White solder pads on through-hole PCB
It's a PCB finish, called Immersion Silver, or IS for short.

Some other common types of finishes:


Hot air-levelled solder (HASL) - Just your normal solder, levelled with hot air knives.
Electroless Nickel Immersion Gold (ENIG) - Your typical matte gold.
Galvanic Gold - Different type of gold finish, looks shiny and polished.
Organic Solderability Preservative (OSP) - Looks like bare copper.