Ionic Contamination: Moisture, Contamination and Cleaning of Assemblies

Moisture and contamination are the two mechanisms that account for most of the electrochemical failures in electronic assemblies, and both are managed by the same discipline: keep the surface clean and keep it dry. The discipline is procedural, and it is easily lost when production pressure rises.

How Moisture Causes Damage

Water on a board does not have to be liquid to cause harm. A thin film of condensed moisture absorbs atmospheric carbon dioxide and becomes weakly conductive, which is enough to create a leakage path between adjacent conductors that are close together.

Where a voltage is present and the water contains ions, a metal can dissolve at one electrode and deposit at the other, growing a dendrite that eventually bridges the gap. The process is slow at first and then rapid, and the resulting short is permanent. Our short circuit notes describe the mechanisms.

Ionic Contamination and Its Measurement

Flux activators, plating residues and handling salts are all ionic, and their presence increases the conductivity of any moisture film. The contamination is measured by extracting the ions from a sample into a known volume of solution and measuring the conductivity.

The result is expressed as an equivalent mass of sodium chloride per unit area, and it is compared with a specification. The measurement is a proxy rather than a direct one, and it should be used as a process monitor rather than as a guarantee.

Cleaning and Its Limits

Cleaning removes the residue and requires that the cleaning agent reaches every surface, including beneath components. A no-clean flux is designed to leave a residue that is harmless, which is a different requirement from one that leaves nothing.

Where water washing is used, the water itself must be clean, because contaminated rinse water deposits the ions it carries. The final rinse is normally deionised water, and its quality is monitored.

Conformal Coating as a Barrier

A coating slows the ingress of moisture and prevents it from forming a continuous film on the surface. It does not stop the process indefinitely, and it must be applied to a clean surface, or it traps the contamination it was meant to exclude.

The coating also has to cover the areas where moisture would otherwise collect: the misalignment gaps around components, the edges of the board and the connector pins. A coating that misses a joint leaves the whole assembly exposed at that point. Our harsh environment notes describe the requirements.

Dendrites and Their Appearance

A growing dendrite appears under magnification as a fine branched structure between two conductors, usually starting from the cathode. Its appearance is distinctive enough that its presence identifies the mechanism without further analysis.

The rate depends on the voltage, the spacing, the contamination and the humidity. Reducing any one of them slows the process, and the design lever is the spacing.

Design Measures That Help

Increasing the spacing between conductors at different potentials is the primary measure, because it lengthens the path a dendrite must grow and reduces the leakage current. Keeping the spacing uniform also avoids the narrow gap that becomes the weakest point.

Where the coating cannot be relied on, a solder mask that covers all the surfaces between conductors helps, provided it is intact and free of voids. A mask over a conductor reduces but does not eliminate the leakage.

Storage and Packaging

The moisture content of a board at the time of assembly is controlled by packaging and by baking. A board stored in a sealed bag with desiccant stays dry; one stored in a humid environment absorbs moisture that must be removed before reflow.

The baking schedule depends on the thickness of the board and on the humidity it has been exposed to. Baking a thick board for too short a time leaves the centre moist, which is why the schedule is specified rather than chosen. Our laminate properties notes describe the mechanism.

Monitoring in Production

The controls that work are simple: a cleanliness measurement on a sample, a record of the rinse water quality, a measurement of the coating thickness and coverage, and a control on the time between cleaning and coating.

The last of those is the one most often lost. A board that is cleaned at the end of a shift and coated at the start of the next has had a night in a humid atmosphere to recontaminate its surface.

Additional Considerations for This Build

Practical attention to rinsing pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating rinsing explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Process Control and Verification

On a design of this kind, conformal coating is the item that decides how the rest of the board is arranged. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.

Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.

Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

Process Control and Verification

On a design of this kind, conformal coating is the item that decides how the rest of the board is arranged. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.

FAQ

Is a no-clean process safe for a humid environment? It is safe when the process is controlled and the residue is verified to be benign for the application. Where the environment is aggressive or the spacing is tight, the residue should be measured rather than assumed to be harmless.

Does a coating make cleaning unnecessary? It makes it more necessary, because the coating traps whatever is on the surface. The cleanliness requirement comes from the coating supplier and is verified by measurement rather than by appearance.

What does gopcb measure before shipping? We measure the ionic cleanliness of a sample where the process requires it, verify the coating thickness and coverage where a coating is applied, and record the rinse water quality. Where a customer specifies a limit we report the measured value against it.