Ok, here's one for discussion...

We have a 68 ceramic leadless chip carrier that routinely yields
insufficient solder at the corner locations.  The details are:
10 mil stencil
1:1 apertures (stencil to board)
Rounded apertures, rounded solder pads
Sn63 solder paste (water soluble flux)
Pretinned components (Sn63)
Reflowed in air

Observations are:
Uniform pretinning solder volume
Uniform HASL on boards
Perfect solder deposits
Perfect component placement
Clean apertures after deposition
Minute solder balls after reflow (nothing out of the ordinary)

Here's the kicker...  we use this same component on another board without
the occurrences of insufficient solder.  The difference is that on this
design the solder pads and stencil apertures are rectangular, not rounded
ends.  I know that your first reaction is going to be "Well, it's obvious,
rounded apertures deposit less solder paste than rectangular ones, thus the
insufficient solder."   I'd say "correct" except that the insufficient
joints appear predominantly on the corners!  BTW, the apertures are the same
size throughout the component.

Any ideas?
Jim Marsico
Senior Engineer
Production Engineering
AIL/Electronics Systems Group
An EDO Company
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