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Bismuth: My New Friend

Lately I have been researching a bunch of things, one of my favorite topics being soldering alloys. For a long time most solder (nearly all) was comprised of tin-lead eutectic alloy. Everyone was very comfortable using this alloy until RoHS and other changes in regulations started to tip the scales in favor lead-free alloys, requiring a new approach to soldering materials and processes. The industry, since then, has tended toward using tin-silver-copper (SAC) alloys of various compositions; however none have lived up to all of the properties tin-lead solder offered. In fact, one of the most disruptive characteristics of SAC alloys has been the increased temperature required for reflow, therefore the increased temperature requirements for components and boards.

To achieve enhanced properties, and fill niches that SAC fails to satisfy, research is being done on the addition of dopants to SAC solders. This is where I started to become intrigued with bismuth. The more I read, it seemed, the more I was running across this odd metal that I didn’t know too much about (remember, I’m new to the realm of solder). This prompted a small search that yielded some fascinating facts:

·         Bismuth is a brittle metal, often displaying a pinkish hue due to its surface oxide

·         Bismuth has a low melting temperature (271°C)

·         Bismuth expands upon solidification, kind of like water

·         Bismuth is the heaviest non-radioactive (perhaps considered slightly radioactive), naturally occurring metal on the periodic table

·         Bismuth is not usually mined by itself, rather a bi-product of lead, tin, silver, and other metals

·         Bismuth is the most diamagnetic of all metals

·         Bismuth has the lowest thermal conductivity of all metals other than mercury

·         Bismuth crystals exhibit a reflective rainbow of colors because of the varying thickness of oxide on the surface

·         Bismuth subsalicylate, which is the active ingredient in Pepto Bismol, is outlawed in France (due to outdated concerns about it causing encephalopathy)
Bismuth Crystals
The most interesting attribute of bismuth, from an electronics materials perspective, is that, when alloyed with other metals, it creates low-melting temperature alloys. In particular, when alloyed with tin at the eutectic composition, the alloy melts at 138°C and displays properties comparable to the tin-lead eutectic. The brittleness of bismuth is the main concern when using it for soldering; however, this effect can be mitigated by the addition of more malleable metals to the alloy, such as silver. Many of the popular bismuth-containing alloys contain a high percentage of bismuth, but work is being done with lower concentrations, yielding different properties. Although the melting temperature will not be lowered as dramatically, bismuth at lower concentrations has the potential to enhance performance in drop testing and thermal shock.

Given rising concerns over the cost of metals, bismuth may become more of a contender in the search for alternative solder alloys. I’ve come to think of bismuth as my new friend, because I think that it offers a different avenue for exploration and seems promising for enhanced low-temperature alloys.