Liquid Silicone Conformal Coating Process for PCBs

Liquid silicone rubber (LSR) or silicone gel used for PCB conformal coating provides excellent protection against moisture, dust, chemicals, thermal shock, and mechanical stress. It is a soft, flexible, and often self-healing material suitable for harsh environments.

1. Pre-Coating Preparation

• PCB Cleaning: Essential for adhesion. Remove flux residues, ionic contaminants, and particulates using:

  • Solvent Cleaning: Using isopropyl alcohol (IPA) or specialized electronics cleaners in vapor degreasers or ultrasonic baths.

  • Aqueous Cleaning: With saponifiers and deionized (DI) water rinses, followed by thorough drying.

• Masking & Taping: Protect areas where coating is not desired (connectors, test points, switches, heatsink surfaces, gold fingers).

  • Use high-temperature polyimide (Kapton) tape, latex or silicone masking boots, or liquid latex masking materials.

• Surface Priming (if required): Some silicone formulations may require a primer to enhance adhesion to specific substrates (e.g., certain plastics or poorly bonding metals). This is less common for standard PCB components.

2. Coating Material Preparation

• Material: Typically a two-part, low-viscosity, addition-cure (platinum-catalyzed) liquid silicone. Key properties: low viscosity (for penetration), specific cure hardness (Shore 00 or A), and defined pot life.

• Mixing: Precisely mix Part A (base) and Part B (catalyst) in the specified ratio (often 1:1). Use:

  • Static Mixer: For manual or dispensing gun applications, a disposable static mixer tube ensures thorough mixing.

  • Automated Meter-Mix-Dispense (MMD) System: For production, using positive displacement pumps for precise ratio control and a dynamic mixer.

• Degassing: Vacuum degassing the mixed material removes entrapped air bubbles that could create voids in the coating. This is critical for optical or high-reliability applications.

3. Coating Application Methods

• Selective Spray Coating (Most Common for Production):

  • Process: Programmable robotic systems with spray valves atomize the mixed silicone into a fine mist.

  • Advantages: High speed, consistent coverage, excellent for complex 3D boards, and easy integration into automated lines.

  • Control Parameters: Spray pattern, flow rate, robot path, atomizing air pressure.

• Dispensing (For Potting or Selective Areas):

  • Process: A bead or dots of silicone are dispensed from a needle. Used for glob-top, edge sealing, or filling specific cavities.

  • Advantages: Precise control over material placement and volume.

• Brush Coating (For Rework or Low Volume):

  • Process: Manual application with a brush. Least consistent method but useful for repairs.

• Dip Coating (Less Common for Silicone):

  • Process: Dipping the entire PCB into a bath of coating material. Can be wasteful and requires meticulous masking. More common for acrylic or urethane coatings.

4. Curing / Cross-linking

• Stage 1: Skin-Over / Gelation: The coated PCB is left at room temperature for a short time (5-30 minutes) to allow viscosity to increase and prevent dripping (slump control).

• Stage 2: Primary Cure:

  • Thermal Cure: The standard method. Boards are transferred to a convection or IR oven.

  • Typical Cure Profile: 5-15 minutes at 80°C to 120°C. The exact time/temperature depends on the silicone's datasheet.

  • The heat accelerates the platinum-catalyzed addition reaction, forming the final elastomeric network.

• Stage 3: Post-Cure (if required): Some formulations may benefit from an extended post-cure (e.g., 1-4 hours at a lower temperature) to achieve final physical properties.

5. Post-Coating Processes

• Demasking: Carefully remove all masking tapes and boots immediately after cure is complete.

• Inspection: Visual inspection for uniformity, bubbles, misses, or contamination. Thickness verification using ultrasonic testers (e.g., PosiTest) or micrometres on coupons.

• Testing & Rework:

  • Electrical Testing: Perform in-circuit or functional tests.

  • Rework: Silicone is challenging to remove due to its chemical inertness. Techniques include:

    • Mechanical Peeling: Using tweezers or picks for thick gels.

    • Specialized Solvents: Swelling agents (e.g., some hydrocarbon solvents) can soften the silicone for removal.

    • Cryogenic Debonding: Freezing with aerosol sprays to make the silicone brittle for mechanical removal.

    • Abrasive/Solvent Combination: Using abrasive pads soaked in solvent.

Key Process Control Parameters & Considerations

Advantages of Liquid Silicone for PCB Coating

• Flexibility & Stress Relief: Remains elastic across a wide temperature range (-55°C to +200°C), accommodating CTE mismatches.

• Excellent Dielectric Properties.

• High Purity & Chemical Inertness: Resistant to oxidation, moisture, and many chemicals.

• Self-Healing: Some soft gels can re-seal after a penetrating object is removed.

This process is widely used in automotive, aerospace, marine, LED, and high-power electronics where long-term reliability under extreme conditions is paramount.