Electrochemical Migration Testing and Prevention Methods

Two conductors a fraction of a millimetre apart, a thin film of moisture, a little ionic residue and a voltage difference. That is all it takes for metal to dissolve from one conductor and grow back as a filament reaching toward the other. Electrochemical migration is one of the most preventable failure mechanisms in electronics, and one of the most frequently overlooked because the conditions it needs are so ordinary.

What Electrochemical Migration Is

Migration is a three part electrochemical process. Metal dissolves at the positively biased conductor and enters the moisture film as ions. Those ions travel through the film toward the negatively biased conductor. At the cathode they are reduced back to metal, which deposits as a filament that grows back toward the anode.

The result is a conductive path that did not exist before assembly and cannot be found by any test performed on a dry board. Because the mechanism requires a liquid film, it is strongly dependent on humidity, and because it requires an electric field, it is strongly dependent on the voltage and the spacing between features.

The Three Necessary Conditions

A continuous moisture film, dissolved ionic species, and a voltage difference must all be present simultaneously. Removing any one of the three stops the process completely, which is why the countermeasures divide cleanly into environmental control, cleanliness and design spacing.

This also explains why the mechanism is so sensitive to small changes. A product that operates in a dry, air conditioned room never accumulates a moisture film, while an identical product in a humid, poorly ventilated enclosure may fail within a year. The board is the same; the environment is not. That sensitivity is the reason a failure investigation should always establish the local humidity history before considering a material change.

<img src="https://www.gopcba.com/wp-content/uploads/2020/11/project_image_11.jpg" alt="Micrograph of a metallic dendrite growing between two biased PCB conductors” />

Dendrite Growth and Its Appearance

A dendrite is the filament formed by the deposited metal. It is typically feathery or tree-like, branching as it grows, and it extends from the cathode toward the anode following the electric field. Under magnification it appears distinctly metallic, unlike a corrosion product, which is darker and more diffuse.

The growth is often intermittent in its electrical effect. A thin dendrite may carry a small leakage current for a long time before it becomes thick enough to sustain a short, and it can burn open under a fault current and then regrow. That behaviour produces faults that appear to fix themselves, which makes diagnosis particularly difficult. Documenting the humidity and bias conditions at the time of the fault is often the only way to confirm the mechanism after the filament has burned open.

Metals That Migrate

Silver is the most aggressive common metal, which is why immersion silver finishes and silver-loaded adhesives attract attention. Copper migrates as well, though more slowly in most conditions, and it is the metal most often involved in practice because copper is everywhere on a board. Tin migrates under some conditions, particularly in the presence of chlorides.

Gold and nickel are considerably more resistant, which is one reason electroless nickel immersion gold performs well in humid and sulfur-bearing environments. Where a design has exposed silver and exposed copper on the same surface, the silver will generally initiate migration first, and the resulting filament may then be joined by copper as the process continues.

Test Methods and Their Severity

Accelerated testing uses a comb pattern with closely spaced fingers, exposed to controlled humidity with a bias voltage applied. The specimen may be as simple as a bare laminate coupon for evaluating a process chemistry, or a fully assembled board when the interaction between components, flux and coating is under investigation.

Severity is governed by the gap, the voltage, the humidity and the duration. A narrow gap with a high voltage in condensing humidity is extremely aggressive, while a wide gap at a few volts in steady humidity is mild. Because the acceleration is not linear, the result should be treated as a comparison between candidates tested identically rather than as a prediction of service life. Reproducing the same severity in two laboratories therefore requires the gap, the voltage, the humidity profile and the coupon construction to match exactly.

Ionic Contamination and Cleanliness

Ionic species are what make the moisture film conductive. Chlorides from handling and plating residues, bromides from flame retardants, and weak organic acids from flux are the usual contributors. Their effect depends on their concentration and on how mobile they are in the water film, which is why some residues are far more damaging than others at the same mass.

Cleanliness is therefore the most direct control. Reducing ionic residue to a defined level by a validated cleaning process removes the electrolyte, and verifying that level by measurement rather than by inspection is what makes the control real. The methodologies used to investigate the resulting faults are described in this overview of short circuit inspection.

Design Measures That Reduce Risk

Increasing the spacing between conductors lowers the electric field and lengthens the distance a dendrite must travel, and both effects reduce risk substantially. Where a fine pitch layout cannot be relaxed, the conductor that is most likely to dissolve should be identified and, where polarity is optional, the bias applied in the less aggressive direction.

Avoiding permanent direct current between closely spaced exposed features is a design decision that costs nothing. Where an interface must be biased, keeping the positive conductor away from its negative neighbour, or interposing a grounded guard trace, changes the field distribution in a way that makes migration less likely.

Coatings, Encapsulation and Their Limits

A conformal coating blocks the moisture film and is therefore an effective barrier where it is continuous and well adhered. Its weakness is coverage: creep paths around the edges of coated regions, voids at component boundaries and coating damage during assembly all provide an entry point. Coating is protection, not immunity.

Coating over contamination is worse than no coating at all, because the residue is sealed in place and the moisture that condenses under the coating cannot escape. Verifying cleanliness before coating is therefore a prerequisite rather than an optional step. Where the coating is applied selectively, the boundary between coated and uncoated regions deserves particular scrutiny, because it concentrates the field gradient. The laminate itself also matters, since a material with high surface insulation resistance provides margin even when the surface film is present, and the relevant properties are described in this guide to laminate material properties.

Specification and Qualification

A specification should state the maximum permissible ionic contamination, the surface insulation resistance requirement under humid bias, the coating coverage requirement and the minimum spacing between conductors of opposite polarity. Together these define the conditions under which migration cannot occur.

Qualification testing should use a pattern and conditions representative of the product rather than a generic coupon exposed to an arbitrary severity. Where a product will operate in a humid environment, the test conditions should reflect it, because a mild test provides confidence that is not justified. Evaluating whether the result is acceptable follows the same discipline used in PCB quality judgement.

FAQ

How quickly can a dendrite form? On a contaminated board in continuously condensing conditions with a bias applied, a bridging filament can grow in a few days. On a clean board in moderate humidity it may never form at all. The timescale depends far more on contamination and humidity than on the metals involved.

Does conformal coating prevent migration? It prevents it where the coating is continuous and well adhered to a clean surface. It does not prevent it where the coating has gaps, where coverage around component edges is incomplete or where contamination was sealed beneath the coating. Coverage and cleanliness both have to be verified.

Why do some faults appear to repair themselves? Because a thin dendrite can burn open under the fault current and then regrow as the electrochemical process continues. The result is an intermittent fault that disappears during investigation and returns later. That behaviour is a strong indication that a migrating filament is the root cause rather than a component.