QFP solder paste volume

2017/2/28 5:17:35

We are running a number of boards with 8-10 25 mil PQFPs on board. Our process is mature with at least 3 years experiance and statistically few opens on the QFPs. Our beloved purchasing department has chased the almighty $ and changed FAB vendors with questionable results. Since the new vendor was properly qualified and the FABs are not technically defective I am stuck with them. The problem ranges from no release to insufficient release on 1 axis caused by HASL height irregularity preventing proper contact and lowering the surface tension to a point where the release is not complete. While the great majority of these boards will print, the paste volume is lower on 1 axis resulting in an unacceptable number of opens. I first thought to increase the stencil thickness from 6 to 7 mil but concluded that this would just exacerbate my surface tension problem. My stencils are electropolished chem etch. Does anyone havea comments about lazer cut stencils with a trapezoidal shape? other suggestions? (other than what to do with purchasing) Thanks