> "Robert M. Wolfe" wrote:
>
> I have a situation where there is a need to provide a dual
> footprint pattern for an alternate package.

> Or would using an OSP coating solve the problem, but I guess that
> coating once gone would leave exposed copper under that part.

I would tread very carefully leaving just an OSP over bare Copper in the
long term.

When having done dual footprints before because of single sourced parts,
I've found using some sort of HASL with two different paste masks to be
a good solution. Well, at least designing two paste masks & getting the
one first needed made - the other paste mask design is then your
fallback solution. Also, if you are considering Alpha Levelling or other
non tin/lead surface treatments, then look at the long term
consequences. If you are doing it for a fallback solution, you could
maybe do it by designing in the alternate footprint but leaving the
second footprint covered by solder mask. Not quite as interchangeable,
but you could generate two topside solder masks as well as paste masks &
still be able to use OSPs etc. It depends on how you'll handle the data
for future use & whether you stockpile PCBs or get them JIT.

I concur that you shouldn't screen solder paste *under* a component that
doesn't use it.

--
Dom Bragge  CID         "The pink building"
Production Manager      Level 1, IBC Bldg
KLAXON IQA              Australian Technology Park
bh+61-2-9209-4404       Eveleigh 1430 NSW
fx+61-2-8374-5030       (Sydney, AUSTRALIA)
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