Creep Corrosion Control on PCB Assemblies: 6 Rules

Creep corrosion is the migration of corrosion products across a surface, away from the metal that produced them. It is not the same as ordinary corrosion, where the damage stays close to the affected area. In creep corrosion the product spreads as a film, crossing the solder mask, bridging conductors and eventually causing leakage or a short.

The phenomenon became prominent with the move to lead-free assembly and to silver-bearing surface finishes, and it is driven by sulfur compounds in the air rather than by water alone. A board can look clean and still develop the condition in a few weeks in the wrong environment.

What Creep Corrosion Is

The mechanism begins with sulfur reacting with silver or copper to form a sulfide. That sulfide is a semiconductor rather than an insulator, and it is also hygroscopic, so it holds a film of moisture on the surface.

Once the film is present, electrochemical reactions can continue along it, and the corrosion product grows outward from the original site. The growth is the defining feature and the reason the effect is called creep.

Because the product is conductive, it can bridge fine pitch conductors and cause leakage currents well before any mechanical damage is visible. On a high-impedance or high-voltage circuit, that leakage is itself the failure.

Sulfur Sources and Mixed Flowing Gas

Elemental sulfur and sulfur compounds arrive from many directions: rubber gaskets and seals, packaging materials, paper, some adhesives and the atmosphere of industrial or agricultural areas. A board inside a sealed enclosure can be exposed if the enclosure itself contains a sulfur source.

Because the effect depends on a mixture of gases, humidity and temperature, the standard method for evaluating it is the mixed flowing gas test, in which coupons are exposed to a controlled blend that includes hydrogen sulfide, chlorine and nitrogen oxides.

The test is a comparison rather than a pass or fail in absolute terms. It ranks finishes and process variations against each other, and a coupon that creeps in the chamber is likely to creep in a real environment of similar severity.

Surface Finish Choices

Silver finishes are the most susceptible because silver sulfide forms readily and is mobile. Immersion silver is still widely used for its solderability and flatness, so the control comes from the rest of the process rather than from avoiding the finish.

Nickel and gold finishes are more resistant, and OSP and tin finishes behave differently again. The choice is a compromise with solderability, cost and the assembly process, so it is made with the environment in mind rather than on the fab line alone.

Where a silver finish is used, thickness and porosity of the coating matter. A thin or porous deposit exposes more area to the sulfur, and contamination under the coating accelerates the reaction. The OSP coating notes describe the alternative approach and its storage limits.

Residue, Cleanliness and Water

A clean surface creeps more slowly than a contaminated one. Flux residue, handling salts and particles all hold moisture and provide ionic paths, and they give the corrosion product somewhere to travel.

Cleaning therefore helps even where the finish is the main variable, and the cleanliness specification should be set with the environment in mind rather than inherited from another product. Ionic contamination measured on the assembly is the practical check.

Handling is part of the same picture, because fingerprints are both ionic and hygroscopic. The contamination control notes describe the handling rules that keep a finished board clean.

Conformal Coating as a Barrier

A conformal coating works by keeping sulfur compounds and moisture away from the metal. Where the coating is continuous, the reaction cannot start, and where it is thin over an edge or a lead, the reaction starts there and creeps under the film.

Coating adhesion is therefore the controlling property rather than coating thickness. A coating that has lifted from the surface provides a channel in which corrosion products can travel quickly, which is worse than no coating at all in some cases.

The coating also has to survive the environment it is protecting against, and a material that becomes brittle in the cold or soft in the heat will crack or creep and lose the barrier. The coating and potting comparison explains when a heavier encapsulation is the better answer.

Test Methods and Records

Mixed flowing gas testing, with a coupon of the same finish and the same cleaning as production, is the usual verification. Coupons are examined for the area of creep, and the result is compared against a reference sample that was known to pass.

Where a product is already in service, the same comparison can be made against coupons exposed in the field, which is often the fastest way to establish whether the environment is severe enough to need a change.

Records should link the finish, the cleaning process, the coating material and the test result, because a change in any one of them can move the outcome. Related reliability effects on tin plated surfaces are described in the tin whisker notes.

Enclosure Design and Field Verification

The environment inside an enclosure is not the same as the environment outside it. A gasket, an adhesive or a moulded part that contains sulfur compounds can raise the concentration around the board even where the outside air is clean.

Ventilation and drainage matter as well. A sealed enclosure that breathes through a small vent draws in humid air as it cools, and the moisture then condenses on the coldest surface, which is often a connector or a shield.

Where a product is deployed in an aggressive location, field coupons of the same finish and the same process give the most direct evidence of what is happening. They are cheap and they remove the guesswork from the question of whether the environment is severe enough to justify a change.

Records should link the location, the exposure time and the coupon result, because the severity of a site is a property of that site and not of the product alone.

Packaging is part of the exposure path as well. Some papers, cardboards and foam inserts contain sulfur compounds that can release vapour inside a sealed bag, and a board stored for months in the wrong packaging can begin to sulfidise before it is ever installed.

That is why the material specification for packaging deserves the same attention as the coating. Shelf-life trials that include the intended packaging, rather than bare coupons, show the combined effect and tell the user how long the finished assembly can be stored.

FAQ

Does creep corrosion only affect silver finishes? No. Silver is the most susceptible, but copper and tin surfaces can also sulfidise and creep. The finish changes the rate rather than removing the risk.

Will conformal coating stop creep corrosion completely? A continuous, well-adhered coating prevents it in the covered area. Coating coverage at edges, leads and connectors is what decides the outcome, and those are the regions where failures are found.

How long does a mixed flowing gas test take? Days to weeks, depending on the severity class being simulated. It is a comparative test, so the value comes from running a known reference alongside the new sample rather than from the absolute result.