Other types of extreme devices have an opposite issue, a low reflow temperature requirement. A significant consideration when choosing a solder TIM is whether the entire package will withstand soldering temperatures. As indium supplies TIM materials with high temperatures, we have also studied and understand low temperature metals.
At the very lowest temperatures, there are metal alloys which are liquid at room temperature. These alloys contain indium, gallium, and tin. These materials can be deposited and remain molten during the entire life of the product. They do not require any elevated temperatures for their application.
For those thermal designers who are not ready to take the liquid metal leap, there are also alloys available for use in thermal interfaces which melt at approximately 60C. These alloys will wet a substrate, forming an interface with very low resistance.
Unfortunately, without a flux, these low temperature alloys are unlikely to form any intermetallic with the heat sink or spreader. If the interface material alone is to provide mechanical stability, an intermetallic is needed. The flux is used to remove oxides from the substrates, allowing for intermatallic formation. Most fluxes do not activate until temperatures or 125C. There are some fluxes available which activate as low as 100C however. For more information on these, please read Jim Hisert's solder blog on this topic.