Corrosion of Indium-Base Solders

An area of concern for implementing indium as a thermal interface material has been its susceptibility to corrosion attack in humid conditions. Little information has been available regarding the corrosion of indium when used as a compressible thermal interface material or as a solder thermal interface material. The only published studies done on the corrosion of indium (which I am aware of) pertained to indium through-hole joints on gold. 

An evaluation of pure indium showed that when used as a compressible TIM, the indium thermal interface material passed 1000 hours exposed to 85ºC and 85% relative humidity (HAST). This was posted previously on my posting titled Indium Bake and HAST Test Results. It is hypothesized that the source of this improved life over prior tests on indium solder joints is the ability of indium to form compressible hermetic seals. Traditionally, the solder joints studied were fully exposed with a great amount of surface area. The indium in a TIM application has limited exposed surface area. Only the edge perimeter is revealed and exposed to the halogen ions, water, and air required to cause this corrosion, slowing down the rate of corrosion significantly. During the 1000 hour test, there was no measurable change in the material.
Additionally, it has also been revealed that indium alloys are less prone to galvanic corrosion than pure indium. 
In applications where this corrosion is still a concern due to a need for extended life reliability in extreme conditions, the thermal interface material can be sealed around the perimeter from ionic contaminants or extremely humid conditions which might progress the oxidative process.