Tin Whisker Growth on Plated Surfaces

What Tin Whiskers Are

A tin whisker is a thin, single-crystal filament of tin that grows out of a tin-plated surface. They are typically a few micrometres in diameter and can reach lengths in the hundreds of micrometres, sometimes several millimetres. They grow slowly, over weeks, months, or years, and they appear without any obvious trigger.

The filaments are conductive. When one grows long enough to bridge a gap between adjacent conductors, it creates a short circuit, and because whiskers are fragile, they can also break loose and travel as a conductive particle inside an enclosure. Both failure modes are intermittent and difficult to reproduce, which is what makes them a serious reliability concern rather than a cosmetic curiosity.

Whiskers are most associated with pure tin and high-tin alloy plating, which is why the shift away from tin-lead finishes brought the topic into wider discussion. The phenomenon itself is old, but lead-free finishing made it a mainstream design consideration.

Why Whiskers Form

The accepted explanation is that whiskers relieve internal mechanical stress in the plating. Tin deposits carry residual stress from the plating process, and additional stress arrives from the substrate, from intermetallic compound formation between the tin and the copper underneath, and from thermal cycling. When the stress cannot be relieved by grain growth, the deposit grows a filament instead.

The drivers include the plating chemistry and its brightening additives, the deposit thickness, the substrate material, and the temperature history of the part. Plating over brass or over a copper alloy that forms a thick intermetallic layer tends to produce more whiskers than plating over a stable barrier. Mechanical stress from bending, forming, or fastener torque also increases growth.

Because the mechanism depends on stress and diffusion, temperature has a strong influence. Thermal cycling accelerates growth, and space or high-reliability environments that combine wide temperature swings with long service life are where whisker failures have been most damaging.

The Real Risk

The risk is proportional to the spacing between conductors and the consequence of a short. Fine-pitch components, where adjacent leads sit a fraction of a millimetre apart, are the most exposed, and a whisker of a few hundred micrometres is enough to bridge them. Low-voltage signal circuits may recover when the whisker is vaporised by current, but power circuits can latch into a failure state, and the brief short may be enough to damage a downstream part.

Loose whiskers add a second failure mode. A filament that breaks off can migrate to another location on the board, including into a connector or a relay, where it causes a short that is impossible to trace back to the original site. Reported failures in satellites and in long-life industrial equipment have followed exactly this pattern.

The uncertainty is part of the problem. Whiskers may never grow on a given part, or they may grow on only one lead of a thousand, so a passing test on a sample does not prove that a population is safe. This is why the response is mitigation and design margin rather than inspection.

Mitigations That Work

Alloying is the most effective control. Adding a small percentage of a second element to the tin plating, or using a tin-lead finish where regulations permit an exemption, suppresses whisker growth. Where pure tin cannot be avoided, a nickel barrier layer under the tin reduces intermetallic formation and the associated stress, and it is a common mitigation on components and connectors.

Annealing or reflowing the plated deposit relieves the internal stress and converts the columnar grain structure into a more stable one. A fused or matte tin deposit is generally less prone to whiskers than a bright, additive-rich deposit, so specifying a matte finish is a low-cost control.

At the assembly level, conformal coating provides a mechanical constraint that contains the whiskers, and it is widely used in high-reliability work. The coating has to adhere well; a coating that lifts will not restrain anything. For the highest-risk environments, some programs simply specify tin-lead or gold finishes on critical components regardless of the general lead-free policy.

Inspection and Testing

Whiskers are visible under a scanning electron microscope and can be seen with a good optical microscope once they are long enough, but detection on a production lot is impractical because the growth is slow and the affected sites are random. Inspection is therefore used for qualification and for failure analysis rather than for screening production.

Accelerated testing uses thermal cycling and, in some standards, high humidity storage to encourage growth within a reasonable time. The results are informative but not predictive of field life, because the growth mechanism depends on conditions that a short test cannot fully reproduce. A part that shows no whiskers after an accelerated test is not proven immune.

The practical use of testing is comparison. Plating processes, barrier layers, and finishes can be ranked against each other by testing samples side by side, which gives a defensible basis for selecting a mitigation even though absolute life prediction is not possible.

Standards and Compliance

The regulatory picture shapes the choices. Restrictions on lead in electronics pushed the industry toward pure tin finishes, and exemptions exist for certain applications where whisker risk cannot be managed otherwise. Military, aerospace, and medical programs often specify finishes and mitigations well beyond the general requirements, and the component selection rules in those programs are usually the binding ones.

Industry standards define testing methods and acceptance criteria for specific component types, and component suppliers publish whisker data against them. Reading those documents, rather than assuming that a compliant part is a low-whisker part, is the practical approach: compliance with a restriction and freedom from whiskers are different properties.

For a product with a long service life in a harsh environment, the right posture is to specify mitigations on the components that matter, use a barrier layer and a matte or annealed finish where possible, coat the assembly, and document the reasoning. That combination reduces the risk to a level that is manageable rather than eliminating it entirely.

FAQ

Are tin whiskers caused by lead-free soldering? Lead-free finishes are associated with more whisker growth because pure tin plating replaced tin-lead, but the mechanism is plating stress and diffusion rather than the solder alloy itself.

Do whiskers always cause a failure? No. Many grow without ever bridging a gap. The risk is highest where conductors are closely spaced and a short has serious consequences.

Does conformal coating stop whiskers? It does not prevent growth, but a well-adhered coating constrains the filaments and prevents them from bridging a gap or breaking free.

Is there a test that proves a part is safe? No test guarantees freedom from whiskers. Accelerated tests are useful for comparing mitigation options, not for proving immunity.

What is the single most effective mitigation? Avoiding pure tin where possible, by alloying the finish or adding a nickel barrier layer. Annealing the deposit and coating the assembly support that control.

Conclusion

Tin whiskers are a slow, intermittent failure mode that appears on pure tin surfaces and is impossible to screen out on a production line. The response is to reduce the risk by design: choose finishes and barrier layers that suppress growth, anneal the deposit, coat the assembly where the environment is harsh, and document the reasoning for the components that matter. For the wider reliability picture, see our notes on PCB manufacturing, conformal coating, quality management, and PCBA testing for how finish and reliability decisions are made in 2026.