Electrochemical Migration (ECM) & Dendrite Growth in Conformal Coating

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Electrochemical migration (ECM) and dendrite growth under conformal coating are among the most difficult reliability mechanisms to diagnose because early-stage failures are often intermittent, humidity-dependent and visually hidden. ECM occurs when ionic contamination + moisture + electrical bias combine to form an electrolyte film that drives metal ion movement and conductive dendrites between conductors, causing leakage currents that progress to hard shorts.

This is not “just a coating defect” — prevention is fundamentally a cleanliness + dry-out + coverage + boundary discipline problem. Coating can slow ECM, but it cannot “fix” contamination already on (or trapped within) the assembly.

For a complete index of defect types and links to each technical article, use the Conformal Coating Defects Hub.

For upstream control (cleaning windows, dry-out strategy, coating process stability), see the Conformal Coating Processes Hub and the Inspection & Quality Hub.

Article Quicklinks

Topic	More
Definitions: ECM, dendrites, leakage currents, insulation resistance	🔗
How it happens: the ECM mechanism under (or through) conformal coating	🔗
Root Causes: contamination sources, moisture pathways and electrical drivers	🔗
Prevention: cleanliness, dry-out, boundary control and control plan	🔗
Troubleshooting & Diagnosis: SIR, humidity-bias replication, IC/FA	🔗
Repair: when to touch-up vs strip & recoat	🔗

What is Electrochemical Migration (ECM) & Dendrite Growth?

• Electrochemical migration (ECM) — metal ions dissolve at one conductor (typically the anode) and redeposit toward another (the cathode) under electrical bias, forming conductive bridges.
• Dendrites — filamentary metallic growth (“trees”) that can span conductor gaps and create leakage currents progressing to hard shorts.
• Insulation resistance (IR/SIR) — a practical measure of how well adjacent conductors remain electrically isolated; ECM typically shows IR degradation over time in humidity + bias exposure.
• Electrolyte film — a thin moisture layer containing ionic contamination (flux residues, salts, cleaning residues, handling contamination) that enables ionic transport.

Even with conformal coating, ECM can occur when moisture finds a pathway (thin edges, voids, interfaces, keep-out boundaries) and ions are present. Coating may slow the mechanism — but it cannot compensate for contamination.

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How ECM Happens Under (or Through) Conformal Coating

• Step 1 — ions remain on the surface: residues persist after assembly/rework, PCB fabrication, handling, or cleaning drift. Under-component residues are the highest-risk because they are hard to remove and hard to inspect.
• Step 2 — moisture is present: humidity cycling, condensation, ingress at edges/boundaries, or moisture trapped pre-coat creates an electrolyte film.
• Step 3 — electrical bias drives movement: leakage current flows; metals dissolve and re-deposit, producing dendrite growth and intermittent-to-hard shorts.
• Step 4 — pathways amplify: thin edges, shadowing, micro-voids, interfaces, and keep-out boundaries concentrate moisture and accelerate migration.

Pattern clue: true ECM typically appears as dendrites bridging biased conductors, often in fine-pitch areas. Where defects present first as voids or pinholes, see pinholes, bubbles & foam. Beading, islands or craters point instead to de-wetting. Lifting at edges or boundaries suggests a delamination-driven root cause rather than migration.

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Root Causes of ECM & Dendrite Growth

Contamination sources (what creates mobile ions)

• Flux residues from assembly or rework (activators, weak acids, salts), especially in tight geometries and under low-standoff parts.
• PCB fabrication residues (process chemistry, etch/plate residues, solder mask by-products, handling contamination).
• Inadequate washing/rinsing leaving ionic or partially soluble residues behind.
• Contaminated wash/rinse systems that redistribute contamination rather than removing it.
• Handling contamination (fingerprint salts, glove powder, dust/FOD) introducing localised conductivity and nucleation sites.

Triggers (what turns residues into failures)

• Moisture exposure (humidity cycling, condensation, wash-water entrapment, insufficient dry-out).
• Electrical bias across conductors (higher bias and smaller spacing increases driving force).
• Coverage weaknesses (thin edges, shadowing, keep-out boundaries, interfaces, poor sealing).
• Process escapes (rework not re-cleaned, uncontrolled queue times, mixed flux/cleaner/coating compatibility problems).

Sanity check (look-alikes):

• if your failures also show corrosion residues or under-film attack patterns, treat ECM and corrosion as linked and route to Corrosion & Ionic Contamination.
• If defects are driven by masking contamination/transfer, see why masking causes many conformal coating defects.

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How to Prevent ECM & Dendrite Growth Under Conformal Coating

1) Control cleanliness (before you coat)

• Validated wash, rinse, and dry processes — define windows, monitor performance, and control drift (don’t rely on “looks clean”).
• Cleaning verification — ROSE can be useful for drift monitoring on comparable builds; ion chromatography supports species identification and root cause on higher-risk products.
• Rework discipline — re-clean after rework; control flux type and application; avoid “no-clean” assumptions for high-reliability assemblies.

2) Control moisture (dry-out + handling + storage)

• Controlled dry-out / pre-bake — remove absorbed moisture from boards/components before coating (validate vs components and assembly constraints).
• Queue-time rules — define max time between clean → coat; control storage conditions to prevent re-absorption and re-contamination.
• Handling controls — gloves, no bare-hand contact, segregation of contamination sources, controlled work surfaces.

3) Control barrier integrity (coverage and boundaries)

• Film build and edge coverage — avoid thin edges, shadowing, and unsealed boundaries that admit moisture.
• Boundary quality — ragged keep-out edges and contaminated masking can create capillary pathways for moisture.
• Inspection plan — combine UV and white-light inspection where needed; verify thickness via coupons/defined checks in the Inspection & Quality Hub.

If ECM is recurring, treat it as a control-plan gap rather than a single defect. Define release criteria for cleanliness, dry-out, and boundary integrity — not just “coated/not coated”.

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Troubleshooting & Diagnosis

1) Confirm the failure signature

• When it appears — humidity dependence, condensation exposure, post-wash events, and “NFF” returns are classic ECM flags.
• Location logic — edges, interfaces, shadow zones, under-component regions, and keep-out boundaries are common moisture pathways.
• Visual confirmation — dendrites may be hidden; local strip + microscopy often required to see bridging filaments.

2) Quantify contamination (don’t guess)

• ROSE testing — useful for process drift monitoring and comparisons on like-for-like builds.
• Ion chromatography — identifies ionic species and helps trace sources (flux activators vs fabrication residues vs rinse chemistry).
• Surface insulation resistance (SIR) / humidity-bias testing — replicate ECM risk under controlled bias and humidity and assess margin.

3) Audit the process chain

• Cleaning audit — chemistry control, filtration, rinse quality, change-out rules, and drying effectiveness.
• Dry-out verification — confirm moisture removal for the actual assembly population, not just bare boards.
• Coverage/boundary audit — check thin edges, shadowing, keep-out boundaries, and ingress paths (UV + white-light where appropriate).

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Repair: When to Touch-Up vs Strip & Recoat

• Localised moisture/coverage weakness with verified clean substrate: controlled local removal, re-clean, and touch-up can be acceptable where access/inspection is robust and the customer spec allows it.
• Confirmed ECM / dendrites: treat as a systemic contamination/moisture control failure. Strip, re-clean, verify cleanliness, then recoat is usually the only reliable approach.
• Hidden/under-component risk: if you cannot inspect/verify the attacked area, assume the mechanism may remain active and escalate to strip/rework.

For removal workflows and best-fit methods, see the Removal & Rework Hub.

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Looking for Other Defect Types?

This page covers electrochemical migration (ECM) & dendrite growth in conformal coating. For the complete index of defect types and links to each technical article:

Explore the Defects Hub ↗

Training on Conformal Coating Defects

SCH delivers practical, standards-driven training covering ECM/dendrites, cleanliness control, inspection discipline, and the wider defects framework used to prevent repeat failures.

Explore Conformal Coating Training ↗

Industry Standards We Work To

SCH Services aligns coating services, training, equipment supply and materials to relevant IPC standards, including:

• IPC-A-610 – Acceptability of Electronic Assemblies
• IPC-CC-830 – Qualification & Performance of Conformal Coatings
• IPC-HDBK-830 – Conformal Coating Handbook (guidance and best practice)

For further details on IPC standards: electronics.org/ipc-standards ↗

Why Choose SCH Services?

You gain a complete, integrated platform for Conformal Coating, Parylene & ProShieldESD—plus equipment, materials and training—backed by decades of hands-on process support.

• 🛠️ End-to-End Support – Selection, cleaning, masking, inspection and troubleshooting.
• ✅ Process Discipline – Recipes, control windows and repeatability.
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Note: This article provides general technical guidance only. Final design, safety, and compliance decisions must be verified by the product manufacturer and validated against the applicable standards.