Introduction
Printed circuit boards (PCBs) are at the heart of electronics equipment providing the pathways for signals and power. However, PCBs are susceptible to contaminants, moisture, and temperature/humidity changes which can cause dendritic growth, corrosion, and short circuits. Applying a protective conformal coating on PCB assemblies safeguards against such damage and enhances reliability.
Silicone conformal coatings have emerged as a popular protective coating for PCBs due to their ability to repel water, provide insulation, and withstand wide temperature variations. This article provides an in-depth look into silicone conformal coatings, their properties, application methods, selection criteria, and benefits for PCB protection.
What is a Conformal Coating?
A conformal coating is a thin polymeric film applied on the PCB assembly to act as a barrier against contaminants like moisture, dust, chemicals, and temperature extremes. The conformal coating “conforms” evenly to the complex contours of PCB components, covering all surfaces.
Conformal coatings are dielectric (non-conductive) in nature to prevent shorting between adjacent traces and components on a crowded PCB. The coatings are applied in a thin layer – typically 25 to 200 microns thick. This protects the PCB without materially increasing weight or dimensions.
Some key functions of conformal coatings on PCBs are:
- Prevent moisture ingress and chemical migration
- Provide thermal and mechanical shock protection
- Isolate high voltage areas on the PCB
- Electrically insulate conductive elements from each other
- Protect against dendritic growth between conductors
- Reduce field failures and extend product life
Why Use a Silicone Conformal Coating?
Silicone conformal coatings provide distinct advantages that make them a popular PCB protection solution:
Hydrophobicity: Silicones have a highly hydrophobic (water repelling) nature. The water contact angle is over 90° for silicones. This property enables excellent moisture and condensation resistance.
Temperature stability: Silicone coatings retain their protective properties over a wide temperature range from -60°C to 200°C. They can withstand soldering temperatures.
Flexibility: Silicones have high elasticity to flex and absorb mechanical and thermal shocks. They resist cracking or chipping.
Dielectric strength: Silicones offer very high dielectric strength (>400V/mil) to insulate high voltage areas on PCBs.
Chemical resistance: Silicones are largely inert and resist a broad range of chemicals like acids, alkalis, and cleaning solvents.
Reparability: Silicone coatings can be selectively removed for rework using solvents without damaging the PCB components.
Processability: Liquid silicones are easy to apply on complex board geometries using spray or brush coating.
UL rating: Silicones can achieve UL 94 V-0 flammability rating – the highest standard.
In summary, the versatility, hydrophobicity, temperature resistance, flexibility, dielectric strength, chemical resistance, and processability make silicone an ideal conformal coating material for PCB protection.
Silicone Chemistry Overview
Silicone or polydimethylsiloxane (PDMS) is an inorganic polymer derived from silica (sand). It consists of a silicone backbone chain with organic methyl groups attached to the silicon atoms:
The Si-O bond gives high temperature stability, while the methyl groups (CH3) provide flexibility, hydrophobicity, and chemical inertness.
Varying the ratio of organic to inorganic content gives different silicone grades. Higher organic content imparts greater flexibility, while higher inorganic content increases hardness and temperature resistance.
Silicones containing reactive chemical groups can undergo further cross-linking to form protective conformal coatings. Moisture curing, UV curing, and thermal curing are popular methods to cure liquid silicones.
Types of Silicone Conformal Coatings
Silicone conformal coatings are available in one-part and two-part systems with different curing mechanisms:
1. One-Part Silicones
One-part silicones consist of siloxane polymers and cross-linking agents in a single container. They cure into solid films upon exposure to atmospheric moisture:
- Acetoxy cure: Releases acetic acid as a byproduct while curing. Used where odor is not a concern.
- Alkoxy cure: Releases alcohol, allowing low odor cure. Used for thick coatings.
- Oxime cure: Cures via oxime linkages. Low odor but slower curing.
2. Two-Part Silicones
Two-part silicones consist of a base resin and a catalyst/curing agent in separate containers. They require mixing before application and cure:
- Condensation cure: Base reacts with a tin, platinum or titanium catalyst. Fast curing, releases alcohol.
- Addition cure: Uses a platinum catalyst to crosslink vinyl-functional polymers. Low odor, no byproducts.
- UV Cure: Base resin cures under UV irradiation. Very fast cure speeds.
One-part silicones cure at room temperature upon exposure to ambient humidity. Two-part silicones provide more control over cure rate and properties.
Silicone Conformal Coating Properties
Silicone conformal coatings can be formulated in different viscosities from low viscosity liquids to thixotropic gels. Key properties include:
Viscosity: Liquid silicones have viscosity ranging from 300 to 1000 cPs, while gels are over 3500 cPs. Viscosity determines the coating thickness achievable.
Hardness: Silicones have Shore A hardness between 20 to 85 after curing. Softer grades are more flexible while harder grades are abrasion resistant.
Dielectric strength: Around 500 Volts/mil for most silicone coatings, enabling insulation of conductors with voltages differences.
Dielectric constant: 2.7 to 3.0 at 1 MHz frequency. Similar to PCB substrate materials.
Dissipation factor: Very low, 0.001 to 0.01 at 1 MHz showing minimal energy losses.
Volume resistivity: >10e15 Ohm-cm ensures electrical insulation.
Temperature range: Protective from -60°C to 200°C enables use under wide temperature swings. Withstands soldering heat.
Tensile strength: 200 psi and above makes silicone coatings tough and resistant to cracks/chips.
Elongation: Up to 150% elasticity allows silicones to bend and absorb stresses.
Flammability: Silicones achieve UL 94 V-0 rating – the highest flammability standard.
Moisture resistance: Silicones are highly hydrophobic with dielectric strength retained after 85°C/85% RH exposure for over 1000 hours. Prevents corrosion and electrical leakage.
Chemical Resistance: Inert to most acids, alkalis, solvents etc. Allows cleaning and rework.
PCB Protection Applications of Silicone Conformal Coatings
Some major applications of silicone conformal coatings for PCB protection include:
Automotive Electronics: Protects against temperature extremes, vibration, moisture, and contaminants in under-the-hood automotive applications.
Power Supplies: Insulates high voltage PCBs and prevents arcing and short circuits in power supplies.
LED Lighting: Protects LED driver PCBs against moisture and corrosion. Maintains insulation resistance.
Control Equipment: Safeguards industrial control units in harsh plant environments against moisture, chemicals and particulate ingress.
Medical Electronics: Provides biocompatible, moisture-proof barrier for PCBs used in medical diagnostic equipment and body-worn devices.
IoT Devices: Extends service life of IoT endpoint devices deployed in uncontrolled ambient conditions exposed to spills, humidity, and pollutants.
Aerospace Avionics: Reliably insulates densely-packed PCBs on aircraft against vibration, shock, temperature swings, altitude changes etc.
Marine Electronics: Protects on-board electronics in marine vessels from salt spray, moisture condensation and water ingress.
Military Equipment: Ruggedizes electronics in battlefield conditions involving wide temperature variations, humidity, sand/dust ingress etc.
5G Telecom Gear: Maintains electrical isolation on sensitive RF PCBs against environmental contaminants.
Thus silicone conformal coatings safeguard PCBs across diverse operating environments and duty cycles throughout product lifecycle.
Benefits of Silicone Conformal Coatings on PCBs
Some key benefits offered by silicone conformal coatings to enhance PCB reliability are:
Moisture Resistance: Silicones provide a hydrophobic barrier preventing short circuits from dew condensation or water ingress.
Corrosion Prevention: The conformal coating isolates metal conductors from oxygen, moisture and contaminants to avoid corrosion.
Thermal Management: Silicones protect components from thermal extremes and minimize thermal shock damage due to their high flexibility.
Vibration/Shock Absorption: Cushions PCB assemblies from mechanical shocks and vibration through its damping properties.
Chemical Protection: Provides a barrier against splashes or leaks of chemicals like acids, solvents, fuels and alkaline cleaning agents.
Dendrite Prevention: Avoids growth of conductive dendrites between adjacent conductors that can cause shorting.
UV Resistance: Protects PCBs from damage by UV exposure during outdoor use.
Flame Retardancy: Silicone coatings achieve UL 94 V-0 rating to minimize fire hazard.
Electrical Isolation: Prevents arcing or short circuiting between closely spaced conductors due to the high dielectric strength.
Reworkability: Allows selective removal for repairs and rework without coating removal from the entire PCB.
Process Control: Liquid silicones are easy to apply on complex board geometries via spraying, brushing, or dipping.
Reliability: collectively, the protective properties enhance field reliability and extend service life of PCB assemblies.
Silicone Conformal Coating Application Process
A typical silicone conformal coating application process involves:
Surface Preparation
The PCB assembly is first cleaned to remove contaminants like dirt, oil or flux residues. An isopropyl alcohol (IPA) rinse cleans organic residues.
Programming Application Parameters
For automated Selective Coating Systems, the application parameters are programmed like coating area, thickness, speed etc.
Masking Sensitive Areas
Connectors, testpoints, and delicate components are masked using latex plugs or polymer tapes to prevent coating.
Mixing Two-Part Silicones
For two-part silicones, the resin and curing agent are precisely mixed just prior to application as per manufacturer instructions.
Coating Application
Liquid silicone is applied via spray coating, brushing, dipping or jetting based on PCB size and geometry. Uniform coverage is ensured.
Curing Process
The silicone coating is allowed to cure as a solid protective film, either at room temperature, with heat, or using UV lamps.
Unmasking and Inspection
After curing, masking materials are removed. The coating is visually inspected under brighter lighting for pinholes, thin spots or other defects.
Testing Protection Levels
The coated assembly may be subjected to insulation resistance, high voltage withstand, or other applicable tests.
Packaging for Shipment
The conformal coated board is then packed with moisture absorbents as needed for shipment to customers.
Selective Coating vs. Total Immersion
For high volume production, PCB assemblies are conformal coated using automated equipment:
Selective Coating uses programmable coating robots or mechanisms to deposit silicone only on the required areas, avoiding connectors or testpoints. This allows easy post-coating access.
Total Immersion dips the entire PCB assembly into a silicone bath to coat all surfaces. Additional steps may be needed to strip the coating from connectors and test areas.
Selective coating provides flexibility to customize the coating area for each board design. It consumes less coating material compared to total immersion. But total immersion allows very fast cycle times for high volume production.
Silicone Conformal Coating Selection Criteria
Key considerations for selecting a silicone conformal coating include:
- Operating temperature range based on thermal loads on the PCB components and the application environment. Wide temperature range silicone coatings are preferred for versatility.
- Flexibility levels based on mechanical stresses on the PCB. Softer, more flexible silicones cushion better against vibrations/shocks.
- Hydrophobicity based on the level of moisture protection required. Higher water contact angle gives better moisture resistance.
- Dielectric insulation needs based on conductor spacing and voltage isolation requirements on the PCB.
- Coating thickness dictated by PCB component heights and conductor clearances. Thinner coatings 20-40 microns are suitable for most applications.
- Viscosity should allow coating intricate PCB geometries. Thinner silicones with 300-1000 cPs viscosity are easier to apply on complex PCBs.
- Cure speed required – one-part silicones for room temperature curing or two-part silicones for faster heat/UV curing.
- Rework needs to remove and recoat certain sections if repairs are envisaged. One-part silicones allow simple rework using solvent stripping.
- Outgassing requirements – for use in vacuum or hermetic environments, low outgassing silicones are chosen.
- Biocompatibility for medical devices – specialized medical grade silicones are used.
- Approvals like UL 94 V-0, IPC CC-830, RoHS compliance. Automotive grade silicones meet IATF 16949 standard.
Key Silicone Conformal Coating Manufacturers
Some leading manufacturers of silicone conformal coatings for PCB protection include:
- Dow – SYLGARD conformal coatings
- Shin-Etsu – KJR and KE Series silicone coatings
- ACC Silicones – SILCOSET range of silicone coatings
- Henkel – LOCTITE conformal coatings
- Dow Corning – SE92XX silicone conformal coatings
- Electrolube – UR5634 Underfill and Conformal Coating
- Dymax Corporation – 9481 and 9482-LV silicones
- MG Chemicals – 419 and 422 Series conformal coatings
These companies offer broad silicone conformal coating product lines catering to different PCB protection requirements across industry sectors with strong global technical support services.
Conclusion
Silicone conformal coatings deliver a unique combination of properties that make them ideally suited for protecting PCB assemblies from environmental and operational stresses. The hydrophobic nature, wide temperature range, high dielectric strength, chemical inertness, shock absorption, and easy application enable silicone coatings to safeguard the reliability of PCBs across diverse use conditions.
Both global manufacturers and local PCB assembly service providers offer a range of silicone coating products to suit specific application needs in terms of viscosity, flexibility, temperature resistance, dielectric rating, outgassing levels etc. By applying the optimal grade using controlled processes, silicone conformal coatings enhance the field reliability and extend service life of electronic systems.
Frequently Asked Questions
Q1. How does a silicone conformal coating protect a PCB?
Silicone coatings protect PCBs by:
- Providing a moisture and chemical barrier
- Electrically insulating high-voltage conductors
- Absorbing mechanical and thermal shocks
- Preventing growth of dendrites between traces
- Shielding against environmental contaminants
- Extending product reliability and lifespan
Q2. What are some key properties of silicone coatings?
Key properties include:
- Hydrophobicity
- Wide temperature range (-60°C to 200°C)
- High flexibility and elongation
- Excellent dielectric strength
- Chemical and UV resistance
- Thermal shock absorption
- Flame retardancy
Q3. When should PCB assemblies be conformal coated?
Conformal coating should be applied when PCBs are used in:
- High-humidity environments
- Applications with large temperature swings
- Presence of destructive contaminants or chemicals
- Conditions prone to vibration/mechanical shock
- High-voltage boards requiring insulation
- Densely-packed boards susceptible to dendrite growth
Q4. How is silicone conformal coating applied to PCBs?
Typical application techniques include:
- Selective spray coating using automated, programmable equipment
- Dipping in silicone baths for total immersion
- Manual brushing for coating selective areas
- Jetting for precise coating of components
- Self-healing silicones via microcapsules for repairable coatings
Q5. Can silicone coatings be removed for repairs/rework?
Yes, silicone coatings can be selectively stripped using solvents like xylene, toluene or THF for rework, without affecting the PCB. This allows easy repair of coated boards.
