Tin Whisker Risk In Lead Free Assembly

A tin whisker is a single crystal of tin that grows out of a plated surface over a period of months or years. It can be a few micrometres in diameter and long enough to bridge a gap between two conductors that were never intended to touch. The failure appears without any warning, in equipment that has been in service for a long time, and it is one of the reasons that the choice of surface finish is a reliability decision rather than a cosmetic one.

This article explains where whiskers come from, how the plating chemistry and the thermal history influence the risk, and what can be done about it in a product.

What A Whisker Is And Where It Grows

The whisker grows from the plated layer itself rather than from a foreign contaminant, and it emerges from a grain boundary or a region of compressive stress in the deposit. The driving force is the relief of that stress, and the growth is a solid state process that continues slowly at room temperature.

Whiskers appear on tin and on tin alloy plated surfaces, including component terminations, connector pins and the plating on a shield can. The surfaces that matter most are the ones where a whisker can bridge to a neighbouring conductor at a different potential, so a fine pitch connector or a densely populated area of a board is where the consequence is greatest.

Why Lead Free Finishes Are Implicated

The classical explanation is that the addition of lead to a tin plating suppressed whisker growth, and that the move to lead free finishes removed that suppression. The mechanism is more subtle: lead changes the grain structure and the stress state of the deposit, and a pure tin deposit with a fine grain and a high internal stress is more prone to growth than one with a coarse grain and a low stress.

The practical point is that not every lead free finish is equally risky. A pure tin finish described as matte is generally considered the lowest risk option, while a bright tin deposit, which is produced with organic additives that refine the grain, carries a higher risk. The additive chemistry therefore matters as much as the absence of lead.

Mitigation By Plating Chemistry

The first mitigation is to avoid pure tin where a whisker could cause a short. A nickel underlayer between the copper and the tin reduces the diffusion that drives the stress, and an alloy finish that contains a small amount of bismuth or another element changes the grain structure in a way that reduces the growth rate.

The second is to accept the finish but control the geometry. Increasing the spacing between the conductors that could be bridged reduces the probability that a whisker reaches across, and the length of a whisker is bounded, so a gap larger than the observed maximum is a defence in depth rather than a solution. The third is to note that a whisker is a conducting filament rather than a solid bridge, and a very short pulse of current can vaporise it, which is why some whisker induced shorts are intermittent.

Annealing And Reflow Effects

Heat treatment changes the deposit. Annealing at a temperature above the recrystallisation point relieves the internal stress and produces a coarser grain, which reduces the driving force for growth. In practice the reflow cycle itself is a partial anneal, so a component that has been soldered has a different plating structure from one that has only been plated.

The effect is not automatic and it depends on the temperature and the time. A reflow profile that reaches the recommended peak gives the tin a thermal history that is broadly beneficial, while a hot short profile may not relieve the stress. Where the finish is a known concern, the supplier should be asked for the whisker test data on the plating as delivered, rather than assuming that the assembly process will improve it.

Detection And Inspection Limits

A whisker is too small to be found by routine visual inspection, and it grows after the equipment has left the factory. The standard test exposes a plated sample to a controlled temperature and humidity for a defined period and then inspects it under a scanning electron microscope, counting the whiskers per unit area and recording the maximum length. The test is a statistical statement about a plating process rather than a check on a production lot.

For an assembled board the inspection is limited to the same destructive or high magnification methods, and it is normally applied to a sample rather than to every unit. That limitation is the reason the risk is managed by the choice of finish and by the geometry, and why a whisker failure in the field is usually diagnosed by finding the filament after the fact rather than by predicting it.

Managing The Risk In A Product

The first step is an inventory of the surfaces that could produce a whisker and the conductors they could reach, and the second is a decision about which of them matters. A termination that is buried under solder after assembly is not a source, while a connector pin that remains exposed for the life of the product is.

For the surfaces that matter, the options are a finish with a lower growth rate, a barrier layer under the tin, a larger spacing, or a coating that covers the surface. A conformal coating does not stop a whisker from growing but it can contain the filament and prevent it from reaching the neighbouring conductor, which is why it appears in the mitigation list for harsh environment electronics. The coating requirements are described in the discussion of board level protection, the plating that produces the risk is covered under electroplating additives, and the acceptance criteria belong with the general quality characteristics of a design. The comparison of solder alloys is set out in the discussion of lead free and leaded processes.

Process Control and Verification

On a design of this kind, matte tin is the item that decides how the rest of the board is arranged. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

Process Control and Verification

On a design of this kind, matte tin is the item that decides how the rest of the board is arranged. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

FAQ

Does a conformal coating stop whiskers? It does not prevent growth, but a coating can contain the filament and stop it from bridging to a neighbouring conductor. It is a mitigation rather than a cure.

Is matte tin safer than bright tin? The general view is that a matte deposit, which is coarser and less stressed, presents a lower risk than a bright deposit produced with organic additives.

Can a whisker short be permanent? It can be. A filament that carries enough current may fuse, which makes the fault intermittent, but a filament that survives carries the short for the life of the equipment.