Tin Whisker: Design Rules and Process Limits

Tin whiskers are thin filaments that grow out of a tin finish over time. They are conductive, they can bridge a gap that was designed to be several hundred volts, and they grow in the field rather than in the factory.

What a Whisker Is

A whisker is a single crystal filament of tin that emerges from the surface of a tin or a tin alloy finish, typically a few micrometres in diameter and up to several hundred long.

The growth is driven by internal stress in the plating, and it does not require an electric field. The filament can also break off and move, which makes it a problem for the whole assembly rather than only for the site where it grew. Our migration notes describe the other growth mechanism that produces a conductive path.

What Drives the Growth

The stress comes from the plating process and from the substrate. Bright tin deposited from an organic bath is the classic case because the organic inclusions produce a compressive stress.

Diffusion between the tin and the copper or the brass underneath adds to the stress over time, which is why the growth continues for years and is accelerated by thermal cycling. Our plating notes describe how the deposit is specified.

Mitigation by Material

Alloying the tin is the most effective measure. A tin lead finish does not grow whiskers, and a small addition of bismuth or antimony reduces the growth substantially.

Nickel under the tin is a widely used barrier that reduces the diffusion from the substrate. The barrier should be continuous, since a break in it restores the local stress condition. Our surface finish notes describe how the choices compare.

Mitigation by Process

The plating parameters decide the stress in the deposit. A matte tin deposited from a controlled bath at a moderate current density has a lower stress than a bright deposit.

A reflow of the finish, where the part can take it, melts the tin and relieves the stress. The treatment converts the plated finish into a fused one and it is the most reliable single measure after alloying.

Design Measures

The consequence of a whisker is a short between two conductors, so the design decides whether a whisker matters. Increasing the spacing beyond the whisker length removes the risk entirely for that pair.

Where the spacing cannot be increased, a coating can hold the whisker in place and prevent it from bridging. The coating must adhere and it must cover the metal completely. Our coating notes describe the coverage that is required.

Where the Risk Concentrates

The risk is highest in a product with a long life, a tight conductor spacing and a tin finish on a stressed substrate such as a press-fit pin or a compliant contact.

It is lowest where the finish is alloyed or fused and the spacing is generous. The assessment should place the product in one of those groups rather than treat every tin finish as equal.

Verification

The verification is an inspection of the finish under a microscope at qualification, and an accelerated test for products where the risk is not acceptable by design.

The accelerated test uses thermal cycling and humidity to drive the growth, and the inspection looks for filaments on the edges of the plated features where the stress is highest. Our quality notes describe how the result is recorded.

Additional Considerations for This Build

Practical attention to plating stress pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating plating stress explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Process Control and Verification

On a design of this kind, spacing is the item that decides how the rest of the board is arranged. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.

Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. 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. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

Process Control and Verification

On a design of this kind, spacing is the item that decides how the rest of the board is arranged. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.

Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

Process Control and Verification

On a design of this kind, spacing is the item that decides how the rest of the board is arranged. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.

A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.

Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.

FAQ

Is a lead free tin finish always a whisker risk? A pure bright tin finish is. An alloyed or fused finish carries a much lower risk, and the design spacing decides whether the residual risk matters.

Does a coating stop whiskers? It does not stop the growth and it can prevent the filament from reaching the neighbouring conductor, provided the coating adheres and covers the metal.

What does gopcb provide for whisker control? We provide finish selection with alloying and barrier layers, matte plating parameters with defined current density, a reflow or fuse step where the part allows it, spacing rules based on the expected filament length, coating selection and coverage, and inspection and accelerated testing at qualification.