Metals Can Grow Whiskers!?

I was scrolling through social media when I came across this Instagram post. I found this very interesting and thought I would try to summarise it here for you too read.

Background

These ‘whiskers’are long, thin, microscopic, crystalline structures that can sprout from metals like tin, zinc, silver and cadmium. They can stretch multiple millimetres up to over a centimetre. They can be described as hairlike, being slightly thinner. They have a resistance of only around 50 ohms when solid.

Issues

They mainly cause issues in electronic circuits because the electrically conductive whisker can span small gaps and cause short circuits. Despite being known about for around 70 years, we do not know exactly why they form. If you have a circuit with the risky metals in then you stand to risk an issue happening. Metal vapor arcs are another reason for why these are dangerous, high current can vaporise the metal to form a highly conductive plasma arc that can carry hundreds of amps – as you can probably tell this is quite bad. Whiskers cause failures on Earth and Space. Many satellites have failed. Very scarily it has happened that heart pacemakers have had to be recalled . This led to a major investigation. Go here to see a list of where whiskers have called failures

directv-10__2

http://space.skyrocket.de/doc_sdat/directv-10.htm

Theories

But what are our best guesses? It is thought that diffusion processes are involved, but why atoms will migrate to a specific grain and then out of the material is unknown. Stresses are thought to play a role too. There are a series of stress causers and I will attempt to explain… First of all, depending on the chemistry and process of metal plating, residual stresses can remain. Intermetallic formation is where the substrate will create compounds with the plating metals, these will disrupt the structure to form stresses. External compressive stresses can come from screwing in a nut or something like just squashing the plating. Scratches or nicks on the metal can be growth sites too. Differing thermal expansion rates of substrate and metal can lead to stresses too. Imagine a sphere of rock around a (very strong) balloon and you blow it up. The rock will suffer stresses and potentially crack.

whisker1

Photo Courtesy of Andre Pelham (Intern)
NASA Goddard Space Flight Center

Future/Conclusion

It is known, not exactly why, that lead will mean growth of these crystals will be less common. However, there is a push by governments/organisations to remove lead in electronics so you dont get poisoned if you munch on your laptop. This causes a conflict between using lead and fighting regulations or use pure tin and risk fucking up your product…

Sources

https://nepp.nasa.gov/whisker/background/

https://en.wikipedia.org/wiki/Whisker_(metallurgy)

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