What a piece of junk. Next.
It's definitely some corrosion related to flux residue.
I've honestly never seen anything like this mess. I'm sorry, the person responsible has no business constructing, using, and especially selling such blatantly faulty, unsafe and potentially dangerous stuff.
Postal and freight companies worldwide need to be warned about accepting these products for possible air-freight.
I always thought run-of-the-mill solder had noncorrosive rosin core flux, no?
I've been doing electronic service for decades and I don't usually clean my work. But I've never seen anything like that.Various dissimilar metals in a sealed box with chemical outgassing, along with potential difference and current flows makes for a real mess. Add to that a plumber's soldering iron and no-idea, and the result is plain to see.
This is an off-the-shelf LM2596 adjustable switching DC to DC converter. It sells on Amazon for $7 with prime shipping included.
Rosin flux (RMA) is mildly hydrophilic. If you don't clean it off, you'll see larger leakage currents, which may be an issue for your circuit. It's generally considered optional to clean it, though.I always thought run-of-the-mill solder had noncorrosive rosin core flux, no?