Gary: As stated by others, what you are seeing is coming from the absorption
of the gold electroplating bath chemistry into the board laminate. This
becomes visible when reacted with heat.

I have seen this in the past primarily on FR4 Sn/Pb boards that were
selectively stripped of Sn/Pb then Ni/Au plated and reflowed.A guess is that
the Sn/pb strip chemistry may be reacting with the laminate buttercoat
making it more susceptible to absorption of the gold plating bath.

Since it is absorbed into the surface structure of the laminate it really
can not be solubilized. Although it can be reacted. We came across a cure
for it strictly by accident. When exposed to high energy levels(joules)of UV
radiation the purple color goes away. An UV oven used for curing UV
soldermask or "bumping" photoimageable soldermask works well. Small handheld
UV light sources are not strong enough. I'm not sure of the actual mechanism
but it may have to do with the UV radiation oxidizing the residue.

Like I said this has worked for us in the past, on our boards, our gold
plate bath and our laminate. If you have a contact in the bare board fab end
of things with an UV oven you may want to give it a try.


regards

Michael Barmuta

Staff Engineer

Fluke Corp.

Everett WA

425-356-6076



-----Original Message-----
From: Gary Camac [mailto:[log in to unmask]]
Sent: Monday, August 28, 2000 11:45 AM
To: [log in to unmask]
Subject: [TN] Staining on edge connector


Good morning all,

I have come across something I have never seen before and thought it would
be a good idea
to see what my panel of experts have to say.  I have some assemblies that
are showing a
purple stain in the laminate around the gold contacts of a board edge
connector.  Under
the microscope,  I see no sign of anything on the laminate on the boards
from stock.   I
took a sample of the stock boards and ran them, unpopulated, through the
reflow oven and -
lo and behold - I've got a stain.  The stain seems to be in the epoxy.  A
little light
scratching with a knife blade takes off something purple.

Any ideas as to what I am looking at?

Gary Camac

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