SIR Failures & Leakage Under Conformal Coating

SIR failures and leakage under conformal coating occur when insulation resistance between conductors degrades enough to create measurable leakage current, instability or intermittent-to-hard shorts. This is not typically caused by the coating alone.

The dominant mechanism is the interaction of ionic contamination, moisture and electrical bias, often amplified by coverage weaknesses such as thin edges, voids and boundary interfaces.

This should be treated as a process control issue involving cleanliness, moisture management, coating coverage and inspection discipline rather than a simple material failure.

What are SIR failures and leakage?

• Insulation resistance (IR): resistance between two conductors intended to be electrically isolated.
• Surface insulation resistance (SIR): IR measured across a surface path, often under humidity and electrical bias conditions.
• Leakage current: unintended current flow between conductors, often presenting as drift, noise or intermittent faults.
• SIR failure: IR drops below acceptable levels or degrades under environmental stress, indicating an active failure mechanism.

SIR failures are often an early stage of corrosion or electrochemical migration and may present as intermittent faults before progressing to hard failures.

How SIR failures develop under coating

• Step 1 – Ionic residues remain: flux, fabrication residues or handling contamination remain on the surface.
• Step 2 – Moisture becomes available: from environment, trapped moisture or ingress pathways.
• Step 3 – Electrical bias drives conduction: ions move within the moisture layer, reducing insulation resistance.
• Step 4 – Escalation: leakage develops into corrosion or dendrite formation under sustained conditions.

Pattern clue: faults that appear under humidity or temperature cycling strongly indicate an electrolyte-driven mechanism.

Root causes

Contamination sources

• Flux residues, particularly under components or in fine-pitch areas
• Fabrication residues and process chemistry remnants
• Cleaning process drift or ineffective rinsing/drying
• Handling contamination such as salts, dust or fibres

Moisture pathways

• Trapped moisture prior to coating
• Ingress at boundaries such as connectors or masking edges
• Permeation over time through coating materials

Coverage weaknesses

• Thin edges and shadowing
• Pinholes and voids
• Capillary interfaces that allow moisture transport

Important: residue behaviour is often misunderstood. Controlled no-clean residues can be stable, but disturbance or partial cleaning can significantly increase risk. For a detailed explanation, see cleaning no-clean flux residues for conformal coating reliability.

Prevention strategy

1) Control cleanliness and residues

• Ensure cleaning processes are validated and stable where used
• Maintain controlled no-clean processes where cleaning is not applied
• Use ion chromatography for contamination diagnosis when required

2) Control moisture

• Implement validated dry-out or bake processes
• Control queue times and storage humidity
• Ensure assemblies are dry before coating

3) Control coverage and boundaries

• Ensure adequate coating thickness at edges
• Maintain clean, well-defined masking boundaries
• Inspect for voids, pinholes and shadowing

SIR failures should be treated as a control plan issue, not a one-off defect.

Troubleshooting and diagnosis

Confirm the failure signature

• Check for humidity-dependent behaviour
• Identify location patterns (edges, boundaries, shadow zones)
• Look for intermittent-to-stable failure progression

Replicate and measure

• Use SIR or humidity-bias testing
• Perform local coating removal and inspection
• Use ion chromatography to identify contamination sources

Audit the process

• Review cleaning effectiveness or residue control strategy
• Verify moisture removal processes
• Check coating coverage and boundary integrity

Repair strategy

• Localised issues: clean, dry and recoat if contamination is controlled and accessible
• Systemic issues: strip, re-clean, validate and recoat
• Hidden areas: escalate if inspection is not possible

For removal workflows, see the Removal & Rework Hub.

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This article is provided as general technical guidance only. Final decisions should be validated against product-specific requirements, process conditions and applicable standards.