Gary,

Steve brings up a good point.  What about the alternatives?  Immersion silver is another hot topic as of late.  It is a single metal finish as is the immersion tin, but silver presents the advantage of no electromigration or "whiskering" which can be a long term reliability issue with the immersion tin.  Silver is also much cheaper than ENIG.  You may be forced to use a high silver solder if this is your choice (I'm not sure about the wettability of the silver with SnPb solder or if there are other issues in assembly).  I know that a silver solder does work fine with immersion silver finishes.  Another thing to ponder!  

Regards,

Joe Aronson 
Wet Process Engineer
ASEM

>>> Steve Underwood <[log in to unmask]> 09/05/99 12:45AM >>>
Gary,

Have you considered Immersion Tin?  We get quite a few comments from
satisfied customers who like the surface flatness.

Steve U.

On Thu, 2 Sep 1999 09:38:58 -0600 Gary Crowell <[log in to unmask]>
writes:
> 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 laboratory, internal company use only.
>
> TIA
>
> Gary Crowell
> Micron Technology
>
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