Humidity and Condensation Testing

Why Humidity Is Tested

Moisture is involved in a large share of field failures that are not mechanical: it accelerates corrosion, it enables electrochemical migration and conductive anodic filament growth, it changes the dielectric properties of the laminate, it degrades the adhesion between the materials, and on a coated board it finds its way through any breach in the coating. A humidity test is an attempt to reproduce the combination of moisture, temperature and electrical bias that produces those failures, in a time short enough to be useful.

Steady State and Cyclic

The two basic test types are steady state, where the chamber is held at a fixed temperature and humidity, and cyclic, where both vary and the assembly is allowed to cool below the dew point so that condensation forms. Steady state is simpler and is used for materials and for ion migration, while cyclic is closer to what a product experiences in service, where a warm enclosure cools overnight and moisture condenses on the surfaces. The cycling is what produces the condensation, and condensation is what drives many of the field failures, so a design that survives steady state can still fail a cyclic test.

Bias and Its Effect

Applying voltage during the test changes the result fundamentally, because the electrochemical mechanisms need a field to drive them. A biased humidity test accelerates corrosion and migration; an unbiased test only measures moisture absorption and material stability. The bias also has to be applied to the structures that are at risk, which is why a test board usually carries a pattern of closely spaced biased conductors or a set of the product’s own nets. Where a test applies bias but at a low voltage, the acceleration may be insufficient to reveal a marginal design, and the test can appear to pass while the field condition is more severe.

Condensation and Dew

Condensation occurs when a surface is cooler than the dew point of the surrounding air, which on a real product happens when a warm, humid enclosure cools or when equipment is moved from a cold store to a warm room. The condensed water is not pure: it contains dissolved ions from the residue on the surface, which makes it a conductive electrolyte and produces exactly the conditions the electrochemical mechanisms need. A test that produces condensation, therefore, is testing both the surface cleanliness and the design’s tolerance of a conductive film, and a board that fails under condensation usually has a cleanliness problem as well as a design margin problem.

What Fails

The failures seen in a humidity test are surface insulation resistance dropping between adjacent conductors, dendritic growth between biased pads, corrosion of a finish or of a joint, loss of adhesion at an interface, and a change in the electrical properties of the laminate. Where the failure is a slow resistance drop, the mechanism is likely to be ion migration or corrosion; where it is a sudden short, the mechanism may be a dendritic bridge or a CAF path inside the board. Distinguishing them requires the failure to be located and cross sectioned, since the electrical symptom is the same for several mechanisms.

Coating and Its Limits

A conformal coating is the main defence against humidity, and it works by keeping the moisture away from the surface rather than by sealing the assembly. Its performance depends on the coverage, the adhesion and the absence of defects, and a test that includes coating is testing all three at once. A coating that is thin at an edge, that has a pinhole or that has poor adhesion at a ridge will fail before the bulk of the film does, which is why the inspection of the coating is as important as its specification. Where a product has to survive condensation, the coating should be combined with a cleanliness specification that limits the ionic residue, because a coating over a contaminated surface traps the electrolyte against the circuit.

Interpreting a Result

The result of a humidity test is a change in the measured parameter over time, so the design of the measurement matters. Insulation resistance should be measured in a way that does not disturb the condition, at a defined voltage and after a defined recovery, and the trend is more informative than a single reading. A failure should be reported with the time at which it appeared, the location, and the mechanism identified by examination, since a failure without a location cannot be corrected. Where a design passes with a large margin, that margin should be recorded, because it is the basis for accepting a subsequent change to the material, the coating or the cleanliness specification.

Humidity and the Design Rules

The design rules that come out of humidity testing are mostly about spacing, cleanliness and protection. Wider spacing between biased conductors reduces the field that drives migration and gives a filament further to travel; a lower voltage on a closely spaced pair reduces the driving force; a coating keeps the electrolyte off the surface; and a cleanliness specification keeps the ions out of the residue. Where a design cannot provide the spacing, the remaining levers are the material, the coating and the process, and the specification should state which one is being relied on. The mistake that appears repeatedly is to assume that a coating makes the spacing rules unnecessary, when in fact a breach in the coating at the very location where the spacing is tight is exactly the combination that produces a field failure.

FAQ

What is the difference between steady state and cyclic humidity testing? Cyclic testing allows the assembly to cool below the dew point, so condensation forms, which is what drives many field failures.

Why apply bias during the test? Because the electrochemical mechanisms need a voltage to drive them, and an unbiased test does not reproduce the failure.

Does a coating prevent humidity failures? It helps substantially, but only where the coverage and adhesion are good, and it must be combined with cleanliness.

What mechanisms does the test reveal? Surface insulation resistance loss, dendritic growth, corrosion, loss of adhesion and conductive anodic filament inside the laminate.

How should a failure be reported? With the time of failure, the location and the mechanism identified by examination, not only the electrical symptom.

Conclusion

Humidity testing reproduces the combination of moisture, temperature and bias that drives corrosion and electrochemical migration, and it only does so when it allows condensation and applies bias. Design the test to include the product’s real nets, measure the trend rather than a single value, and identify the mechanism of any failure. Environmental reliability belongs to quality management, the coating that defends against moisture is described under conformal coating, and the design rules belong to PCB design and layout. Environmental testing is normally introduced alongside prototype PCB assembly in 2026.