We're doing our first board using high pincount BGA's, and I understand
that we should be an immersion gold finish to get the flatness needed
for BGA assembly.  My question is, how do I specify this type of finish
to the fab house?  I assume thickness is one of the factors; what is
typical and what are the considerations in thickness spec?  What else
needs to be specified?

The board in question is 9x14", 18 layers, a prototype build on a board
that will never see high volumes (about 100 units max lifetime
production).  Environment is labratory, internal company use only.

TIA

Gary Crowell
Micron Technology

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