Hello all,

I hope I can relate what is happening here so all of you experts can lead me
down a path! (Am I asking for it or what?)

25mil and 20 mil QFP's on a double sided PWB. We are seeing variances in the
total height of the pad and resulting HASL finish. Specifically maybe 5-10
pads out of the 84 on the QFP exhibit an increase in HASL result in that the
solder is mounded on top of the copper. The rest of the pads all are flat in
nature with good HASL finish observed. We are being told that the resultant
solder height is not controllable. Is this true? Is it unexpected to see all
84 pads looking the same?

The situation came to light at the post printing process where a visual
inspection detected abnormal paste placement. We ended up with either more
solder per pad (potential shorts) or less solder than normal (potential
reduced reflow) per pad. When I queried TechNet some time ago about stencil
aperture sizes, the bulk of the results came back that apertures were
ordered at some percent smaller than the actual pad size. In the
neighborhood of 5-20% reduction.

If apertures are smaller than the pad size wont this add to problems of
uneven pad heights, assuming pad heights are not very well controlled?

As usual, Thanks in advance.

Richard Hamilton
Clemar Mfg. / Rain Bird
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