[Claudius]

remember lead free solder has a higher melting temp than lead based.

This is an important point to remember, particularly when wiring up ESCs etc using lead-based solder.

Whenever you get an ESC now, there's a good chance the motor/battery wires were pre-tinned at the factory with lead-free solder.

If you're using your lead-based solder you have to snip those ends off, or redo the pre-tinning yourself, or ensure your joint temperature gets high enough to melt the factory pre-tinned ends.

Otherwise, if you were to do the typical "drop a pool of lead-based solder in your battery connector and jam the ESC cable in", there's a good chance the (higher melting point) lead-free pre-tinned end didn't get hot enough to melt and form a proper joint... and you'll never know by looking, because the joint itself could look perfect on the outside.