Hi Doug,

I am a PCB fabricator in the UK and when we started using the electroless
Nickel and Immersion Gold process we noticed a curious phenomenon. If the
panels are immersed in DI water or mains water after processing, then
exposed to air for a minute or two before drying, the gold has reddish brown
stains in random areas. Although this did not seem to affect solderability
to any noticeable extent, it was not cosmetically attractive.

It is easy to restore the appearance of bare boards by using a fresh
solution of mild acidic cleaner and rinsing thoroughly before drying without
any delay, but I would not do this on assembled pcb's. You need a cleaner
which will not affect the solder or components so perhaps someone could
advise on a suitable one to use.

To prevent this, we keep panels immersed in DI water until ready to dry then
feed through a conveyorised rinse drying machine using mains water spray
rinsing, chamois squeegee rollers, and then hot air dry. Check your process
to see how long the boards are exposed to air when wet and try to minimise
it.

I would like to have an explanation for this mechanism. The immersion gold
is 0.15 microns max (6 millionths of an inch) over 4 microns (0.00016") of
electroless nickel. As this occurs with DI or mains water, contamination of
the water does not seem to be a factor, but simply exposure to air at room
temperature when the surface is wet.

Hope this helps.

-----Original Message-----
From: Hooper Doug <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: 03 July 1998 15:33
Subject: [TN] Au/Ni/Cu PROBLEM


I am experiencing a problem with my Au/Ni/Cu circuit boards. During the
cleaning process in warm (120f) DI water some of them are exhibiting a
"contamination". In certain areas (random) I am getting a discoloration
on the pads. These are non-soldered connections of the board. Other areas
on the board are soldered (hand) and have excellent
solderability/wettability/solder coverage are generally bright, smooth
and shiny. Sometimes this does not occur until the assembly has been
washed 3 or 4 times throughout the production process. The discoloration
appears brown, orange (like rust) and even dark blue to black. It also
appears to be removed with an eraser. Does anyone know what is going on
that would cause this condition to occur? I am concerned about the
condition with respect to contact resistance (keypad/dome application).
Will it get worse even if I can rub it off?

My circuit board supplier is on plant shut-down today. Any input on this
matter would be greatly appreciated! Technet or otherwise.

Douglas A. Hooper, Sr.
Process Engineer
Luminescent Systems, Inc.
[log in to unmask]
(716)655-0800 x163
(716)655-0309 fax

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