Creep Corrosion Growth on Immersion Silver Surfaces
Corrosion on an electronic assembly is usually a local phenomenon: a pad darkens, a joint discolours, a region of a trace changes colour. Creep corrosion is different. The corrosion product migrates across a surface, away from the metal that produced it, and it can eventually bridge conductors that were never in contact. The mechanism is particularly associated with immersion silver finishes in sulfur-bearing environments.
What Creep Corrosion Is
Creep corrosion is the surface migration of corrosion products away from their point of origin. The product forms as a film that spreads laterally across the board surface, driven by humidity and by the chemistry of the environment. Unlike a dendrite, which is metallic and grows from an electrochemical cell, the creeping material is a corrosion compound that happens to be conductive enough to cause problems.
The visual signature is a dark, matte film spreading across the solder mask and laminate around affected pads. It does not respect conductors or mask boundaries, and it can travel several millimetres from the source. Because it is a surface phenomenon, it is invisible to X-ray and can only be assessed visually or by surface analysis.
Why Immersion Silver Is Vulnerable
Silver reacts readily with sulfur compounds in the air, forming silver sulfide, which is dark and electrically conductive. The reaction is thermodynamically favourable and proceeds at room temperature, which is why silver surfaces tarnish even in indoor environments with modest pollution levels. The tarnish layer itself is usually stable, but under the right humidity conditions the products migrate.
Other finishes corrode in sulfur-bearing environments as well, but their products tend to remain in place. Copper forms sulfides that stay local, and gold is largely inert. Immersion silver is uniquely problematic because its corrosion product is both mobile and conductive, which is the combination that produces creep.
Sulfur and the Source of Attack
Sulfur reaches assemblies from several sources. Industrial pollution, rubber gaskets and seals, some packaging materials, certain adhesives and even some paper products release sulfur compounds. In an enclosed cabinet, a small source can generate a concentration high enough to attack a large area of board over time.
The concentration required is low, which is what makes the problem so difficult to design against. A product installed in a clean office may never show the effect, while the same product installed in a facility with rubber seals and poor ventilation can fail within a couple of years. The environment, not the board, is the dominant variable. Identifying and removing the source is often cheaper than changing the finish, and it is the first thing worth investigating when creep appears in one installation rather than across a whole product line.
Humidity and the Role of Surface Films
Humidity is the other necessary ingredient. A dry surface allows the corrosion product to form but not to move, so tarnished silver in a dry environment remains largely local. Once a water film is present, the corrosion products become mobile and the creeping behaviour begins.
Cycling humidity is worse than constant humidity for the same reason it is worse in other electrochemical mechanisms. Each wetting cycle mobilises the products, and each drying cycle concentrates them, depositing material further from the source. That progressive advance is what allows the film to cover a large area over a long service life.
Creep Across and Under Conformal Coating
Conformal coating can slow creep corrosion considerably because it blocks the surface film necessary for migration. However, the protection is only as good as the coverage. Creep has been observed advancing under a coating where adhesion is marginal, and around the edges of coated regions where the surface remains exposed.
The failure mode is therefore a coating defect rather than a coating failure. Contamination on the surface before coating, insufficient coverage at the mask edges or around component bodies, and coating damage during assembly all create pathways. Where the product environment contains sulfur, the coating process needs verification of both coverage and adhesion rather than an assumption of protection.
Effects on Fine Pitch Assemblies
The damage mechanism is a bridge between adjacent conductors, so the risk is inversely related to the spacing. A creeping film that covers two millimetres on a coarse board may be harmless, while the same growth on a fine pitch component can connect two pads and short them. Fine pitch assemblies are therefore the first to show the symptom.
The resulting failure often presents as leakage rather than a hard short, because the film is resistive. Analogue measurements drift, sensor readings become noisy and current consumption rises. That gradual onset is what makes the mechanism difficult to attribute, since a device that drifts can be blamed on many causes. Comparing the film distribution with the circuit layout usually identifies which gap closed first. Established approaches to short circuit investigation help narrow it down.
Test Methods for Creep Corrosion
Accelerated testing uses a chamber with controlled temperature, humidity and a sulfur-bearing gas such as hydrogen sulfide or a mixed flowing gas blend. Coupons with representative fine pitch geometry are exposed for a defined period, and the distance the corrosion product travels from the source is measured. The method is standardised, but the correlation with field life is approximate.
Field assessment is equally important, because the results depend heavily on local sulfur concentration. A product that survives an accelerated test may still fail in a specific installation, and one that fails in the test may perform acceptably in a clean environment. The useful output is a comparison between candidate finishes and coating strategies under identical conditions. The finish comparison itself is covered in this guide to surface finish selection.
Mitigation through Finish and Coating Choices
The most direct mitigation is to avoid immersion silver where the environment contains sulfur. Electroless nickel immersion gold is far more resistant because gold does not react with sulfur compounds at normal temperatures, and hot air levelled finishes are also less susceptible because the surface is tin-based and the corrosion products are less mobile.
Where immersion silver must be used, the combination that works best is a clean surface with a well adhered conformal coating of sufficient thickness to survive assembly. The clean surface matters because contamination under the coating creates a pathway for creep along the interface. Coating thickness and coverage at mask edges are the two variables most worth verifying.
Specification and Field Experience
A specification should state the finish, the coating requirement if any, and the environmental conditions the product is expected to see. Where the installation environment includes rubber seals, industrial pollution or poor ventilation, that information should reach the design team before the finish is chosen rather than after a field failure.
Recording field experience closes the loop. When a returned unit shows a creeping film, identifying the corrosion product by analysis and comparing its distribution with the layout shows which clearances were critical. That evidence is what justifies a finish change and what supports the decision when the change carries a cost. Assessment of whether a returned board is acceptable or not follows the same reasoning used in PCB quality judgement.
FAQ
Is tarnished silver the same as creep corrosion? No. Tarnish is a localised discolouration from silver sulfide that forms on the metal surface. Creep corrosion occurs when those products migrate across the board away from the source, and it is the migration that creates the risk of bridging adjacent conductors.
Does conformal coating prevent creep corrosion? It significantly slows it by blocking the surface moisture film, but it does not guarantee protection if coverage is incomplete or adhesion is poor. Creep has been observed advancing under marginally adhered coatings and around the edges of coated regions, so coverage and adhesion both require verification.
Which finish should be used in a sulfur environment? Electroless nickel immersion gold is generally the most resistant option because gold does not react with sulfur compounds at normal temperatures. Where immersion silver is retained for other reasons, a well adhered coating over a verified clean surface is the practical alternative.
