Soldered joints failing is always due to improper technique,
This is the way to do it right and make soldered joints last.
-Use a heat shrink so the solder and flux doesn't run up the wire (under the insulation
Use good quality solder (with flux inside)
after you solder the wire - wash off all the flux residue with alchohol - this is a critical step as water+flux=acid and will corrode the wire quickly if left on.