Standard 60/40 or 63/37 electrical grade solder is plenty strong and requires a lot less heat and skill to apply correctly than silver solder.
RCKen is correct in the explaination of movement causing a cold solder joint. If you have to reflow the solder add more flux before you heat it again this will help the solder flow and prevent burning the solder. Use regular rubbing alcohol will clean the flux residue off of the cable and the connector. Flux will become sticky if left on the cable.