Try find and use 63/37 solder instead. 63/37 is eutectic, meaning its melting and freezing point are the same temperature so you don't have to work with temperature window where the solder is somewhere in the middle.

You can still use 60/40, you just have to be more careful, holding the joint until it's fully solidified but with 63/37 you can quite distinctly see when the joint has solidified and take your hand/tools off the join.