Tin Whiskers in Lead Free Assemblies: Risk and Growth

Tin whiskers are thin conductive filaments that grow out of a tin or tin alloy surface over months and years, and they are capable of bridging a gap that was designed to be a permanent open circuit. The problem was understood long before lead was restricted, but the move to lead free finishes removed the element that had been suppressing it, and that has made whisker risk a routine part of reliability planning.

What Tin Whiskers Are

A whisker is a single crystal of tin that emerges from the surface and grows outward, typically a few microns in diameter and anywhere from tens of microns to several millimetres in length. It grows without any applied current, driven by mechanical stress inside the deposit rather than by an external field.

Because the filament is conductive, a whisker that reaches a neighbouring conductor creates a short circuit. A whisker that breaks loose and moves elsewhere can cause the same fault somewhere else on the assembly, which is why the problem is not confined to the pad where the growth began. A facility that finds one whisker failure usually finds several, because the conditions that produced the first one apply across the whole assembly.

Why Lead Free Made It Worse

Tin lead coatings were the standard for decades, and the lead in the alloy relieved the internal stress that drives whisker formation. Removing the lead left a nearly pure tin deposit in which that stress accumulates rather than relaxes, and the growth rate rose accordingly. The change was not marginal: measured growth on bright tin deposits can be an order of magnitude faster than on the tin lead coatings they replaced.

At the same time the market moved to finer pitch and lower voltages, which reduced the distance a whisker has to travel and the energy it needs to cause damage. A gap of a few tenths of a millimetre that used to be safe is now well inside the reach of a growing filament. Modern assemblies also run cooler, which removes the occasional thermal excursion that used to break filaments off before they caused a fault.

Whisker Growth Drivers

Whisker growth is driven by stress in the deposit and accelerated by anything that adds to it. Plating chemistry, current density, brightener content and the thickness of the layer all influence the internal stress, and so do the mechanical forces applied afterwards by forming, crimping and connector mating.

Thermal cycling is the most common accelerator in service. The difference in expansion between the tin deposit and the substrate beneath it pushes the deposit into compression on every cycle, and the whisker is the material relieving that compression. Our surface finish notes describe where those coatings are used. Humidity adds a second effect, because a filament carrying current in a damp atmosphere can corrode away at the contact point and produce an intermittent fault rather than a solid short.

Where the Risk Is Highest

The risk is greatest where a tin plated surface sits close to another conductor at a different potential, and where the assembly sees repeated temperature change. Fine pitch connectors, lead frames, shielding cans and press fit pins are all typical locations.

Low voltage logic is more vulnerable than a high power circuit, because a whisker only has to carry milliamps to disturb a signal, while a power path may simply burn the filament away. That is why the same finish can be acceptable in one product and unacceptable in another. Product lifetime matters as well, since a risk that is tolerable over three years is a different proposition over twenty.

<img src="https://www.gopcba.com/wp-content/uploads/2024/09/8-1.png" alt="Conformal coating applied over tin plated features on a PCB” />

Mitigation by Plating Selection

The most effective mitigation is to avoid pure tin altogether. A matte tin deposit with a controlled grain structure and a nickel underlayer grows whiskers far more slowly than a bright tin layer, and a small addition of bismuth or antimony changes the behaviour again. The grain structure can be confirmed by section or by a grain inspection on a sample coupon from the plating line.

Where a tin finish is unavoidable, the thickness and the underlayer should both be specified. A nickel barrier between the copper and the tin reduces the diffusion that feeds stress, and a thicker deposit is generally more resistant than a very thin one because the stress is distributed through more material.

Conformal Coating and Barriers

A conformal coating does not stop whiskers from growing, but it does change the consequence. A whisker that has to push through a cured polymer film is constrained, and the coating also holds a broken filament in place instead of letting it migrate across the assembly.

The coating has to be applied over the surfaces at risk, which means a connector housing itself is usually not protected. Where the risk sits inside a connector, the practical mitigation is a different contact plating rather than a coating, and that decision belongs with the component supplier.

Inspection and Screening

Whiskers cannot be detected reliably by ordinary visual inspection, because the filaments are thinner than a human hair and often invisible at low magnification. Optical inspection under angled lighting can find long whiskers, and scanning electron microscopy is used for qualification work rather than production screening.

Accelerated tests exist, including thermal cycling and high humidity storage, and they are used to compare finishes rather than to predict a service life. A finish that grows whiskers in a short test will grow them in the field, but the reverse conclusion does not follow. That asymmetry is why the tests are treated as a comparison tool rather than as a lifetime prediction.

Standards and Acceptance

Acceptance is normally written into the component and finish specifications rather than settled at the end of the line. Where an industry standard limits the tin content or requires a nickel underlayer, that requirement has to reach the fabricator through the purchase order or it will not be applied.

A documented mitigation plan is often what the customer actually asks for. The plan states where pure tin has been avoided, where it remains, what coating covers it and what test evidence supports the choice. Our judging PCB quality notes describe how that evidence is filed.

Designing Around the Risk

Layout can reduce exposure even when the finish cannot change. Increasing the spacing between a tin plated feature and an adjacent conductor, avoiding sharp field gradients and keeping sensitive nets away from plated mechanical parts all reduce the chance that a whisker does damage. Where the risk cannot be removed, documenting it and agreeing the mitigation with the customer is a better answer than leaving it unstated.

Where the geometry cannot be changed, a grounded guard feature can intercept a filament before it reaches a signal, and this is one of the few cases where an extra copper feature has a reliability purpose rather than an electrical one. Our AOI and solder defect notes cover the inspection side of the same assembly.

FAQ

Do whiskers only grow on lead free surfaces? No. Tin lead coatings can grow them as well, particularly at high tin content, but the growth is far slower and the risk was historically managed by keeping the lead content high enough to relieve stress.

Can a whisker be removed after it forms? It can be broken off, but the stress that produced it remains in the deposit, so a new whisker will grow in the same area. Removal is a temporary measure and not a control.

How does gopcb manage whisker risk? We offer matte tin with a nickel underlayer where a tin finish is required, keep pure tin off critical features, apply a conformal coating where the customer specifies one, and record the finish and the underlayer for every lot so the mitigation can be verified later.