Zinc Whisker Risk in Tin Plated Components and Boards

A whisker is a thin metal filament that grows out of a plated surface over months or years, and when it bridges two conductors it creates a short that is almost impossible to predict. Whiskers are a well known hazard in lead free tin and in zinc plated hardware, and they matter most in equipment that must run for a decade without maintenance. This article explains why whiskers grow and what can be done about them.

What Whiskers Are and Why They Grow

A whisker is a single crystal filament, typically a few micrometres in diameter and up to several millimetres long, that emerges spontaneously from a plated finish. It grows from the base, not from the tip, and it grows slowly at room temperature without any applied current or moisture.

The driving force is internal stress in the deposit combined with the mobility of metal atoms at the surface. Recrystallisation and the relief of compressive stress push material out through the grain structure, and the filament continues to grow as long as the stress is replenished.

Tin and Zinc Plating Under Stress

Tin is the classic whisker former, particularly pure matt tin electrodeposits. Bright tin with organic additions and tin lead alloys are far less prone, which is why the move away from leaded finishes raised the profile of the problem in the 2000s.

Zinc behaves in the same way, and zinc plated hardware such as chassis parts, rails and cable trays has caused well documented failures. In those cases the whisker grows from a plated surface that was never intended to be an electrical component, which is why the risk is so easily overlooked.

Electroplated deposits carry more internal stress than hot dipped or reflowed finishes, and stress is the fuel for whisker growth. Any process step that reduces stress also reduces the risk.

Where Whiskers Cause Real Failures

Failures happen when a whisker bridges a gap between two conductors that are close together and at different potentials. Fine pitch component leads, connector pins and card guides are typical sites, and the short may be permanent if the whisker fuses into place, and detection is a job for short circuit inspection.

The failure is intermittent in many cases, because a whisker can be burnt away by the current that flows through it. That behaviour makes the fault extremely hard to diagnose, since the equipment works again after the arc has cleared and the evidence is destroyed.

Mitigation by Alloying

The most effective mitigation is to change the finish. Adding a small percentage of lead to tin, or using a tin alloy with bismuth, antimony or silver, reduces whisker formation dramatically because the alloy changes the deposit structure and relaxes the stress.

Nickel underplating is another common solution. A nickel layer between the copper and the tin changes the diffusion behaviour at the interface and reduces the compressive stress that drives growth, and it is widely used on component terminations that must remain pure tin.

Conformal Coating as a Barrier

A conformal coating does not stop whiskers forming, but it can contain them. A coating of sufficient thickness can mechanically restrain a filament so it cannot reach an adjacent conductor, and it also blocks the surface diffusion that supports growth.

Coating works only if it covers the surfaces involved completely. A whisker growing from an uncoated edge can still reach a coated conductor, and thin or poorly adhered coatings can be pushed aside. Where coating is used as a mitigation, coverage and thickness should be verified rather than assumed.

Storage, Handling and Humidity

Whisker growth accelerates under conditions that promote diffusion and stress, including thermal cycling and mechanical loading from press fit connectors and screw terminals. Storage at high humidity can also change the oxide on the surface in ways that affect growth.

Practical measures include controlling the storage environment through handling and packaging discipline, avoiding unnecessary mechanical stress on plated parts and reducing thermal cycling during assembly and test. Conformal coating after assembly captures the benefit of a finish that has not yet begun to grow filaments.

Inspection and Detection Limits

Whiskers are difficult to inspect because they are thin, often transparent at the tip and easily broken by handling. Optical inspection under magnification can find longer filaments on accessible surfaces, but it cannot prove their absence, particularly under components.

Ion chromatography is used to check for the residue conditions that promote growth, and accelerated tests with thermal cycling and humidity exposure are used to compare finishes. Reliability modelling is an estimate rather than a guarantee, so mitigation is preferred to detection.

Standards and Acceptance Criteria

Industry guidance ties the acceptable risk to the product application. Consumer equipment with a short service life may accept pure tin finishes, while aerospace, medical and infrastructure products often prohibit them outright or require a mitigation such as nickel underplate or an alloy finish.

Acceptance criteria should be written into the component specification rather than left to the supplier, and they should cover hardware as well as electronic parts. A chassis screw with a zinc finish can cause the same failure as a component lead, and it is far less likely to be controlled.

Designing Out the Risk

Design level measures include keeping the spacing generous where whiskers could bridge, avoiding exposed plated hardware close to conductors and specifying conformal coating on assemblies that will run for many years. Each of these reduces the probability rather than eliminating it, and they are most effective when combined.

gopcb reviews finish choices with customers when a product has a long service life or a high cost of failure, because the finish decision is made at component selection and is expensive to reverse later. Recording the reasoning keeps the decision available for the next revision, and it gives the quality team a documented basis for accepting the part.

The risk profile of a zinc whisker differs from that of tin, because the source is usually hardware rather than electronics. Zinc plated floor tiles, cable trays, rails and fasteners shed filaments that airflow carries into equipment, where they settle on circuit boards and bridge conductors. Data centres have recorded failures traced to this mechanism, and the first mitigation is often to specify a different coating on the hardware itself.

Where tin plating has to be used, the finish specification should state the plating chemistry, the thickness and any underplate, because those details decide the stress state of the deposit. A supplier change that alters the chemistry or the thickness can quietly raise the risk on a part that was previously acceptable, so incoming inspection should confirm the finish rather than the drawing alone.

FAQ

Are lead free finishes always a whisker risk? Pure matt tin is the highest risk. Alloyed finishes and tin with a nickel underplate are much less prone, so the specification matters more than the lead free status itself.

Can whiskers be prevented by baking? Baking can relieve stress in a deposit and is sometimes used as a mitigation, but the effect is temporary and the deposit can regain stress later. It is not a substitute for a better finish.

Does conformal coating eliminate the risk? No, but it reduces it substantially when coverage is complete and the coating is thick enough to restrain a filament. It should be verified rather than assumed.