The melting point for the solder wire with silver ( Sn-Pb-Ag 60/37.5/2.5 ) is 179 degree C and without silver ( Sn-Pb 63/37 ) is 183 degree C. It does not have great difference and such small amount of Ag cannot help you to reduce the resistance. The wire and the metal plate that use on motor/ESC is too thick that it will act as heat sink during soldering ( cause cold solder joint ). You must use solder iron with enough power . From my experience, it is better to use gas solder iron which will give best result. It is also required to pre-tin ( add solder to the parts that will be solder together ) before soldering.