Salt Spray Testing and Corrosion Resistance of PCB Finishes
A salt spray chamber is one of the oldest tools in corrosion testing, and it remains one of the most requested by customers. It produces a repeatable, visually striking result in a short time, which makes it attractive for specification. It is also frequently misinterpreted, because salt spray is a comparative test rather than a predictor of service life. Understanding what it does and does not measure prevents both false confidence and unfair rejection.
What Salt Spray Testing Measures
The test exposes a sample to a continuously atomised saline fog at a controlled temperature. Any exposed metal corrodes, and the extent of the corrosion is assessed visually or by measuring the area affected. For a plated or coated surface, the test reveals the presence of pores, thin areas and mechanical damage that would allow the underlying metal to be attacked.
What it does not do is reproduce a real environment. A coastal installation experiences wetting and drying cycles, variable temperature and a mixture of contaminants, none of which the continuous fog reproduces. The test is best used to compare candidate finishes under identical conditions, not to promise a number of years of service.
Test Conditions and Standards
The most common configuration is a five percent sodium chloride solution atomised at thirty-five degrees Celsius, with the sample held at a defined angle and the fog deposition rate controlled. Duration ranges from a few hours to several hundred hours depending on the requirement, and the solution pH is controlled because acidity significantly changes the attack rate.
Reproducibility depends on details that are easy to overlook. The deposition rate must be measured with a collection funnel rather than assumed from the nozzle setting, the salt concentration must be verified, and the chamber must be cleaned between runs to avoid contamination from previous tests. Samples must be positioned so that one does not drip onto another.
Neutral, Acetic and Cyclic Variants
Neutral salt spray uses a pH around seven and is the standard general purpose test. Acetic acid salt spray lowers the pH and is considerably more aggressive, and it is used where the product will encounter acidic conditions or where a more severe screen is wanted. Copper accelerated acetic acid salt spray adds copper chloride to accelerate attack on plated deposits further.
Cyclic salt spray alternates fog exposure with drying and sometimes with humidity stages. It is a closer approximation of a real outdoor environment because the drying phase allows corrosion products to concentrate and the wetting phase renews the electrolyte, which reproduces the accelerated attack seen in coastal service far better than continuous fog.
How the Different Finishes Behave
Hot air levelled tin and lead-free finishes perform poorly in salt spray, because the surface is non-noble and corrodes readily. Electroless nickel immersion gold performs well, as the gold layer is inert and the nickel beneath is resistant, provided the gold is thick enough to be continuous. Immersion silver lies between the two, darkening quickly but usually without losing solderability.
Organic solderability preservatives are the weakest in this test because the protective layer is thin and designed to be removed during soldering rather than to resist the environment. The comparison between finishes is only valid if the thicknesses are comparable and applied to identical substrates, so a test that compares a thick gold against a thin silver proves very little.
Porosity and Its Role
Almost all plated deposits contain pores, and corrosion in salt spray typically begins at those pores rather than at the surface generally. The pore density depends on the plating process, the thickness and the substrate condition, which is why the same nominal finish can perform very differently on two boards.
A pore allows the electrolyte to reach the underlying metal, and the resulting galvanic couple accelerates attack at that point. This is why increasing thickness improves performance: it does not eliminate pores, but it reduces the probability that a pore penetrates through to the substrate. Reporting thickness alongside the test result allows a divergence to be attributed to the deposit rather than to the substrate or the cleaning. Measuring plating thickness and reporting it with any salt spray result makes the comparison meaningful, and the methods are described in this guide to plating thickness verification.
Sample Preparation and Edges
Edges and cut surfaces corrode first because the plated layer is thin or absent there. A board tested in the as-routed condition will show edge attack within hours, which is a genuine observation but says nothing about the performance of the finish on the face of the board. How edges are treated is therefore part of the test definition.
Cleaning before the test matters equally. Handling residues, fingerprints and flux left on the surface introduce chloride and organic contamination that changes the result substantially. Samples should be cleaned to a defined standard, dried and handled with gloves afterwards, and the cleaning method recorded so that results remain comparable between runs.
Evaluating and Rating Results
Rating is normally done against a standard photographic scale or by measuring the percentage of area affected. Both approaches have weaknesses: the visual scale is subjective and the area measurement depends on how the boundary of a corroded region is defined. Whichever method is used, the same assessor methodology should be applied to every sample in the comparison.
Where the product function depends on electrical performance, a functional check after the test adds useful information. Contact resistance, insulation resistance or solderability may change before visible corrosion becomes extensive, and those changes are more relevant to the product than the appearance of the surface. Post-test solderability is assessed using the methods described in this solderability test guide.
Limitations of the Method
The most important limitation is the lack of correlation with real service life. Two finishes that behave identically at forty-eight hours may differ substantially after years of wetting and drying, and a finish that fails quickly in continuous fog may perform acceptably in a sheltered environment where condensation is rare.
The second limitation is the sensitivity to test details. Deposition rate, pH, angle, cleaning and edge treatment all influence the outcome, so results from two laboratories are comparable only when the methods match. Quoting a duration without the conditions carries almost no information. That is why a salt spray certificate quoted without the full test conditions has limited value in a supplier qualification argument.
Specification and Acceptance
A useful specification states the test variant, the solution concentration and pH, the deposition rate, the duration, the sample preparation and the rating method. It should also state what the test is being used to decide, because a comparative screen and an acceptance criterion require different levels of rigour.
Acceptance limits are best set from a baseline established on a known-good finish from the same supplier, rather than from an absolute figure that appears in a generic standard. Comparing a candidate finish against a qualified reference under identical conditions produces an actionable result. The finish choices themselves are compared in this guide to surface finish selection.
FAQ
Does salt spray testing predict service life? No. It compares finishes under an aggressive, fixed condition and reveals porosity and thin areas, but it does not reproduce the wetting and drying cycles, temperature variation and contaminant mix of a real environment. Results should be treated as a ranking rather than a lifespan prediction.
Which PCB finish performs best in salt spray? Electroless nickel immersion gold generally performs best because the gold is inert and the nickel beneath is corrosion resistant, provided the gold is continuous. Hot air levelled tin performs worst. Immersion silver falls between the two, darkening without necessarily losing solderability.
Why do edges corrode first? Because the plated or coated layer is thin, damaged or absent at a cut edge, leaving the base metal exposed. That is a real weakness for exposed edges, but it also means an edge failure can dominate the result and obscure the performance of the finish on the board face.
