I  agree with John and Charles on this.   Use a 6 mil SS stencil (laser
cut), Stainless steel blades, run a 1 in./sec squeegee speed, under cut
your apertures by 10% (from the pad size), make sure your paste is good and
not dried out, OA is more aggressive and offers better soldering, but we
are all no-clean with Alpha UP-78 and are getting great results (solder
joints look as good as the OA joints I have looked at for 10 years).  Also
be sure your profile is in line with what the paste manufacturer calls for.
Are your parts being placed 50%+ on the pads?  Are the operators tweaking
them?  If so this will cause many problems.

I once had yields similar to yours.  I switched to the stainless steel
blades and laser cut 6mm stencil and yield instantly went to 100%.  That
may sound far fetched but it is true.

Brad Kendall
Hella Electronics.

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