Hi Glenn,

     A bi-level gold approach, i.e., 10 to 20 u" over everything, and 70 U'
     min. at the wire bond sites, is the simplest way to go, if you can
     tolerate the 10 to 20 u" of gold in your solder process.  At least,
     this approach requires the least amount of imaging or masking steps
     and generally provides better yields at the board fab level.  It most
     often is the least expensive approach as well, unless there very large
     features that are consuming gold.

     Hope this helps.

     If you want to discuss this further, you can contact me off-line.

     Andy Slade
     HADCO Tech Center East
     978-556-6310


______________________________ Reply Separator _________________________________
Subject: [TN] Fab:  Multiple surface finishes
Author:  glenn pelkey <[log in to unmask]> at SMTPLink-Hadco
Date:    09/30/98 10:31 AM


Hello all,

     I'm looking for information on how to build a board with both soft
gold and solder as outer layer surface finishes.  We want to make sure
it is manufacturable before designing it.  The only thing I have now are
examples with edge connectors.  The gold and solder would be at select
locations throughout the board.

     In case you're wondering why, I'll tell you.  We want to increase
the gold thickness to greater than 70 microinches for wire bonding.
But, this product also has screened solder for SMT.  Since sceening
limits the amount of solder, gold embrittlement becomes an issue.  I
calculated 3.7% of gold by weight at 70 microinches.  I know opinions
vary here, but I think everyone can agree that this is at least a
concern.

      What do you think?  How hard is this to do?  Contamination,
increased cost, other reliabilty or quality issues to trade off?

Thanks for all your help.

Glenn Pelkey
Maxtek Components Corp.
[log in to unmask]
[log in to unmask]

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