Hello All!

I lost the original post on this subject, so I will use Dougal Stewarts'
post to reply.

I haven't seen this one (purple stain) for quite awhile!

In the "early days" of reflowed tin/lead, we put the nickel/gold onto
edge card tabs right after etching but before reflow. The plated
tin/lead was easy to strip off the fingers and the process went well.

With the advent of HASL, it was a pain to nickel/gold plate prior to
HASL because of the infamous finger taping issues prior to HASL. So we
nickel/gold plated after HASL. HASL solder was quite difficult to
completely remove without both chemicals AND abrasion in the inline tab
plater.

We began to get the purple stain which turned up at soldermask bake and
got worse as the board saw more and more heat.
Our labs tested the product and found that in some rare instances that
there could be an ionic issue but not as disastrous as the cosmetic
issue. We decided to eliminate the cosmetic issue entirely by finding a
better way of taping the nickel/gold prior to HASL.

The purple stain itself is caused by a complexing of the Cobalt in the
gold with the Bromine in the laminate under heat. We suspected that
somehow the aggressive HASL solder strip chemistry combined with
abrasion provided a better path for the gold salts into the laminate.

We don't make boards anymore, but occasionally we get a few supplier
questions regarding acceptance.

I hope this helps.

Gerry



-----Original Message-----
From:   Stewart, Dougal [SMTP:[log in to unmask]]
Sent:   Thursday, May 21, 1998 4:05 AM
To:     [log in to unmask]
Subject:        Re: [TN] DISCOLORATION

Purple discoloration on gold plated boards is due to the formation of
'purple of Cassius', which yu should be able to find out about in your
chemistry books. I believe that it is due to some contamination absorbed
into the resin of the laminate. I remember this problem 15 years ago,
and a change of laminate supplier had the problem go away.

Dougal
Dougal Stewart
Product Development Manager
Viasystems Selkirk Ltd
Selkirk, Scotland, TD7 5EJ
Tel: +44 1750 21601
Fax:+44 1750 22513
email   [log in to unmask]

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