Hi Ben
A couple of suggestions.
- is the solder paste well aligned with the board? If not, it would
be consistent in direction.
- is the pick and place machine placing the parts evenly in the paste
deposits?
If the part is more in one paste deposit compared to the other, it may
lift from the other.
- have you looked at orientation going through the oven? E.G. if you
reflow a board and always get tombstones lifted on the west side, and
then run the next board rotated 180 degrees, do the tombstones match?
If so, can you run it rotated 90 degrees? Ideally you want both pads to
reflow at the same time, so if the part is oriented perpendicular to the
direction of travel you should get best results.
Pad geometry can also contribute. We have a board with 0603s where the
pads are about 50% bigger than per IPC SM782, and the tombstoning rate
is relatively high, whereas we have 0402s on properly proportioned pads
and they never tombstone.
We also find that, with our particular paste (SnPb, RMA flux), running
without nitrogen pretty much eliminates tombstones because it slows down
the wetting.
Good luck.
regards,
Graham Collins
Process Engineer,
L-3 Communications Electronic Systems Inc.
Halifax
(902) 873-2000 ext 6215
>>> [log in to unmask] 06/08/05 09:09AM >>>
Technetters,
I have several SMT board styles which give me some trouble with 0306
capacitor tombstones, and through many attempts to reduce the number
of
these defects, I have found a condition that I can not seem to
attribute
a cause to. I have found that across a board, all the tombstones tend
to
lift in the same direction (for components in the same orientation).
I have looked at the build process, but different board styles result
in
tombstones in different directions, so I believe the cause to be board
related. I have looked at solder mask registration (misregistration
occurs in various directions), power/ground plane connections by pad
(changes from location to location), and solder bumps due to HASL
(wicked pads have similar results), and nothing has led me to a root
cause.
Any ideas???
Thanks,
Ben Gumpert
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