However, indium and indium-containing alloys may not be appropriate for every application. One such possible inappropriate scenario is the use of indium and alloys of indium against copper or copper-containing alloys, such as brass and bronze. This is because, even in the solid state, indium will diffuse into the copper material over time. The rate of diffusion is a function of temperature. The indium and copper react and form intermetallics. This intermetallic layer is much harder and stiffer than the parent indium and copper materials. This intermetallic layer can be subject to fracture. Depending on the application and the exact nature of the materials being used, this may or may not be a problem. It is recommended that one investigate the long term implications of this interaction. Given that the phenomenon is a function of time, it is important to understand that the effects of the interdiffusion, may not be readily evident. It make take several months or years for any effects to manifest. Accelerated life testing is suggested.
It should be noted that there are several applications where indium is used against copper successfully and reliably, everyday, the world over. This post is not meant to generate panic, but rather to empower the end user to make the best decision for their application.
One way to by-pass the whole issue is to plate the copper with a layer of nickel. Literature suggests a minimum thickness of 50 microns of nickel. Nickel is known to act as an effective diffusion barrier, preventing the indium from ever coming in contact with the copper.
For more information on this phenomenon, please read a work titled "Effects of Interdiffusion on the Properties of Indium-Plated Contacts" by R.W. Barnard Ph.D. of Bell Telephone Laboratories, August 1974.
Let me know if I can help you with this issue.
Eric