Dendrite Growth on Assembled Boards: 5 Drivers
Dendrite growth is the formation of metal filaments between two conductors on an assembled board, driven by voltage, moisture and ionic contamination. A dendrite grows across the surface rather than through the laminate, and it starts as a faint trail that raises leakage current long before it becomes a short circuit. The failure often appears after months in the field rather than at test.
Electrochemical migration is the mechanism behind it, and it needs three things at once: a voltage difference, an electrolyte and a path between the conductors. Remove any one of the three and the growth stops. This note explains where the conditions come from and how to keep them apart.
What Dendrite Growth Is
Metal dissolves at the anode, migrates through the electrolyte as ions and deposits at the cathode as a fine, branching crystal. On a PCB assembly the metal is usually copper or silver, and the electrolyte is a thin film of water carrying dissolved ions. The filament grows towards the anode and eventually bridges the gap.
The growth is not the same as a short caused by a stray solder ball or a whisker. A dendrite forms in service, follows the field between the conductors, and is surrounded by evidence of corrosion. A solder ball is present from the start, and the difference matters when a failure is being investigated.
The Three Conditions It Needs
Voltage is always present on an operating board, so it is the condition that cannot be removed. What can be controlled is the spacing between conductors and the field strength at the surface, which is set by design and by layout. Conformal coating also removes the electrolyte from the surface and is the most direct control.
The electrolyte and the ionic path are the conditions that a manufacturer can attack. Both come from residue left on the surface, from the environment and from the materials used in assembly, which is why cleanliness is the central control for electrochemical migration.
Where It Starts on an Assembly
Growth starts where residue collects and where the field is strongest, which in practice means under components, between fine pitch leads and at the edges of a coating. Under a large body the residue from flux is trapped, humidity condenses in the gap and the surface never fully dries, so the area stays wet long after the rest of the board has dried.
Vias that are tented but not plugged, tight component gaps and connectors with exposed contacts are other common starting points. The pattern in a batch of failures usually points at a single station: if all the affected boards came from one line, the residue is local, and if the failures are spread across lines, the design or the environment is the cause.
Flux Residue and Ionic Contamination
Flux residue is the most common electrolyte on an assembled board. A no-clean flux leaves a residue that is intended to be benign, and it usually is, provided the residue stays dry and the ionic content stays low. Add moisture and the same residue becomes conductive, which is why no-clean is a qualification rather than an exemption from cleanliness control.
The level of residue is measured rather than judged visually. Ionic contamination testing extracts the soluble species and reports them as an equivalent of sodium chloride per unit area, and the extraction method and limits are described in our notes on ionic contamination testing. Where the reading is close to the limit, the flux chemistry and the cleaning process are both worth reviewing, and our page on flux residue covers the chemistry side.
Humidity and Condensation
Electrochemical migration needs a continuous film of water, and a film forms long before visible condensation appears. As relative humidity rises above about 60 percent, adsorbed water on a contaminated surface becomes continuous enough to carry ions. The threshold falls as the ionic load rises, so a dirty board can support migration in a room that a clean board tolerates.
Temperature cycles make it worse. A board that cools overnight draws moisture into every crevice, and a board that is powered up in a humid room can sit below the dew point for minutes at a time. Enclosure design, heaters and desiccants all affect the local humidity, and those are the practical levers where the environment cannot be controlled directly.
Voltage Bias and Conductor Spacing
Migration rate rises with the applied voltage and falls with the gap between conductors. A design that runs 3 volts across a 0.15 mm gap has a modest field, while the same gap at 50 volts is a strong driver. The layout rules that keep spacing proportional to voltage exist for this reason, and they apply to the surface as well as through the laminate.
Time under bias matters as much as voltage. A board that is powered continuously in a humid environment accumulates growth faster than one that is switched on for an hour a day, which is why field failures often concentrate in equipment that runs around the clock. Duty cycle belongs in the reliability assessment.
Diagnosing a Dendrite Failure
The first evidence is usually a leakage current that rises over time, followed by a short or an intermittent fault. Where the board is still available, inspect under magnification at the location of the fault before cleaning anything. A dendrite is delicate and is easily destroyed by handling, and once it is gone the diagnosis rests on the residue around it.
Confirm the mechanism rather than assuming it. Sectioning, ion chromatography of the residue and a review of the environment history all build the case, and the residue analysis is what identifies the source. Our notes on no clean residue risk describe the conditions under which a benign residue stops being benign.
Preventing Dendrite Growth in Production
Start with cleanliness. Establish an ionic contamination limit for the product, measure it on every process change and clean where the measurement says it is needed. Where a no-clean process is retained, verify the limit by test rather than by the datasheet, and keep the boards dry between operations.
Then protect the surface. Conformal coating removes the electrolyte from the most vulnerable areas, provided it covers the gap between conductors and adheres to a clean surface. Coverage at the edges of a keep-out is where coatings usually fail, and the inspection method is set out in our conformal coating inspection guide. The accepted test methods for cleanliness and coating, published by IPC, give the thresholds that a customer will accept.
Design Choices That Widen the Margin
Layout is the cheapest place to defeat migration, because spacing costs nothing once it is drawn. Keep the surface gap between conductors proportional to the working voltage, avoid long parallel runs at high potential difference and place the highest voltage nets away from the tightest gaps. Where a gap has to be small, a routed slot or a coating dam is more effective than extra clearance elsewhere on the board.
Material choices matter as well. A laminate with a lower moisture uptake, a solder mask with high surface resistivity and a coating that adheres to a clean surface all raise the threshold at which migration begins. Where a product must survive a humid environment, these are the decisions that decide whether the board lasts ten years or two, and they should be recorded in the design file rather than rediscovered during a field failure.
FAQ
Does a dendrite always cause a hard short? No. Early growth raises leakage and can disturb a high impedance circuit long before it bridges the gap. In low current analogue circuits the first symptom is often drift rather than failure.
Can a dendrite form on a board that was never powered? It needs a voltage difference, so a board in storage does not grow one. Corrosion of a similar kind can still occur from residues and moisture, which is why clean and dry storage matters even before first power.
Is lead free assembly more prone to migration? The alloy is not the main factor. Flux chemistry, residue and coating coverage dominate, and a lead free process with a well controlled cleaning and coating step performs as well as any other.
