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


***********************************************************************  
****
* TechNet mail list is provided as a service by IPC using SmartList 
v3.05 *
***********************************************************************  
****
* To unsubscribe from this list at any time, send a message to: 
          *
* [log in to unmask] with <subject: unsubscribe> and no text. 
       *
***********************************************************************  
****
* If you are having a problem with the IPC TechNet forum please 
contact   *
* Dmitriy Sklyar at 847-509-9700 ext. 311 or email at [log in to unmask] 
     *
***********************************************************************  
****


***********************************************************************  
****
* TechNet mail list is provided as a service by IPC using SmartList 
v3.05 *
***********************************************************************  
****
* To unsubscribe from this list at any time, send a message to: 
          *
* [log in to unmask] with <subject: unsubscribe> and no text. 
       *
***********************************************************************  
****
* If you are having a problem with the IPC TechNet forum please 
contact   *
* Dmitriy Sklyar at 847-509-9700 ext. 311 or email at [log in to unmask] 
     *
***********************************************************************  
****


***************************************************************************
* TechNet mail list is provided as a service by IPC using SmartList v3.05 *
***************************************************************************
* To unsubscribe from this list at any time, send a message to:           *
* [log in to unmask] with <subject: unsubscribe> and no text.        *
***************************************************************************
* If you are having a problem with the IPC TechNet forum please contact   *
* Dmitriy Sklyar at 847-509-9700 ext. 311 or email at [log in to unmask]      *
***************************************************************************