Hey Bob,
Boy, you are lucky to have a laboratory, and now colorful pix!!!
The only silver IMC that you will see are: * Ag6Sn, much less likely at 'normal' soldering temperatures * Ag3Sn, much more likely
Allowable silver concentration: * Below 2% will not cause deterioration in solder connection. * At 4% IZOD impact strength begins to decline from ~80 in-lb to ~15 in-lb at 6% [like falling off a cliff, eh?]
So ... * How thick is your IMC? * What level of silver do you have?
Observations are: * Be concerned that component manufacturers, in their rush to be lead-free, have resorted to using thick, 100% silver lead finishes. [You said 'no-lead', you don't anything about silver. Gawd the @#%#!! Eurolanders are making everyone's products unreliable, even if we don't use no-lead!!!] * While you are correct to be concerned about the Ag3Sn IMC, your solder system will have IMC other than silver.
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