Conformal coating is a protective coating that is applied to printed circuit boards (PCBs) to shield them against moisture, dust, chemicals, and temperature extremes. This coating acts as an insulating barrier and helps extend the life and reliability of PCBs. With the miniaturization of electronics and the advancement of technology, conformal coatings have become indispensable in the electronics industry.
Conformal coatings, also known as conformal films, are thin polymeric coatings applied on printed circuit boards (PCB) and electronic assemblies. The purpose of conformal coating is to protect the components and circuitry from moisture, dust, chemicals, and temperature extremes that can cause current leakages, electrical failures, and corrosion.
Conformal coatings are called “conformal” because they conform to the contours of components and circuit boards, creating a protective covering. The coatings are made of insulating polymeric materials like acrylic, silicone, urethane, perylene, epoxy, etc. When applied correctly, they form a uniform coating just a few millimeters thick over the entire assembly.
The benefits of conformal coating include:
– Increased resistance to moisture, fungi, dust, and corrosion
– Protection against electrical shorts from condensation and surface contamination
– Prevention of dendritic growth and tin whiskering
– Improved product reliability and lifespan
– Aid in meeting safety requirements and industry standards
There are several types of conformal coatings available, and the choice depends on the application, operating environment, and requirements. The common types include:
Acrylic conformal coatings provide good moisture and chemical resistance at an affordable cost. They cure rapidly at room temperature and can be easily repaired. Acrylics are suitable for most PCB applications but have limited resistance to solvents.
Silicone conformal coatings offer excellent moisture, chemical, thermal, and UV resistance. They retain their flexibility over a wide temperature range but require elevated curing temperatures. Silicones can be difficult to repair and require primers on some surfaces.
Urethane conformal coatings provide high abrasion and chemical resistance. They have good dielectric properties over a wide temperature range. However, urethanes are relatively expensive and require long curing times.
Parylene conformal coatings are deposited through vapor deposition, enabling uniform thin films over complex surfaces. They offer excellent penetration, dielectric strength, and moisture resistance. However, parylene deposition requires specialized equipment.
Epoxy coatings provide superb moisture, chemical, and abrasion resistance. They have good dielectric properties but require elevated curing temperatures. Epoxies are relatively rigid coatings with limited repairability.
Applying Conformal Coatings
Conformal coatings can be applied on PCBs through several techniques:
Dipping involves submerging the PCB in a conformal coating bath. It allows the coating of complex geometries but can result in uneven coatings. Drip drying and fixture rotation help achieve uniform coats.
Spray coating using pressurized spray guns allows fast application on high-volume production lines. Automated spray systems with programmable spray patterns provide excellent control of coating thickness and coverage.
Manual brushing lets accurate and selective application on specific components or areas of a PCB. Brushing is suitable for prototypes, rework, and touch-up applications. However, it can result in uneven coats.
Automated dispensing systems use programmable needle valves to deposit precise dots or beads of conformal coating. Dispensing allows accurate coating of selective components and leads to little material waste.
Parylene coatings are applied through vapor deposition under vacuum conditions, enabling excellent penetration and pinhole-free coatings of uniform thickness. However, it requires costly specialized equipment.
Selecting Conformal Coating Materials
The selection of conformal coating material depends on several factors:
Operating environment – Coating must be compatible with temperature, humidity, chemicals, etc. Silicon and parylene suit high temperatures while acrylic suits room temperatures.
PCB material and components – Compatibility of coating material with PCB substrate, components, solders, wire insulation, etc must be ensured.
Coating thickness required – Thinner coatings (25-75 microns) suffice for most applications while thicker coats (75-175 microns) suit extreme conditions.
Repairability – Acrylic and some urethanes are easiest to repair while silicone and perylene are difficult to repair.
Process considerations – Coating materials must suit the application method used. For example, viscosity must suit spray, dip, or dispense application.
Safety approvals – Coating must meet flammability ratings like UL94 V-0 and IPC standards.
Cost – Acrylics are economical while parylene and some urethanes are expensive.
The component datasheets, material compatibility charts, and conformal coating supplier recommendations should be referred to for selecting the right coating.
Step-by-Step Conformal Coating Application
Follow these key steps for proper conformal coating application:
1. Surface Preparation
The PCB must be thoroughly cleaned to remove contaminants like moisture, grease, and particulates which can cause coating defects. Solvent cleaning and plasma/laser surface activation help improve coating adhesion.
Areas that should not be coated like connectors, test points, switches, etc. must be masked using plugs, caps, tapes, and latex. Masking tackles problems like inserted connectors and avoids rework.
3. Coating Material Preparation
Coating materials like acrylics, urethanes, and silicones typically contain two parts (resin & catalyst) that must be mixed as per the manufacturer’s instructions before application. Pre-heating the PCB helps lower coating viscosity for uniform coats.
4. Coating Application
The mixed coating must be applied according to the selected application method – dip, spray, brush, dispense, etc. Proper fixturing ensures boards remain static during coating. Automated precision spray and dispense systems allow accurate coating thickness.
The applied coating must be cured as per the manufacturer’s recommended procedure to ensure full cross-linking. Insufficient curing can lead to tacky and weak coating unable to protect the PCB. Curing is done by heat in ovens, or at room temperature.
6. Unmasking and Touch-up
After curing, all masking materials must be removed. Any minor defects should be repaired with a touch-up coating. The coated board should be visually inspected under adequate lighting for any flaws.
Additional heat curing for a few hours may be recommended by the manufacturer to ensure a complete cure. Post-curing enhances the protective properties of the coating.
Conformal Coating Safety Precautions
Conformal coatings contain volatile organic compounds (VOCs) and solvents that require safe handling. Take the following precautions:
– Wear nitrile gloves, eye protection, and respirators to prevent skin/eye contact and vapor inhalation.
– Coating application and curing must be done in well-ventilated areas to prevent vapor buildup.
– Flammable solvent-based conformal coatings require explosion-proof environments and spark-free tools.
– Coating containers must be properly closed to prevent evaporation and ignition hazards.
– Avoid contact of coatings with water (moisture catalysts cure reactions).
– Hazardous uncured coatings should be disposed of per local environmental regulations.
With proper handling, conformal coatings provide excellent protection for PCB assemblies. However, no coating can prevent damage from extreme over-voltages or physical damage. Regular inspection of coated boards for damage is recommended to achieve maximum reliability.
Types of Conformal Coating machine