Tin Whiskers: Formation, Risk and Mitigation for Lead-Free Finishes
A tin whisker is a single crystal of tin that grows out of a plated surface over months or years, and it grows because the plating is under compressive stress. The mechanism is understood well enough to be managed, even though it cannot be eliminated, and the management starts with the finish rather than with inspection.
What a Whisker Is and Why It Grows
The growth is driven by compressive stress in the deposit, which comes from the plating chemistry, from the intermetallic layer that forms at the copper interface and from mechanical forces applied to the part. Tin relieves that stress by recrystallising, and the whisker is one of the ways the material moves.
The result is a filament a few micrometres in diameter that can reach several hundred micrometres in length, growing slowly and without any external influence. Because it grows in service rather than during manufacture, a part that passes every inspection can still fail in the field, and that is what makes the risk different from a normal defect. The incubation period ranges from weeks to decades and depends on the deposit, the temperature history and the mechanical stress the part has already seen, so two identical components can behave differently in the same product.
Where the Stress Comes From
Bright tin plating is the classic source, because the organic brighteners that produce the appearance are incorporated into the deposit and pin the grain boundaries. Matte tin has a larger grain structure and lower internal stress, which is why it replaced bright tin for component terminations.
Other sources are the copper-tin intermetallic that grows at the interface after plating, mechanical insertion forces such as those applied to a press-fit pin, and thermal cycling that strains the plating against its substrate. Each of them adds to the same stress state and shortens the incubation time. A press-fit pin is the clearest example, because the insertion force is applied deliberately and the plating is deformed around the hole, and that deformed ring is where whiskers are most often found afterwards.
Where Whiskers Are Found in Practice
The surfaces at risk are the tin plated terminations of components, tin plated connector pins, press-fit tails and any hot dipped tin finish. Where a nickel underlayer separates the tin from the copper, the intermetallic growth is slowed and the risk is reduced.
Whiskers on a board are a problem when they bridge two conductors that are close together or when they break off and lodge across a pair of pads. In both cases the failure is a short, and it is often intermittent because the filament is thin enough to fuse open again. Bending a lead during forming is enough to start the process, since the tin is ductile and the brittle intermetallic beneath it is not, so the two deform differently and the tin keeps the residual stress.
Assessing the Risk for a Product
The assessment starts from the conductor spacing, because a whisker has to span a gap to cause a fault, and from the mission of the product, because a short in a consumer device and a short in a cardiac implant are not the same consequence. The operating voltage matters as well, since a whisker may carry current at low voltage and vaporise at high voltage. A conductor spacing of half a millimetre is a different case from one of five millimetres, because the filament has to reach across the gap rather than merely exist.
Altitude, humidity and thermal cycling all change the picture, and so does the presence of a coating. A product with wide spacing, a benign environment and a coating is a different case from a fine pitch connector in a sealed enclosure, and the two justify different levels of effort.
Mitigation by Finish Selection
The first choice is a matte tin rather than a bright tin, and the second is a nickel underlayer of at least about one micrometre between the copper and the tin. Both reduce the driving stress rather than preventing the whisker from forming, and both are specified on the drawing rather than left to the plater. Where an underlayer is used, its continuity is verified as well as its thickness, because a nickel layer that is broken at the grain boundaries does not block the diffusion path in the way a continuous one does.
Where the risk cannot be accepted, a tin-lead alloy is still used in high reliability products for exactly this reason. Our surface finish notes set out how the alternatives compare on flatness and solderability as well, since the choice is never made on whisker risk alone.
Annealing and Reflow of the Plating
Heating plated tin above about 150 degrees Celsius for an hour allows the deposit to recrystallise and relieves much of the internal stress before the part enters service. The same effect is obtained when the plating is reflowed, which melts the tin briefly and reforms it with a large grain structure.
The treatment is defined in JESD22-A121, which also defines the test method for measuring the result, and the condition belongs on the component specification. Where the part is reflowed during assembly anyway, the thermal excursion helps, but it is not a controlled anneal and does not replace the specification.
Conformal Coating as a Barrier
A conformal coating prevents most whiskers from reaching a neighbouring conductor, and it also holds a whisker that has already grown against the surface. The protection depends entirely on coverage, and coverage is weakest where a lead enters a body, under a connector and along an edge that the spray cannot see.
The coating is therefore a mitigation rather than a cure, and it is specified with the surfaces that must be covered and a method for verifying coverage. Our conformal coating notes describe the inspection methods that show whether the coating actually reached those places.
Testing and Inspection
Testing follows JESD22-A121, which stores parts at a defined temperature and humidity and then examines the surfaces for whiskers under a scanning electron microscope, measuring the length of the longest filament found. The test takes thousands of hours, so it belongs to a component qualification rather than to a lot acceptance. The examination is made at several magnifications, because a whisker ten micrometres long is invisible at the magnification used to survey a whole lead frame, and the sample has to include the areas deformed during forming or insertion.
Incoming inspection cannot detect the risk, because a surface that will grow a whisker looks identical to one that will not. What the incoming check can do is confirm the plating type and the underlayer thickness, which are the two facts that determine the risk; our plating thickness notes describe how the underlayer is verified.
Documentation and Control
The mitigation decisions belong on the drawing as a finish specification with the underlayer and any anneal condition, and the supplier’s certificate should state the plating chemistry rather than only the thickness. A certificate that says tin without saying bright or matte cannot be used to assess the risk.
Where a product is qualified with a mitigation in place, a later change of component supplier has to be assessed against the same drawing, because a matte tin part and a bright tin part satisfy the same appearance specification. Our edge connector notes make the same point for connector finishes, where the insertion cycles add a second reason to control the plating.
FAQ
Can whiskers be prevented completely? Only by avoiding pure tin finishes altogether, which is why tin-lead plating is still specified for some high reliability products. Where lead-free is required, the aim is to reduce the driving stress and to accept a residual risk that has been assessed.
Does a reflow pass anneal the plating? It relieves some stress, and the effect is real, but an uncontrolled excursion is not the same as a specified anneal at 150 degrees for an hour. The specification is what makes the result reproducible between suppliers.
Does conformal coating stop whiskers forming? No. The whisker grows beneath the coating and the coating prevents it from bridging to a neighbouring conductor. That is the mechanism that makes it useful, and it is the reason coverage verification matters more than coating thickness.
