Material Compatibility – silicones, labels, adhesives, flux residues

Material compatibility in conformal coating is one of the most overlooked design risks in PCB protection. Silicones, labels, adhesives and flux residues can all inhibit adhesion, cause de-wetting and fish-eyes, or lead to delamination and corrosion over life.

This guide explains which materials are safe, which are risky, and what PCB designers should consider early in layout.

• Adhesion failure: Silicones, oils and plasticisers migrate and prevent coatings bonding to solder mask, copper and components.
• Chemical softening: Some labels and adhesives swell or dissolve in coating solvents or thinners.
• Outgassing and bubbles: Foams, tapes and low-cost labels can outgas during cure, creating voids and porosity.
• Residue entrapment: Flux residues and handling contaminants can be locked under coating, driving electrochemical corrosion.
• Rework and scrap: Incompatible materials often show up as field failures or late-stage rework rather than simple visual defects.

Silicones (RTV sealants, gaskets, keypads, lubricants, potting compounds) are the most common cause of conformal coating de-wetting. Even tiny amounts can spread across the surface and create large “craters” where conformal coating will not wet.

• High-risk sources: RTV gaskets, silicone greases, silicone conformal coatings, silicone keypads and keyboard coatings.
• Failure modes: Fish-eyes, cratered areas, exposed copper / solder joints, poor adhesion around silicone contact zones.
• Design guidance: Keep silicone-based parts outside coated regions wherever possible, or create defined keep-out areas and masking plans.

Alternative options to discuss with your materials engineer:

• Use urethane or epoxy for potting where coating must overlap onto surrounding PCB areas.
• Consider fluorosilicone O-rings for lower transfer where a silicone-based seal is mandatory.
• Use PFPE-type lubricants instead of silicone greases on mechanical components within coated zones.

PCB labels carry critical traceability data, but not all label materials are compatible with conformal coating or Parylene. Poor label selection can lead to peeling, bubbling, or unreadable barcodes after processing.

• Substrates: Prefer polyimide labels designed for solder reflow and conformal coating environments.
• Adhesives: Specify silicone-free, high-temperature adhesives suitable for solvents and elevated cure temperatures.
• Printing: Use inks and printing systems validated for reflow and coating exposure (no smearing or loss of contrast).
• Placement: Keep labels away from tight gaps, connectors and high-voltage keep-out areas to avoid masking complexities.
• Parylene note: Parylene will fully encapsulate labels unless they are masked; decide whether labels should remain exposed and accessible.

Structural and staking adhesives must be compatible with both the conformal coating chemistry and the cleaning process. Some adhesives soften or outgas, creating cosmetic and functional defects.

• Cyanoacrylates (superglues): Can bloom, become brittle and show poor long-term stability under heat and humidity. Use with care near coated areas.
• Epoxy adhesives: Generally good mechanical stability and compatibility with many conformal coatings if fully cured.
• Acrylic adhesives: Common in tapes and labels; some grades swell in solvents or soften during coating cure.
• UV-cure adhesives: Can be an excellent choice where thoroughly cured; check for oxygen inhibition and potential outgassing.

Design tips:

• Standardise a short list of approved adhesives with your coating supplier and purchasing team.
• Define bonding zones on the PCB where staking or structural adhesive is allowed and compatible with coating flow.
• Request compatibility testing if a new adhesive is introduced mid-project.

Flux chemistry and the level of residual contamination have a direct impact on coating adhesion and long-term
electrochemical reliability. Even “no-clean” fluxes can leave residues that are incompatible with certain coatings.

• Water-soluble fluxes: Must be thoroughly cleaned and rinsed; ionic residues are a high risk under coating.
• No-clean fluxes: May still need selective or full cleaning to meet coating adhesion and ionic cleanliness targets.
• Rosin-based fluxes: Can be difficult to remove and may interfere with coating wetting if left in place.

Design and process alignment:

• Agree early whether boards will be cleaned or truly “no-clean” before conformal coating.
• Specify acceptable flux types and cleaning requirements in the design specification and drawing notes.
• Work with your coating supplier to define cleanliness limits and test methods (e.g. ionic contamination, visual criteria).

• Foams and spacers: Check for outgassing and compatibility with both coating chemistry and cleaning media.
• Cable ties and clips: Some plastics can crack or embrittle with solvent exposure; align with approved materials lists.
• Encapsulants and underfills: Ensure these are fully cured before coating or Parylene deposition.
• Oils and lubricants: Avoid mineral and silicone oils in coated regions; specify low-migration alternatives.

Use this quick checklist when releasing a design into a conformal coating or Parylene process:

• Silicones are kept out of coated zones, or clearly controlled with keep-outs and masking.
• Labels use high-temperature, silicone-free constructions approved for coating.
• Adhesives are selected from an approved material list and fully cured before coating.
• Flux types and cleaning requirements are specified and agreed with manufacturing and coating teams.
• Any new materials (foams, tapes, lubricants) are reviewed for compatibility before introduction.
• Design documents capture key notes on material restrictions and keep-out regions for the PCB assembly drawing.

Your conformal coating or Parylene provider can help validate material compatibility in conformal coating and advise on preferred constructions. Sharing information early usually avoids late-stage surprises.

• Share BOM details for silicones, labels, adhesives and fluxes at the quotation stage.
• Request sample builds or coupon tests if critical materials are new or unproven.
• Agree how material changes will be communicated so coating processes can be re-validated if needed.

Building material compatibility into your design rules greatly reduces defects, rework and reliability risk in harsh environments.

Why Choose SCH Services?

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• ✈️ 25+ Years of Expertise – Specialists in coating technologies trusted worldwide.
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Note: This article provides general design guidance for conformal coating and Parylene. It does not replace product-specific standards, OEM requirements or safety approvals. Always validate final layouts and electrical spacings against the relevant IEC/IPC standards, customer specifications and safety agency rules for your application.