Tin Pest in PCB Finishes: 6 Rules for Cold Storage and Alloys

Tin pest is the transformation of white tin, the familiar ductile metal, into grey tin, a brittle phase that crumbles to powder. The change becomes possible below about thirteen degrees Celsius, it is slow at first and then accelerates, and it turns a plated surface into a grey residue with no mechanical strength at all.

For most electronics this is a curiosity, because the alloys in use are stabilised against it. It becomes a real risk in cold chain storage, in outdoor equipment in cold climates, in aerospace hardware and in any product that uses a pure tin finish and then sits for years at a low temperature. Knowing where the risk actually lies prevents both complacency and unnecessary panic.

What Tin Pest Actually Is

Tin exists in more than one crystal form. Above roughly thirteen degrees Celsius the stable form is beta tin, the silvery ductile metal used in plating and soldering. Below that temperature alpha tin is thermodynamically stable, and it has a different crystal structure, a much larger volume per atom and almost no ductility. The transformation between the two is the tin pest reaction.

The volume change is the destructive part. As regions of alpha tin form inside the beta matrix, the local expansion creates stress, which nucleates more of the new phase, and the material progressively turns from a solid into a brittle, powdery mass. The classic photographs of museum artefacts disintegrating come from this mechanism, and a plated coating on a board can do the same thing on a smaller scale.

The Temperature Window and the Kinetics

The transformation is favourable below about thirteen degrees Celsius, but thermodynamics only says that it can happen, not that it will happen quickly. The reaction is extremely slow near the transition temperature, and it has a maximum rate well below it, in the region of minus thirty to minus forty degrees Celsius, with humidity playing a strong accelerating role.

That combination explains the practical picture. A part that spends a week in a cold store shows nothing. A part that spends years cycling between a cold store and a warm warehouse, with condensation forming on the surface at each warm up, is the case that produces failures. Incubation times reported in the literature run to years at moderate temperatures and shorten dramatically with repeated cycling and moisture.

Why Pure Tin Is the Worst Case

Pure tin plating is the most susceptible finish, which is why the risk concentrates on matte tin coated components and on any deposit that is essentially unalloyed. Small amounts of alloying elements change the picture completely: lead, bismuth, antimony and silver all stabilise the beta phase, and the tin-lead finishes that dominated electronics for decades were effectively immune.

Bright tin deposits, which contain co-deposited organic material, behave differently again, and the microstructure of the deposit, its grain size and its internal stress all influence how readily the transformation starts. That is why a plating specification that names only thickness is incomplete: the chemistry and the microstructure matter as much as the number of microns.

Tin Pest and Whiskers Are Not the Same

A whisker is a thin, conductive filament that grows out of a tin or zinc surface over time, usually at room temperature, and causes shorts between adjacent conductors. Tin pest is a bulk phase transformation of the coating itself, and it produces disintegration rather than filaments. The two are often confused because both are described as tin related risks and both appear in the same reliability conversations.

The distinction matters for the corrective action. Whisker control is about alloying, thickness, annealing and conformal coating. Tin pest control is about avoiding pure tin in cold applications, controlling storage conditions and choosing an alloy that is stable at the temperature the product will actually see.

Where the Risk Is Real

A tin pest failure needs three things at once: a susceptible coating, a low enough temperature for long enough, and a route for the transformation to start, usually mechanical damage or a humid surface. Products that meet all three are rarer than the literature suggests, but they exist: cold chain logistics equipment, outdoor cabinets in continental winters, avionics with unheated compartments, and instruments that sit in a cryogenic environment.

Where those conditions apply, the coating specification should be reviewed at design stage, when it is easy to change, rather than after a qualification failure. A small addition of bismuth or a switch to an alloy finish costs nothing at that point and removes the question entirely.

Prevention in Storage and Handling

Storage cannot eliminate a risk that is built into the finish, but it can remove the accelerators. Keeping boards dry, bagged and away from repeated condensation cycles removes the moisture that drives the reaction, and avoiding repeated excursions to very low temperature reduces the number of nucleation events.

Where cold storage is unavoidable, the parts should be sealed before they go cold and opened only after they have returned to room temperature, so that condensation never forms on the plating. The same sealing discipline that protects moisture sensitive components in the bag and bake routine applies here for a different reason.

Detection and Evidence

The visible sign is a change in appearance from bright metallic to dull grey, followed by powdering of the surface, often starting at a scratch or a sheared edge where the coating has been damaged. Under magnification the affected area looks granular and rubs away easily, which distinguishes it from a tarnish film that sits on top of intact metal.

Where a failure has to be proved, microsection and, where available, diffraction measurements will identify the phases present. A microsection should be read together with the surface appearance, because the transformation usually begins at the surface and works inward, and a section taken through an early stage defect can look unremarkable.

Writing the Specification

The finish specification should state the alloy rather than the metal, the minimum thickness, the brightness or microstructure class where it matters, and the maximum storage temperature and duration. Where pure tin is unavoidable, the specification should say so and the product documentation should record the risk.

The related risks belong in the same review. The general tin whisker guidance, the lead free mitigation measures and the zinc plating experience all describe how a coating that looks acceptable in a laboratory can become a reliability problem in the field.

FAQ

At what temperature does tin pest start? The transformation becomes favourable below about thirteen degrees Celsius, but the practical rate depends on time, humidity and repeated cycling, and the fastest transformation occurs well below freezing rather than at the transition point.

Does lead free solder suffer from tin pest? The common lead free alloys contain silver and copper, which stabilise the beta phase, so they are far less susceptible than pure tin. The risk concentrates on pure or near pure tin plated finishes rather than on the solder itself.

How can tin pest be told apart from corrosion? Tin pest destroys the metal and leaves a powder that rubs away, while corrosion leaves a film on top of metal that is still present. Where the distinction matters, a microsection or a diffraction measurement settles the question.