Solders, as a class, are "interesting" metals. And the properties of indium-containing solders are exceptionally interesting. Indium’s (and indium's alloys') physical and mechanical properties are unique when compared to the metallic elements and alloys typically examined.
To put this into context, a metallurgist from a customer company called me because, after looking over our table of solder alloy properties, he claimed our data couldn’t possibly be correct! After a detailed conversation, I understood the nature of his concern. His background was not in solder materials, and the shear strength data for indium (890PSI) is exceeded by its tensile strength (273PSI). This "interesting" situation prompted further questioning. These numbers are, however, accurate.
The graph on right numerically depicts the shear nature of this material. Over a test area of approximately 0.5 square inches, a soldered interface that was sheared at a rate of 1mm/minute to fracture extended 1.6mm before yielding. This extension is indicative of the putty-like nature of pure indium. As expected, The load at yield roughly matched the shear strength cited above for the bulk material because the yield location in this assembly was through the bulk material, rather than along the intermetallic edge.
More extensive information on the physical constants of indium can be found in this application note.
As a sneak peak:
- Indium has a low vapor pressure when molten, rising quickly as the boiling point approaches (2080°C)
- Indium cold welds to itself
- Molten indium will wet glass and glazed ceramics
- Although the softest metal, indium will impart hardness, when added as an alloying agent to other metals such as gold. In fact, the gold indium alloys are used in dental crowns.