Corrosion & Ionic Contamination in Conformal Coating

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Corrosion and ionic contamination under conformal coating occur when residual ions on the assembly combine with moisture and (often) electrical bias to form an electrolyte film. This can drive metal degradation, leakage currents, and, in more severe cases, electrochemical migration (ECM) / dendrite growth.

This is rarely “just a coating problem” — prevention is fundamentally a cleanliness + dry-out + coverage + boundary + inspection discipline problem. Coating can slow moisture access, but it cannot compensate for contamination already present on (or trapped within) the assembly.

Routing sanity check:

• if your main symptom is measured IR/SIR loss or leakage, start with SIR failures & leakage.
• If you are seeing metallic bridges/dendrites, go to ECM & dendrite growth.
• If the failure is internal to the PCB laminate (subsurface, via-to-via, hard to “clean away”), go to CAF under coating & solder mask.

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: corrosion, ionic contamination, leakage/SIR, ECM & dendrites	🔗
How it happens: ions + moisture + (often) bias under coatings	🔗
Root Causes: contamination sources, moisture pathways, boundary amplifiers	🔗
Prevention: cleanliness, dry-out, edges & boundaries, control plan	🔗
Troubleshooting & Diagnosis: ROSE, ion chromatography, SIR/humidity-bias, FA	🔗
Repair: when to touch-up vs strip & recoat	🔗

What is Corrosion & Ionic Contamination Under Conformal Coating?

• Ionic contamination — residual salts/ions (often from flux residues, handling, fabrication chemistry, or inadequate rinsing) that dissolve in moisture to form an electrolyte.
• Corrosion — electrochemical degradation of metals (copper, tin, nickel, silver, etc.) enabled by moisture and accelerated by ionic residues and sometimes electrical bias.
• SIR/IR degradation (symptom) — a measurable reduction in insulation resistance or increased leakage; this can be caused by moisture films over ions, corrosion products, ECM, or (in some cases) CAF.
• ECM / dendrites (mechanism) — metal dissolves at one node and deposits toward another under bias, creating conductive dendrites and bridges between conductors.

Even with conformal coating, failures occur when moisture finds a pathway (thin edges, voids, interfaces, keep-out boundaries) and ions are present. Coating may slow moisture access — but it cannot “fix” contamination already present.

Key separation: this page focuses on surface/under-film corrosion + ionic contamination. If your root cause is actually internal laminate growth (CAF), coating changes and cleaning changes will not resolve it.

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How Corrosion and ECM Develop Under (or Through) Conformal Coating

• Step 1 — ions remain on the assembly: residues persist after soldering/rework, PCB fabrication, or cleaning drift. Under-component residues are highest-risk because they are difficult to remove and inspect.
• Step 2 — moisture becomes available: humidity cycling, condensation, trapped moisture pre-coat, or ingress at boundaries creates an electrolyte film.
• Step 3 — electrochemical reactions start: the electrolyte enables ionic conduction and metal reactions, lowering IR and increasing leakage; corrosion products may form under-film.
• Step 4 — escalation under bias: sustained bias can evolve leakage into ECM/dendrite growth and intermittent-to-hard shorts, especially in fine-pitch regions.
• Step 5 — pathways amplify: thin edges, micro-voids, interfaces, and keep-out boundaries concentrate moisture and accelerate attack.

Pattern clue:

• If behaviour is strongly humidity dependent (works dry, fails damp), suspect an electrolyte mechanism (ions + moisture) first. If you can confirm filamentary metallic bridges between biased conductors, route to the dedicated ECM & dendrite growth page.
• Where coating defects appear first as voids/pinholes, see pinholes, bubbles & foam.
• Where you see beading, craters or islands, see de-wetting.
• Where corrosion is edge-driven with lifting, consider delamination.

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Root Causes of Corrosion & Ionic Contamination

Contamination sources (what creates ions)

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

Moisture pathways (how water reaches the ions)

• Trapped moisture pre-coat (insufficient dry-out after cleaning or from ambient absorption in laminate/mask).
• Ingress at boundaries (masking edges, keep-out zones, connectors, enclosure interfaces).
• Permeation over time (all polymers transmit some moisture; long exposure + ions matters).

Amplifiers (what makes it worse)

• Electrical bias across conductors (higher bias and smaller spacing increases driving force).
• Coverage weaknesses (thin edges, shadowing, voids/pinholes, poor sealing at interfaces).
• Process escapes (rework not re-cleaned, uncontrolled queue times, mixed flux/cleaner/coating compatibility problems).

Sanity check (look-alikes):

• If you see corrosion starting at a mask boundary where coating lifted, consider delamination.
• If failures are primarily IR/SIR drop and leakage, route to SIR failures & leakage.
• If failure is subsurface/internal, route to CAF.
• If defects are driven by masking contamination/transfer, see why masking causes many conformal coating defects.

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How to Prevent Corrosion & Ionic Contamination 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 on high-reliability assemblies.

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

• Controlled dry-out / pre-bake — remove absorbed/trapped moisture before coating (validate profiles vs components and assemblies).
• Queue-time rules — define max time between clean → coat; control storage RH to prevent re-absorption and re-contamination.
• Packaging discipline — don’t bag/ship parts before full cure and dry-out; use barrier packaging where required.

3) Control barrier integrity (coverage and boundaries)

• Film build and edge coverage — avoid thin edges and shadowing in high-risk nets; verify thickness using coupons/defined checks.
• 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; focus on edges, keep-outs, connectors and under-component shadow zones.

If corrosion/ionic issues recur, treat them as a control-plan gap. 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

• Humidity dependence — if leakage rises with RH/condensation, you are almost certainly dealing with an electrolyte-based mechanism.
• Location logic — edges, interfaces, shadow zones and keep-out boundaries are common moisture pathways and residue traps.
• Visual mapping — under-film corrosion often tracks along interfaces/edges; dendrites (ECM) bridge between biased conductors.

2) Replicate and measure (don’t guess)

• SIR / humidity-bias testing — apply bias under controlled humidity and trend IR/leakage over time.
• Local strip + microscopy — remove coating in the suspect zone and inspect for residues, corrosion products, dendrites, or boundary-driven ingress.
• Ion chromatography — identify ionic species and trace sources (flux activators vs fabrication residues vs rinse chemistry).

3) Use clear confirmation criteria

• ECM confirmed: dendrites/filaments bridging biased conductors (often after local strip) plus IR/SIR trending down under humidity + bias.
• Corrosion confirmed: under-film attack products/pitting/residue patterns consistent with moisture pathways, often concentrated at edges/interfaces/boundaries.
• CAF suspected/confirmed: no surface bridge/path after strip + failure remains; microsectioning reveals internal filament growth (route to CAF page).

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Repair: When to Touch-Up vs Strip & Recoat

• Localised corrosion with verified clean substrate: controlled local removal, re-clean, dry-out and touch-up can be acceptable where access/inspection is robust and the customer spec allows it.
• ECM/dendrites or repeated corrosion: treat as systemic contamination/moisture control failure; strip, re-clean, verify, then recoat is usually required.
• Hidden/under-component risk: if you cannot access/inspect the attacked area (e.g., under QFN/BGA), assume the mechanism may persist and escalate to strip/rework or scrap per customer rules.

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

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Looking for Other Defect Types?

This page covers corrosion & ionic contamination under 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 corrosion/ionic contamination, 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?

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• 🛠️ End-to-End Support – Selection, masking, inspection and troubleshooting.
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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.