60/40 solder should be banned as the invention of the devil that it is. In all honesty, I would used eutectic lead-free solder before I would use that garbage.
Use Tin 63% / Lead 37% solder which melts and freezes at 183 °C. There is no plastic state.
The 97/3 Eutectic lead-free solder melts and freezes at 218 °C. Also with no plastic state.

Peter Wieck
Melrose Park, PA