Following from the EMPF HelpLine in Indianapolis, prepared by John Greaves:
We like to use an etch of hydrogen peroxide, water, and ammonium hydroxide

Usally as follows:
50% NH OH
     3
50% Water and Peroxide (H O )
                   2 2
(note:  text formating will probably not Email well)

We use
20 mL Ammonium Hydroxide
19 mL Water
1 mL 30% Hydrogen Peroxide

Also, acetic acid can be used to attack anything with lead in it.  It also 
darkens the surface which probably won't hurt if you are looking at 
intermetallics.

Jim Nelson at Buehler is considered at expert at electronic microsection 
preparation (probably etching as well) (unfortunately, I believe he retired, 
or will be soon)

John Greaves can be contacted through the EMPF HelpLine at 
[log in to unmask] or 317.226.5616
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Subject: Sn-Pb etch
Date:  24 Jan 96 12:13PM

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Can anyone recommend a method to completely remove the 63-37 solder from a
surface, leaving the underlying intermetallics untouched?   I have tried
hydrochloric acid; it works for solder reflowed on Cu, but in joints with
gold it apparently re-precipitates gold artifacts.  Any help would be
greatly appreciated.

Edwin Bradley
Motorola