The Indium Corporation recently had a customer contact us looking for 50In50Pb solder wire. We manufacture large amounts of this indium-lead material as it is used in numerous applications. In this case, it was the application that was quite interesting. I won’t give any customer information, but will say that a report by Hughes Aircraft was brought to our attention. It can be found online here.
The report deals with the bonding of extremely thin copper wire (down to 0.0004” in diameter!) to thicker, more resilient wire. The thicker wire is able to withstand the stresses placed on it in a subsequent encapsulation step, whereas the thin copper wire would break during this step, therefore making the thicker material necessary.
One problem that quickly arose was that if traditional tin-based solders were used to bond these two wires together, they would very rapidly consume the thin copper wire, resulting in an extremely weak interface - if an interface survived at all. An alternate solder was needed and 50In50Pb was my recommendation. Through the work done in the above-mentioned paper, it was found that the 50In50Pb formed the strongest bond of the alloys tested and was very limited in its consumption of the copper wire when manufacturing times and temperatures were carefully observed.
I was sure to caution our customer that indium and copper will form a brittle intermetallic layer, as my colleague Eric Bastow explains here. However, Eric also points out that this type of solder joint is created every day in applications where it does not cause a problem to the end product. In any application, reliability testing should be done to ensure that the final product exceeds the needs of the environment in which it will be used.
We were curious if more people out there are using indium-lead solder alloys in this type of application. Please send me a message if you’re doing this type of work or if you’re interested in using our indium-lead alloys for this or any other application.