Salt Spray: Design Rules and Process Limits

Coastal installations, vehicle underbody modules, and outdoor equipment all expose electronics to chloride, and chloride attacks metal faster than almost anything else. A salt spray test reproduces that exposure in a chamber and compresses months of service into days. The test is easy to run and easy to misinterpret, so it helps to know what it does and does not demonstrate.

What a Salt Spray Test Reproduces

The chamber creates a fine fog of sodium chloride solution that settles on the specimen, keeps it wet, and holds it at a controlled temperature. The chloride ion is small and mobile, and it breaks down passive oxide layers on copper, nickel, and aluminium. Once the oxide is breached, an electrochemical cell forms and the metal dissolves at the anode while hydrogen or oxygen is reduced at the cathode.

What the test does not reproduce is the full range of field conditions. Real exposure includes drying cycles, temperature swings, pollutants such as sulphur dioxide, and mechanical wear. A board that survives a neutral salt spray test may still corrode in an environment with alternating wet and dry periods, because the drying phase concentrates the salt and drives the reaction. The test is a comparison tool rather than a service life prediction.

Test Conditions and Standards

The classic neutral salt spray test uses a 5 percent sodium chloride solution at 35 degrees Celsius, with the fog deposited at a controlled rate and the specimen oriented at a defined angle. Acidified and copper accelerated variants increase the severity, and a cyclic variant alternates wet and dry periods to better represent an outdoor environment. The conditions should be quoted with any result, because they change the outcome substantially.

Duration is the next variable. Twenty four hours is enough to separate a well protected board from a bare one, while several hundred hours are needed to differentiate between two comparable finishes. The acceptance criteria matter as much as the duration: some programmes require no visible corrosion, others allow a defined percentage of the surface to be affected, and a few require an electrical test after exposure.

Which Surface Finishes Survive

Finishes differ widely in their corrosion resistance. Electroless nickel immersion gold provides good protection because the nickel layer is a barrier, although a thin or porous nickel can allow attack at the copper beneath. Immersion silver corrodes readily and forms sulphide and chloride films, so it is usually avoided in coastal applications. Organic solderability preservatives offer almost no protection once the assembly is complete, and hot air solder levelling protects only where the solder covers the copper.

Exposed copper is the weakest point in any design, and copper that is only covered by solder mask at the edge of a pad will corrode from that edge inward. This is why the finish specification for a corrosive environment usually includes a requirement that no copper be exposed at a pad boundary, and why the laminate and mask adhesion matter as much as the metal itself.

Standards for the test define the solution concentration, the chamber temperature, the fog collection rate, and the specimen angle, and any deviation changes the severity. It is worth checking which variant a customer specification actually calls for, because a neutral test and a cyclic test with the same duration will rank two candidate finishes differently. The report should record the standard, the duration, and the inspection criteria together, so that a comparison between two suppliers is meaningful.

Coating, Sealing, and Edge Protection

A conformal coating is the main defence, and its performance depends on adhesion and on coverage rather than on thickness alone. A coating that lifts at an edge allows chloride solution to creep underneath, where it is trapped against the surface and the corrosion is worse than on an uncoated board. Cleaning before coating, correct cure, and full coverage over the board edge are the controlling factors.

Sealing and potting offer more protection where the design allows it. A potted assembly excludes the environment almost completely, provided the potting compound adheres to every surface it touches and the enclosure does not flex enough to open a path. Where a connector must remain accessible, a gasket and a sealed housing usually outperform any coating applied to the connector itself.

Reading the Results

Inspection after the test should be systematic. Photograph the specimen before and after, record the location and the extent of corrosion, and classify the findings rather than describing them loosely. A single rust spot at an edge and a general attack across a plane mean very different things, and the location usually identifies the mechanism.

Electrical measurements add information that visual inspection misses. Insulation resistance and continuity measured after exposure detect corrosion under components and inside holes, which is invisible from the surface. Where the boards are to be coated, the test should be repeated on coated samples, because a coating changes the failure mode from surface attack to undercut corrosion and can hide the damage until it is severe.

This is also where process control pays off: a board with a clean surface, a properly cured mask, and a sound finish has far less corrosion variability between lots than one where those parameters drift.

Corrosion Mechanisms on Assembled Boards

Galvanic corrosion occurs when two dissimilar metals are connected by an electrolyte. A gold plated pad next to solder, or a nickel barrier next to exposed copper, forms a cell in the presence of chloride, and the less noble metal corrodes. The rate depends on the area ratio, so a small anodic area next to a large cathodic area is the worst combination.

Electrochemical migration is a second mechanism and is more insidious. Metal ions dissolve at one conductor, travel through the electrolyte, and deposit as a dendrite at the other, which grows until it bridges the gap. The process needs only a thin film of moisture and a small bias, and it can occur inside a connector or under a coating. Keeping ionic residue low and using a coating that adheres well are the two most effective countermeasures.

Design Measures for Corrosive Environments

Design measures start with the geometry. Keep spacing generous so that a partially corroded trace still has margin, avoid exposed copper at pad edges, and place the most sensitive circuits away from the board edge where salt accumulates. Where a connector is required, choose one with a sealed interface and provide a drain path so that water does not pool against the contacts.

Material choices complete the picture. A laminate with low moisture absorption, a solder mask that adheres well after thermal cycling, and a finish selected for the environment all contribute. Where aluminium or a metal core is used, the cut edges should be protected because a bare aluminium edge corrodes quickly and can undermine the coating around it.

FAQ

How long should a salt spray test run? Twenty four to 96 hours is typical for a comparison between designs or finishes, while a specific standard may require several hundred hours. State the duration and the conditions with any result.

Does conformal coating always prevent corrosion? No. It delays it where coverage and adhesion are good, but a coating defect or an uncoated edge allows undercut corrosion that can be worse than no coating at all.

Is a passing result a guarantee of field life? No. Salt spray is a comparative test. Field life depends on the wet and dry cycle, the pollutants present, and the electrical bias, so a passing board should still be evaluated in a representative environment.