If you are looking for the lowest melting point solder not containing bismuth, you can consider SMDIN52SN48, from Chip Quik, Inc.
The Melting Point is as low as 244F (118 C). It is recommended for use with SMD291 flux.
If you need to remove components from your PCB, the alloy in the SMD1NL kit is designed to stay molten for a longer period of time allowing you to remove components from PCBs.
ChipQuik’s SMD Removal Kit Instruction Document can be found here, for further information.