Electrochemical Migration And Dendrite Growth On PCBs

A board can pass every electrical test, be clean enough to satisfy a conductivity measurement, and still fail in the field because a metal filament grew across a gap that nobody was measuring. The filament is the visible result of electrochemical migration, a process in which a water film, a small amount of ionic contamination, and a direct voltage combine to dissolve metal at one conductor and deposit it at another. The result is a short circuit that develops slowly and appears suddenly.

This article explains the mechanism, the conditions that make it possible, the design and process measures that prevent it, and how the risk is evaluated by test.

The Mechanism

Three conditions have to be present at the same time. There must be a film of water on the surface, which can be as thin as a few molecular layers and does not require a visible droplet. There must be ionic contamination in that film, which may come from flux residue, from plating chemistry, from handling, or from the environment. And there must be a direct voltage difference between two conductors that are close enough for the film to bridge.

When those conditions are met, the water film becomes an electrolyte. Metal at the positively biased conductor dissolves into it, the metal ions travel through the film under the electric field, and they are deposited as metal at the negatively biased conductor. The deposit grows back towards the anode as a branched structure called a dendrite, and when the two conductors touch, the circuit shorts. The damage is not limited to the short: the metal that dissolved leaves the conductor thinner, so a second failure can follow even after the dendrite is removed.

Which Metals Migrate

Silver migrates most readily of the metals used on a board, and it does so at low activation energy, which is why an assembly with immersion silver, silver filled adhesive, or silver bearing solder is watched closely. Copper migrates as well, particularly where copper is exposed at a via, at a test point, or along the edge of a mask opening, and the copper salts that form are conductive. Tin and lead migrate more slowly, and gold is comparatively resistant.

The presence of halides accelerates the process. A flux that contains chlorides leaves a residue with a high ionic content, and the same residue holds moisture more strongly than a clean surface does. A metal whisker, which is a metallurgical phenomenon rather than an electrochemical one, is a different failure mode that produces a similar outcome, and the difference matters because the two are prevented by different measures.

What Triggers It In Service

Humidity is the first trigger. Above about sixty percent relative humidity a surface begins to hold a thin water film, and the film becomes continuous when the surface is contaminated, because contamination lowers the contact angle and lets the water spread. Condensation, which happens when a product is moved between a cold and a warm environment, is far more dangerous than a steady high humidity.

The electrical and geometric conditions set the rate. A higher bias voltage drives the ions faster, a smaller gap is bridged sooner, and a higher temperature accelerates every step of the chemistry. That combination is why the standard qualification tests apply a bias voltage at an elevated temperature and a high humidity, and why a board that survives a dry environment can fail in a condensing one with the same circuit and the same contamination.

Design Measures

The most effective measure is spacing. Increasing the distance between two conductors raises the length that a dendrite has to grow before it touches, and the industry spacing rules for high humidity applications are derived from that requirement rather than from the manufacturing limit. The rules for spacing between a via and a neighbouring trace are described under via to trace clearance, and they apply directly to this failure mode.

The second measure is a barrier. A conformal coating interrupts the water film, and it is the single most effective protection for a circuit that must operate in a humid environment, provided it is applied over a clean surface and provides continuous coverage. Solder mask helps but does not solve the problem, because a dendrite can grow along the surface between mask features and can also form under a mask that has lifted. Where exposed metal of different potential is unavoidable, as at a test point beside a ground pad, the spacing and the coating have to be chosen together. The coating process is described under board level protection.

Process Measures

Cleanliness is the controllable variable. Removing flux residue, avoiding fingerprints, and controlling the ionic content of the surface all raise the threshold at which migration begins, and the cleaning decision is therefore a reliability decision rather than a cosmetic one. Where a no clean process is used, the residue has to be the specific material that was qualified for the application, applied at the specified volume, and processed within the profile it was designed for.

Handling is the second. A fingerprint is a mixture of salts and oils that provides both the ionic content and the surface energy that holds a water film, and a board that is handled with bare hands in a humid climate is a different reliability proposition from the same board handled with gloves. Storage in a controlled humidity and packing in a sealed bag with a desiccant remove the exposure during the period when the board is most vulnerable.

Testing The Risk

The classic test is a surface insulation resistance measurement under bias, in which a comb pattern of closely spaced conductors is held at a voltage in a humid chamber while the leakage between them is monitored. A fall in resistance indicates that a conductive path is forming, and the test can be run on a coupon that is processed exactly like the product, which makes it a useful qualification tool as well as a research method.

Accelerated versions run at a higher temperature and humidity, which shortens the test at the cost of changing the failure mechanism if the conditions become too severe. The flux classification tests, which include the copper mirror and the silver chromate paper methods, describe the aggressiveness of a residue rather than the migration risk of a design, so they answer a different question. The practical approach is to qualify the process with a biased humidity test, and then to control the two variables that the test has shown to matter. The wider framework is described under PCB design quality characteristics.

FAQ

Is electrochemical migration the same as a tin whisker? No. A dendrite grows through an electrolyte from dissolved metal, while a whisker is a metallurgical growth that emerges from a plated surface without moisture or bias. The two require different prevention measures.

Does solder mask prevent migration? It reduces the exposed area and slows the process, but it is not a barrier. A dendrite can grow across the surface between mask features and beneath a mask that has lost adhesion.

Which applications are most at risk? Those with a high humidity environment, a continuous bias, small conductor spacing, and exposed silver or copper. Condensing conditions and unfiltered outdoor air raise the risk further.