Josee
The metallurgical micro etchant you want to use on your microsection is
2% Nital 2ml of Nitric Acid in 98 ml of Ethanol or Methanol
This should be swabbed on for no more than 3 seconds - you will have to experiment to get the time right depending on pressure and the mount surface.
The Sn/Cu intermetallic will be the white layer next to the copper.
Before using the Nital we like to microetch with 50/50 3% hydrogen peroxide/ammonium hydroxide to removed the smeared copper.
Hope that helps.
Susan Mansilla
Robisan Lab