This is a good question. In the past, I have processed boards with both gold
and tin-lead deposits on the same boards through an alkaline etchant and a
hydrogen peroxide sulfuric acid based etch. We never detected any staining
or darkening of the gold. However, the extent of undercut on the
copper/nickel/gold plated traces was significantly greater than on the
tin-lead/copper traces.
We can attribute that to the galvanic reaction, but do not know if that is
the reason for gold staining.

Best regards,

Mike Carano

> -----Original Message-----
> From: doug smith [SMTP:[log in to unmask]]
> Sent: Thursday, February 18, 1999 11:49 AM
> To:   [log in to unmask]
> Subject:      [TN] Galvanic reactions
>
> In PCB fabrication process SnPb is used as an etch resist. On some boards
> gold is selectively plated, for example on a BGA pattern. In this case
> both the SnPb and the Au are being used as the etch resist.
>
> The etch process removes unwanted copper leaving the plated circuits and
> component lands.
>
> Can copper staining on gold be the result of a galvanic cell that is
> formed when the nickel/gold and tin/lead are used as an etch resist?
>
> Doug Smith
> Technical Support Engineer
> Yamamoto Mfg. (USA), Inc.
>
> (408)944-8308
> [log in to unmask]
>
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