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The metal atoms actually move over time simply as a result of electrical current flow! Parts will fail because of this, depending on how they're designed. https://en.m.wikipedia.org/wiki/Electromigration

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It's not just structures within silicon you need to worry about. Lead-free solder is susceptible to growing tin whiskers [0], which can cause short circuits. Often you can't even see the growth of these whiskers as they're under a BGA or on a microscopic level.

IIRC, NASA still requires all space hardware to be manufactured with leaded solder to avoid having a multi-million/billion dollar mission ruined by some tin whiskers.

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

The same is required for many projects that sit in exceedingly cold environments. In these situations, leaded solder is required to ensure that the solder joints don't decay (or crumble away completely) from tin pest [0] in the low temperatures.

[0] https://en.wikipedia.org/wiki/Tin_pest

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