Part 1 of a primer on types and uses of these protective materials.

A conformal coating is defined as a thin polymeric material that covers the surface of an electronics assembly. These coatings are used to provide an electrically insulative and environmentally protective seal or cover to a completed PCB.

Conformal coatings protect the PCB from solids, vapors and fluids. They offer physical protection from assembly debris such as wire clippings, loose hardware, and solder splatter, and can help protect the PCB from vibration damage. Given a single exposure, they will usually protect the surface against vapors. Different chemistries repel contaminants at different levels; material compatibility studies must be performed to determine the correct one for the application. Conformal coatings are divided into five classifications based on their chemistry.

Acrylic resin (AR) is a relatively hard material with a smooth, glossy finish. It has lower abrasion resistance and yields readily to scraping, chipping and flaking. Most acrylics can be softened by heat resulting in a gummy residue. These coatings are dissolved in solvent in the fully cured state, and as a result, they can be easily reworked. Acrylics can be applied by brush, spray or dip coating.

Epoxy resins (ER) offer good humidity resistance, high abrasion and chemical resistance, as well as good electrical properties (low dielectric). They are one of the hardest coatings and form a hard, smooth surface that will resist chipping, peeling or cracking, and form one of the strongest surface adhesion bonds. They are difficult to rework and require a soldering iron to penetrate the coating when replacing a component. Epoxy resins are usually available as a two-part system and can be applied by brush, spray or dip coating.

Silicon resins (SR) have excellent dielectric strength and high arc resistance. Cured, they are rubbery and pliable. Adhesive strengths range from easily detachable to tightly bonded. They have good moisture, humidity, UV resistance and thermal endurance. They are difficult to rework because they are resistant to most common solvents. Custom thinners and solvents are available from the manufacturer. Silicon resins can be applied by brush, spray or dip coating.

Polyurethane resins (UR) offer good humidity and chemical resistance and have high dielectric properties. The coating surface finish is smooth, glossy and nonporous. Coating hardness ranges from extremely hard to relatively soft. They have medium bond strength and tend to peel or flake off in large pieces. Heat at or near the solder melting temperatures will soften polyurethanes and make them pliable. They can be soldered through for rework, but this produces an unsightly brown residue. Polyurethane resins can be applied by brush, spray or dip coat application.

Parylene (XY) coatings have good dielectric strength, low thermal expansion, good abrasion resistance and outstanding chemical resistance. These coatings form a strong surface bond and provide a thin, uniform coverage that conforms fully to the PCB contour. They are used to protect circuits against high humidity, intermittent immersion, salt fog, pollution and aggressive solvents. They are FDA-approved for use in medical applications. They are effective in high-voltage applications because they can coat sharp edges. Parylene coatings are applied by vacuum deposition.

Specifications and Standards

MIL-I-46058, Insulating Compound, Electrical for Coating Printed Circuit Assemblies, is an older military specification that lists technical criteria for conformal coating characteristics. It also lists the quality assurance tests and how they are to be performed. A companion document, QPL-46058, lists coating materials that are in compliance with MIL-I-46058 and is used by the federal government for acquisition purposes.

IPC-CC-830B (with Amendment 1), Qualification and Performance of Electrical Insulating Compounds for Printed Board Assemblies, was derived from MIL-I-46058 and establishes qualification and performance requirements and characteristics.

IPC-HDBK-830, Guideline for Design, Selection and Application of Conformal Coatings, was written to assist in conformal coating selection. It outlines typical properties of each coating type and how they impact end-use performance. It also outlines processing steps to ensure proper coating application.

Next month: The coating process and acceptance criteria.

The American Competitiveness Institute (aciusa.org) is a scientific research corporation dedicated to the advancement of electronics manufacturing processes and materials for the Department of Defense and industry. This column appears monthly.