How wide is the solder mask dam at the via. Is possible the solder is going
under, rather than over, the dam?
If not, then consider the weight of the part versus the surface tension of
the solder balls. Perhaps those little guys just can't hold the part up. It
seems kind of intuitive to me that, given equal ball and land pattern
diameters the heavier part with fewer balls will end up closer to the board
than the lighter part with more balls.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Edwin Louis
Sent: Friday, July 08, 2005 12:41 PM
To: [log in to unmask]
Subject: [TN] s::Ball Grid Array Reflow


We have a 256 leaded BGA with pads on the perimeter in which the solder
flows over the stringer solder mask barrier and fills up the attached via
hole and pad and causes a short circuit. The component pad size is 0.6mm
dia., the ball dia. Is 0.75mm, the pwb pads are nsmd and 0.71mm dia. This
part weighs 8 grams. Another plastic BGA on the board weighing 2 grams has a
0.6-0.9mm ball and 0.584mm pads. The PWB pads are the same as the other part
but the 357 leads are in a square center pattern. The 256 leaded part ends
up 0.46mm off the board while the 357 leaded part ends up 0.74mm off the
board. Can someone give an idea why this is happening?


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