First qurstion to my mind is if the board is SMOBC (solder mask
over bare copper).  Right off I can't think of any other reason the
copper would move around under the mask, especially if the
circuitry is wide.

Larry Campbell
BFGoodrich, Avionics Systems

>>> David Hoover <[log in to unmask]> 10/15/97 09:24pm
>>>
A customer asked me some questions about his
product. (Didn't have a clue!)
So........I was curious (like always) if someone
out there might have some ideas as to possible
answers to these.

Product:
2 sided FR-4 PCB .028 1/1 (very small in size).
SMOBC with HASL finish. The part has a couple
of very small components. Then it gets set into a
small plastic housing (1 side) and has pins (component
leads) soldered into place. The whole assembly then
gets potted in a black epoxy.
Part of their customer requirements is to test this
assembly in 85/85 (Temp/Humidity) and apply
constant bias voltage. They need to meet at
least 1000 hrs. The problem they have ran into is
that the copper traces (big guys) are migrating
under the soldermask (LPI) to other traces
and are failing the test. When they rip apart
the assembly they are seeing both traces and
soldermask separating both ways.
The question that pops up is FR-4 a wise choice
for this type of product?
Is soldermask beneficial or would bare traces
encapsulate better rendering a more electromigration
resistant bond?
Is a solder finish the best for the potting medium to
bond to?

Just Curious.

The above comments are of my own opinion, and
not necessarily that of my employer.

http:\www.hadco.com:8080

Dave Hoover (aka, Groovy)
NPI Mgr HADCO TC2
Watsonville, CA

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