Here are a couple of images of lead-free solder that I took with an optical microscope back in 2005. At the time I was studying the reflow properties and behavior of the NEMI-recommended solder composition Sn3.9Ag0.6Cu (95.5% tin, 3.9% silver, 0.6% copper).
One interesting thing about this solder alloy is that, when the cooling rate is slow and controlled, it requires quite a bit of undercooling to recrystallize after being melted. What this means is that my samples needed to be cooled to 15-20° below their melting points before they would recrystallize.
Above is a 200x dark field image, and following are 100x images taken using bright field and polarized bright field illumination. The sample is about 1.0 mm in diameter, or about 0.04″.
The samples were cross-sectioned, polished, and etched.