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When I first started in PWB manufacturing, we used gold as an etch
resist exclusively. If my memory serves me correctly we used 20
microinches. We used it because immersion tin wasn't viable for us
(slivers/solderability/shelf life/etc.) and there wasn't a stable
tin/lead bath on the market at that time. We also used pyrophosphate
copper because acid copper sulfate was relatively new to high volume
production. We were predominantly screening our innerlayer etch
resists because again, the photoimageable stuff was relatively new. We
used the Dupont "Riston process" as it was called only for "fineline"
jobs (i.e. 6 mil line 10 mil space).
At the time, big advantage to using gold - veeery simple process and
flow. Still would be as simple a process today.
What killed it?
1) Cost - I remember when gold was <$40 an ounce. It seemed to
double, then triple, then 10X. I remember hedging gold "futures" for
electroplate at $800 per ounce for 2-4 years!!!!! No rocket science to
figure this one out! Also no rocket science to figure out why gold
plated edge contacts all-of-a-sudden went from 100 microinches minimum
to 70 to 50 - to wherever it is now. Amazing how all the reliability
data seemed to support the reductions. Sort of made us feel stupid to
have been plating 100+ microinches on edge card contacts!!!!
2) Slivers - could never really get rid of them with gold unless the
fold was stripped. Reduce slivers yes, but not eliminate. Could never
really get rid of slivers for immersion tin either unless it was
stripped. We scrubbed by machine & by hand. Ughhhh.
3) Solder joint issues with gold- Yes, I had to live through that one
too! Very difficult for a boards person to understand at that time.
Enough History.
Gerry
=======================================================================
=============
----------
From: [log in to unmask][SMTP:[log in to unmask]]
Sent: Wednesday, January 29, 1997 6:39 PM
To: [log in to unmask]
Subject: Re: Gold as Etch Resist
So, if you use the Ni/Au as an etch resist, what protects the trace
edges
corrosion? Do you plan to plate it twice or strip the gold? I think
the photo resist is cheaper.
Chuck Brummer, Acuson.
______________________________ Reply Separator
_________________________________
Subject: Gold as Etch Resist
Author: [log in to unmask] at ccUnix
Date: 1/29/97 3:35 PM
Hello,
Please post information, experiences, and anecdotes, on using
electroplated gold as an etch resist. In particular how is the
etch
rate controlled. How can the undercut be reduced to eliminate the
chance of slivers.
(Gold/Nickel/Copper/FR4 :The copper gets etched)
Thanks
Rich
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