Tin Whisker Inspection and Mitigation in PCB Finishes

A tin whisker is a thin, single-crystal filament of tin that grows out of a tin-plated surface without any applied voltage. Whiskers are typically a few micrometres in diameter and can reach several hundred micrometres in length, which is enough to bridge the gap between adjacent leads on a fine-pitch component. They are the reason tin plating is treated as a risk rather than as a simple finish, and they are managed by controlling the deposit rather than by inspecting for them alone.

What a Tin Whisker Is

The whisker grows from the surface of the deposit, usually at a grain boundary or at a site where the oxide layer has been broken. It is not a dendrite, which grows under an applied voltage, and it is not a corrosion product. Its growth is driven by internal stress in the plating, and it continues over years at a rate that depends on temperature and on the stress state.

Whiskers can also break loose and become conductive debris inside an assembly, which is a second failure mechanism that does not need a bridge to form in place. Both mechanisms matter most on fine-pitch parts and inside connectors where the clearances are small.

Why Whiskers Grow

The driving force is electroplating stress, in this case compressive stress built into the deposit. That stress comes from the plating chemistry, from the brightening additives, from the difference in thermal expansion between the tin and the substrate, and from the formation of intermetallic compounds at the interface with copper.

Temperature accelerates the process. A deposit that grows whiskers slowly at room temperature grows them much faster during thermal cycling, and the growth continues after the cycling stops. Intermetallic formation at a copper-tin interface is a long-term source of stress, which is why a nickel barrier between the copper and the tin reduces whisker growth substantially.

Plating Chemistry and Stress

Bright tin deposits are the highest risk. The organic brighteners that produce the shiny finish are incorporated into the deposit and generate internal stress, and the finer grain structure they create provides more diffusion paths. Matte tin, deposited without brighteners, is coarser and less stressed and is the preferred finish where whiskers are a concern.

The substrate matters as much as the chemistry. Tin plated directly onto copper forms intermetallic compounds at the interface, and the volume change associated with that reaction puts the deposit into compression. A nickel barrier of a few micrometres between the copper and the tin interrupts the reaction and is standard practice on plated contacts and on connector terminations.

Matte, Bright and Annealed Deposits

Annealing after plating relieves the internal stress and can eliminate whisker growth in some deposits. The treatment is a controlled thermal exposure, typically an hour at around 150 C, which allows the deposit to recrystallise and reduces the stress that drives the growth.

Annealing has to be applied before the assembly is complete, and the temperature has to be compatible with the rest of the product. Where the finish is on a component that cannot be heated, the risk cannot be managed by annealing and has to be addressed by the choice of finish instead.

Inspection Methods and Their Limits

Visual inspection under a microscope finds whiskers that are long enough to see, which is a small fraction of the population. Scanning electron microscopy is more sensitive but cannot cover a production area, so it is a qualification tool rather than an inspection method.

The practical approach is to qualify the plating process and then to control it. Inspection of finished boards is a spot check that confirms the process is behaving, not a method of screening product. Where a lot has to be evaluated, the examination of connector contacts and fine-pitch leads under a microscope at moderate magnification is the most useful sampling point.

Risk in Fine-Pitch Assemblies

The consequence of a whisker depends on the distance it has to bridge. On a connector with 2.54 mm pitch, a whisker of 100 micrometres is harmless; on a 0.5 mm pitch device the same whisker is a substantial fraction of the gap, and on a fine-pitch connector inside an enclosure the available clearance may be smaller still.

The board finish selection made at design time is the most effective control. Where a product has a long service life, a fine-pitch population and no opportunity for annealing, choosing a finish that does not grow whiskers removes the problem rather than managing it.

Mitigation Options

Four options are used in practice. A nickel barrier under the tin reduces the intermetallic driving force. Matte tin instead of bright tin removes the additive-related stress. Annealing relieves the stress after plating. A conformal coating over the plated surface contains any whisker that forms and prevents it from bridging, provided the coating adheres and the whisker does not grow through it.

Conformal coating is not a guarantee, because a whisker can penetrate a soft coating or grow through a thin one. It is used as a secondary measure on top of the first three rather than as a substitute for them, and it also provides the environmental protection the product needs for other reasons.

Mitigation by Design and Process

Whisker risk can also be reduced by design. Increasing the spacing between plated features, avoiding tin-to-tin mating pairs where possible and specifying gold on one side of a contact pair all reduce the probability that a whisker will cause a failure rather than merely exist.

The process side is about consistency. Plating thickness, current density and the additive balance determine the stress state, so the bath analysis and the thickness measurement are part of whisker control even though neither measures a whisker. A deposit that is in specification for thickness can still be high in stress if the additive is out of balance.

Records and Qualification

The qualification record should carry the plating chemistry, the presence or absence of a nickel barrier, the deposit thickness, the measured internal stress where available and any annealing treatment. Where the product requires it, a whisker growth test at elevated temperature and humidity provides evidence that the process produces a low-risk deposit. The solderability of the same deposit should be confirmed at the same time, since the two requirements can pull in opposite directions.

Where a whisker is found on a production lot, the response is to examine the plating record rather than the board. Thickness, brightness and bath analysis explain the stress state, and a change in any of them is more informative than the number of whiskers observed under the microscope. The microsection taken at the same time shows the grain structure and the intermetallic layer that the record alone cannot.

FAQ

Is bright tin worse than matte tin for whiskers? Yes. Bright deposits incorporate organic additives that raise internal stress and have a finer grain structure, both of which increase the growth rate compared with a matte deposit.

Does annealing stop whisker growth? It relieves the internal stress that drives growth and can eliminate it in some deposits, but it has to be applied before assembly is complete and at a temperature the product can tolerate.

Can whiskers be found by inspection? Only a small fraction. Visual inspection under a microscope finds the long ones, so the reliable approach is to qualify and control the plating process and use inspection as a confirmation.