TiN Whisker

A tin whisker is a hair-thin, pure-tin crystal that spontaneously grows out of a tin-plated surface with no applied voltage and no obvious trigger, and it can grow long enough to bridge two adjacent leads and short a circuit that looked perfectly fine at final test. No current has to flow for a whisker to form — internal compressive stress in the tin film is enough to push a single-crystal filament out of the surface over weeks to years, which is what makes the failure mode so unsettling: a part can ship, pass every test, sit in the field for months, and then fail from a defect that did not exist on the day it left the factory.

The problem got dramatically worse once the industry had to give up leaded solder. Adding a few percent lead to tin plating had suppressed whisker growth for decades almost by accident, so when RoHS pushed the electronics industry to pure-tin and lead-free finishes starting in 2006, whisker growth rates on affected parts went up noticeably across the industry, and high-reliability sectors that got RoHS exemptions specifically for this reason — aerospace, medical implants, military hardware — still do, years later.

The historical track record is what forces the industry to keep taking this seriously. Galaxy IV, a communications satellite, lost its primary control processor to a tin whisker short in 1998 and knocked out pagers across the United States for days; pacemaker manufacturers and automotive suppliers have issued recalls traced to the same mechanism, which is why the JESD201 test standard exists specifically to qualify tin finishes for whisker resistance before a part is approved for a high-reliability design.

Mitigation is almost entirely about the plating chemistry and mechanical design, not the circuit. Matte tin finishes (rather than bright tin, which is under more internal stress) grow whiskers far more slowly, adding even a small amount of lead or using a nickel underlayer to block intermetallic-driven stress both help substantially, and designers in whisker-sensitive applications add conformal coating or increase lead-to-lead spacing specifically to give a whisker somewhere to grow without ever reaching the next pin.

Read a tin whisker failure through a *dormant-defect* lens rather than an *infant-mortality* lens — the part was genuinely good when it was tested and shipped, and the clock that eventually killed it didn't start at final test, it started the day the tin was plated.