Cara,
I'm a firm believer (after seeing many
microsections) that most blowholes are caused by poor drilling at the fab house
(too many hits on the drill?).
I have observed the area where the blowhole
penetrates the holewall nearly always adjacent to uneven fibers in the glass
strands. My theory is that after drilling with a dull bit, thereby breaking the
fiber bundles, then e-plating the Cu in a plating BATH (keyword) causes an
entrapment behind the barrel, just waiting for your soldering machine to boil it
out and create a small volcano.
The bad news is even if you CAN proe that to your
PCB supplier, he'll only be willing to refund or replace the bare board, even
though you've done all that value added parts/labor and now everything is
"stuck" in the assembly.
Pre-solder baking can help, allowing the volatiles
to escape slowly and completely (however still may have ruptured the hole wall,
but since it's now dry, the effect is dimminished significantly).
I have an idea about an incoming inspection test
that could be of interrest if you find out what I described above is the cause.
Regards,
Ed Popielarski
QTA Machine
10 Mc Laren, Ste.
D
Irvine, Ca. 92618
Ph: 949-581-6601
Fx:
949-581-2448
http://www.qta.net
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