We recently had circuit card assembly sent out for cross sectioning.
One of the leads for a thru-hole device has voids -- perfectly round
voids.  The component body prevents us from seeing the top of the solder
joint (which, if we could have seen, we would have seen a void).  The
bottomside of the joint has a void, but, would not have been visible
without cross sectioning.  The board was sent out for cross sectioning
to evaluate a different problem, but, cross sectioning was done on a
sample basis to evaluate the entire board.  In other words, this
specific component has not been a problem (yet).
 
My questions are as follows:
1.  What causes the voids over the wave?  My first guess is insufficient
preheat or dwell time.  My second guess is materials (moisture in the
board).  Are these valid root causes?  What else should I consider?
2.  Are the voids detrimental to the reliability of the board?  We use
OA flux.  I bet there is flux entrapped.  That can't be good.
3.  What is the acceptance criteria for voids in Thru-Hole parts?
IPC-A-610 sec 6 covers circumferential wetting as well as percent fill
as these are visible without cross sectioning.  Is there a spec that
covers what a thru-hole solder joint should look like cross sectioned?
 
TIA,
Carrie
 
 
 

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