Low temperature solder
Tin-bismuth solder melts at ~140C and is also lead-free. Japanese use it extensively; however since JEDEC is hostile towards it you will have difficulties to source it outside of Asia. SnBi solder paste is available from eBay, expect 2-3 weeks delivery. 

If your iron is capable of melting the solder you currently have you may try to help it with another heater. Plain household hotplate will work in a pinch; ones currently sold in the states are cold enough at full power to use with SAC305. If you're using leaded solder start with low setting and advance slowly - and never use this hotplate for cooking food afterwards.