SIR Failures & Leakage Under Conformal Coating

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SIR failures and leakage under conformal coating occur when insulation resistance between conductors degrades enough to create measurable leakage current, noise, drift or intermittent-to-hard shorts. The most common drivers are ionic contamination + moisture + electrical bias, amplified by coverage weaknesses (thin edges, voids, boundaries and interfaces). This is rarely “just a coating problem” — it is a cleanliness + dry-out + coverage + inspection discipline problem.

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: SIR/IR, leakage, intermittent shorts	🔗
How it happens: moisture + ions + bias under coating	🔗
Root Causes: contamination, moisture pathways, coverage weaknesses	🔗
Prevention: control plan for cleanliness, dry-out, edges & boundaries	🔗
Troubleshooting & Diagnosis: SIR testing, humidity-bias, FA	🔗
Repair: when to touch-up vs strip & recoat	🔗

What are SIR Failures & Leakage Under Conformal Coating?

• Insulation resistance (IR) — resistance between two conductors intended to be electrically isolated. Low IR means increased leakage risk.
• Surface insulation resistance (SIR) — IR measured across a surface path (often via contamination + moisture films), typically under controlled humidity and bias for verification.
• Leakage current — unintended current flow between conductors. In practice, it can cause drift, noise, unstable logic, sensor offset and intermittent behaviour long before a hard short occurs.
• SIR failure — IR falls below an acceptable threshold (or trends down under humidity + bias), indicating a mechanism is active (often ionic contamination and moisture under coating).

SIR failures are often the “early warning stage” of ECM/dendrite growth and corrosion. Many field issues present as intermittent faults because the leakage path strengthens/weaken with humidity and temperature cycling.

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How SIR Failures Happen Under (or Through) Conformal Coating

• Step 1 — ions remain on the assembly: flux residues, fabrication chemistry, handling salts, or cleaning drift leave conductive ionic species on the surface (often under components and at interfaces).
• Step 2 — moisture becomes available: high RH, condensation cycles, trapped moisture pre-coat, or ingress at boundaries creates a thin electrolyte layer.
• Step 3 — bias drives leakage: the electrolyte enables ionic conduction between biased conductors, reducing IR and creating measurable leakage current.
• Step 4 — mechanisms escalate: sustained bias can evolve leakage into ECM/dendrites or under-film corrosion, shifting from intermittent leakage to hard shorts.

Pattern clue: humidity-sensitive behaviour (works dry, fails damp) strongly suggests an electrolyte-based leakage path rather than a purely electrical design issue. Where failures align with coating voids, edges or keep-out boundaries, treat it as a coverage/boundary amplifier on top of contamination and moisture.

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Root Causes of SIR Failures & Leakage Under Coating

Contamination sources (what creates the electrolyte)

• Flux residues (assembly or rework), especially under low-standoff parts and fine pitch regions.
• Fabrication residues (etch/plate chemistry remnants, solder mask residues, handling contamination).
• Cleaning process drift (exhausted chemistry, insufficient wash energy, poor rinse quality, inadequate drying).
• Handling & FOD (fingerprint salts, dust, glove contamination, fibres) creating localised leakage pathways.

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 + bias matters).

Coverage weaknesses (what amplifies risk)

• Thin edges, shadowing, and incomplete coverage near high-risk nets.
• Pinholes/voids that shorten the barrier path and admit moisture locally.
• Capillary interfaces that transport moisture under-film (see wicking below).

Sanity check (related mechanisms): if you also see corrosion residues, route to corrosion & ionic contamination. If moisture clearly tracks along interfaces/edges, also see capillary wicking. If voids dominate, see pinholes, bubbles & foam.

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How to Prevent SIR Failures & Leakage Under Conformal Coating

1) Control cleanliness (before you coat)

• Validated wash, rinse and dry processes — define parameters, monitor drift, and control change-out rules.
• Rework discipline — re-clean after rework; prevent local flux concentration and under-component residue retention.
• Verification — use ROSE for drift monitoring where appropriate; use ion chromatography for species identification on critical builds.

2) Control moisture (dry-out + queue times + storage)

• Dry-out / pre-bake — remove absorbed and trapped moisture before coating (validate profiles vs components and assemblies).
• Queue-time rules — define max time from clean → coat; control storage RH to prevent re-absorption.
• 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)

• Edge coverage & thickness — avoid thin edges and shadowing in high-risk nets; verify thickness using coupons/defined checks.
• Boundary quality — keep masking edges clean and well-defined; ragged boundaries can create capillary pathways.
• Inspection plan — UV + white light inspection as appropriate; focus on edges, keep-outs, connectors and under-component shadow zones.

If SIR failures 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.
• Trend behaviour — intermittent faults often precede hard shorts; log leakage/IR vs time and environment.

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, dendrites, corrosion tracks or boundary-driven ingress.
• Ion chromatography — identify ionic species and trace sources (flux activators vs fabrication residues vs rinse chemistry).

3) Audit the process chain

• Cleaning audit — chemistry control, loading, filtration, rinse quality, drying effectiveness.
• Dry-out verification — confirm moisture removal for the assembled population, not just bare boards.
• Coverage audit — confirm edge thickness, shadowing, voids/pinholes, boundary quality and ingress routes.

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Repair: When to Touch-Up vs Strip & Recoat

• Single, localised coverage escape with verified cleanliness: controlled local removal, re-clean, dry-out and touch-up can be acceptable where inspection is robust and the customer spec allows it.
• Confirmed SIR degradation across multiple sites: treat as systemic contamination/moisture control failure; strip, re-clean, verify, then recoat is usually required.
• Under-component risk: if you cannot access/inspect the leakage path (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 SIR failures & leakage 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 SIR/leakage mechanisms, 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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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.