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Date: | Thu, 25 Apr 96 13:22:12 CST |
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We have had a problem in the past with OSP/bare copper boards with
gold fingers. It might be related. It occurs along the gold/copper
interface. When the board goes through the microetch prior to the OSP
coating the bare copper traces entering the gold fingers were over
etched at the gold/copper interface.
For some reason, the term the "experts" used was "galvonic etch", the
microetch at the interface becomes too agressive resulting in etch
outs at the interface. Very difficult to see. Since electrical test
is before the OSP treatment we have had opens in final product as a
result of this..
Our design solution was to make sure the soldermask overlaps the gold
fingers and also that there was enough room to insure the taping prior
to gold plating completely covered any bare copper.
Never saw this in HASL boards with gold fingers. The exposed
copper/nickel/gold interface seemed to be the cause.
Someone else will have to explain the chemistry.
[log in to unmask]
______________________________ Reply Separator _________________________________
Subject: Re:FAB gold plating problem
Author: [log in to unmask] at Dell_UNIX
Date: 4/25/96 11:25 AM
I'm in the same boat! Identical process. Tried everything under the
Sun. No go - No way. <For me> The gold lands must be completely
isolated with no outerlayer traces. (Solder to gold demarcation line)
The gold plating basically poisons the solder which disappears in
the etcher (or is it itcher?) and wul-ah, an open.
Groovy
BTW-Isn't the itcher located next to the board stretcher?
Or was it the on the shelf by the box of tooling holes?
<Poor guy. I guess everybody gets initiated into the buis
one way or another.>
______________________________ Reply Separator _________________________________
Subject: gold plating problem
Author: [log in to unmask] at SMTPLINK-HADCO
Date: 4/24/96 8:18 PM
Our Process
Isolated tabs in the center of the board to be gold plated are defined
by a primary layer of resist. After copper and tin-lead plating them,
we vacuum laminate 3 mil dry film resist.(over the plated tin-lead and
over the primary layer or resist) This top layer of resist is
selectively imaged to produce an opening exposing the area to be gold
plated (which now has tin-lead on it). Then the tin-lead is stripped,
and the nickel and gold are plated using our usual sulfamate
nickel/soft gold process.
The Problem
The plated nickel peels from the copper surface. The peeling is
always near traces connecting to the tabs.
Therefore I suspect that tin-lead stripper is being trapped under the
top layer of resist and coming out during plating.
Solutions?
I don't suspect the nickel/gold process is the problem, as the usual
yield on products without this tin-lead stripping step is nearly 100%.
We have no option to change the design of the board to facilitate
manufacturing. ie: No bussing of the tabs is possible therefore base
copper must be used as a conductive path prior to etching.
We've tried: 140 degree F water rinse after stripping, conveyorized
nitric tin-lead stripper, immersion and conveyorized peroxide tin-lead
strip, a second strip cycle, UV bump (160mJ), among other things.
Any help would be appreciated.
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