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Reply To: | TechNet Mail Forum. |
Date: | Mon, 18 Aug 1997 20:20:59 -0500 |
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Address,
Be careful when identifying the cause of defect, since blowholes can
be attributed to either fabrication or assembly processes.
Identifying this type of defect is quite difficult.
FAB:
1. Plating pockets caused by a dull drill bit pulling laminate/prepreg
from the hole wall versus cutting.
2. Closed or open pockets where the plating solutions are
encapsulated.
3. Thin plating in hole wall
ASSY:
1. A prebake (1 to 2 hrs @ 110°C to 120°C) is sometimes recommended
prior to assembling (caution is recommend at the elevated
temperatures, due to material Tg). Doing so will eliminate any
moisture the laminate has absorbed. Remember the laminate if
hygroscopic and if left exposed, no desiccant or proper packing in
a controlled environment, will absorb any moisture.
The moisture typically escapes through the PTH and there are enough
cavities present, will work its way to the surface.
However, the baking will not eliminate the existing cavities
created at the fab level, which can not be removed until the solder
is brought to its melting point.
2. Monitor your wave process carefully. I typically found that the
preheat was too high, the conveyor too slow, insufficient flux or
any combination of these variables would present blowholes. If
there is not enough flux present at the wave, then the solder will
solidify prematurely, hence, blowholes or cavities. Play with the
above variables and sizzle the flux in wave and you may be able to
minimize if not eliminate its occurrences.
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