symmetrical, for example six-layer plates, 1-2 and 5 ~ 6 layer thickness and prepreg of the paper should be consistent, otherwise laminating after easy warping.
B. multilayer cores and prepreg should use the same vendor products.
C. outer surface and A B surface line should be as close as possible to the graphics area. If A surface is a large copper surface, while the B side only a few lines, the PCBin etching after easy warping. If both sides of the line area too significant, rare side with some independent grid for balance.
2. the material before baking plates:

CCL material before baking plate (150 ° c, time 8 ± 2 hours) aims to remove the moisture in the plate, while the boards of resin fully cured, the further elimination of remaining stress for sheet metal, which prevent the MLB is helpful. At present, many double-sided, multi-layer boards still persist in the material before or after this step baking plates. But there are some exceptions, the sheet metal plant various PCB factory baking plate of time provisions are also inconsistent, from 4-10 hours, suggested that, in accordance with the production of printed circuit board grades and customer requirements on the warp. Cut into pieces after drying or block large material drying after cutting, the second method is feasible, it is recommended that after baking sheet metal shear. Inner plate should bake plate.
3. the prepreg of knit to:

Prepreg laminates after meridional and zonal shrinkage, Blanking and laminated must distinguish the meridional and weft. Otherwise, the laminate can easily lead to the finished product after MLB, even with pressure drying plate is also very difficult to correct. Multilayer PCB warpage of reasons, many are laminated at prepreg of knit to not distinguish, stacked.

How to distinguish between knit to?% volume of prepreg roll direction is the direction to, and the width is zonal; on copper plates, long edge zonal, short side is subject to, if not sure to the manufacturer or supplier queries.
4. after the stress of laminated:

Multilayer PCB in complete remove the cold heat pressure, shear, or after cutting away the rough edges, then placed in the oven to 150 degrees Celsius within 4 hours of drying to make the plate of stress releasing and resin fully cured, this step is omitted.
5. sheet metal plating requires straightening:
0.4 ~ 0.6mm ultrathin multilayer PCB for plate surface plating and pattern plating should be making special clamping roller, automatic plating line flying bus clip on the sheet, with a round stick the entire flying coach Chuck roll on up to straightening rolls up all of the Board, so after plating will not fail. If no such measures, the plating to 30 µm copper layer, thin plates will bend and difficult to remedy.
6. HASL board cooling after:

PCB hot air solder pot through the whole usual (approximately 250 ° c), thermal shock, after removing should be placed to the formation of marble or natural cooling on the plate, to make cleaning after processing. This anti warped Board is very good. Some factories to improve lead-Tin surface brightness, HASL board immediately put into cold water, after a few seconds to remove in the post-processing, a hot and cold shock, on some models of the Board is very likely to produce warping, hierarchical or blistering. Additional devices can be equipped with flotation bed for cooling.
7. handling of warping Board:

Management of the factory, printed circuit board in the final inspection is made of 100 per cent of flatness check. Where non-conformity of the Board will be singled out, placed in the oven at 150 ° c within and under the pressure of 3-6 hours of drying and heavy pressure natural cooling. And then remove the pressure put on the Board, the decision was made to check flatness to save part of the Board, the Board made two to three times before leveling drying. pressure Shanghai Hua Fort agent pneumatic warp back straight machine used by the Shanghai Bell in remedial PCB warpage have very good results. If the above involving anti-warping process measures are not implemented, partly it's dry pressure on the Board, only scrap.

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