The warpage of the circuit board will cause the inaccurate positioning of the components.When the PCB is bent at SMT and THT process, the foot of the components are not in line, which will bring many difficulties to the assembly and installation work.
The maximum warpage or distortion of the IPC-6012, SMB–SMT’s PCB is 0.75%, and the other board is generally not more than 1.5%.The allowable warpage (double-sided / multilayer) of the electronic assembly plant is usually 0.70—0.75% (1.6mm plate thickness). In fact, many boards, such as SMB and BGA boards, require warpage less than 0.5%, and some factories even less than 0.3%; PC-TM-650 2.4.22B.
Calculation of warpage = warping height / edge length
Prevention of warpage of PCB:
- Engineering design: interlayer prepreg arrangement should be corresponded; multilayer board and prepreg should use the same supplier product; outer layer C/S side copper area as close as possible, independent grid can be adopted.
- Bake the PCB material before sheet cuting:
- Outlayer and innter layer need excluded the moisture in the plate and solidify the resin and eliminate the stress in the plate with 6-10 hours in 150 degree before material baking.
- Attention the prepreg direction of latitude and longitude before the multilayer lamination:
- The shrink ratio is different of latitude and longitude. Attention the prepreg and core direction of latitude and longitude before sheet cuting.In general curable sheet direction is longitude, and the long direction copper clad plate is latitude.
- Cold pressure,trim the edge after lamination.
- Baking the plate with 4 hours by 150 degree before drilling process.
- Thin PCB is better no mechanical abrasion, and chemical cleaning is recommended. Special fixture is used to prevent bending and folding of plate.
- HASL PCBs naturally cooled to room temperature or air floatation bed to be cleaned after cooling.
Warping board treatment:
Use smooth steel plate heavy pressure with 150 degrees or hot press 3–6 hours, 2-3 times baking.
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