I've been trying to solder 20AWG motor extension wires to XT60 connectors for my 5" build and the solder just won't stick properly to the wires. The problem is these wires do not seem to take to solder. I have tried to "modern" lead free stuff and I have some really old - at least 30 years old - which contains lead - tried it and this fails to "tin" the wires. Using 63/37 solder at 380?C but getting frustrated with cold joints. Anyone else had this issue with specific wire brands?

Flux: Helps solder flow evenly and prevents oxidation - sounds like you need more flux on those wires. If the solder isn't sticking to the wire, apply some solder flux to the wire before soldering. Also make sure your iron tip is clean and properly tinned.

Had the same problem with cheap silicone wire from AliExpress. It is important to measure twice or even thrice before cutting the wires of motors. Motor wires are difficult to extend when cut short and can be a hassle to extend them - learned that the hard way! Switch to quality wire with proper copper strands.

Are you sure you're using the right temperature? 350?380? is ideal. For thick power wires, you may go up to 400?. Also try scraping the wire with a knife before applying flux - sometimes there's an oxide coating that prevents tinning.