Conformal coatings are polymeric materials which protect electronic assemblies from environmental contamination, and serve as insulative protection and a physical barrier. There are five main types of conformal coatings categorized by their chemical composition: acrylic (AR), epoxy (ER), urethane (UR), silicone (SR), and poly(para-xylylene) (XY). Fluorescent compounds are incorporated into the coatings for ease of inspection under UV light, as shown in Figure 3-1. Each coating has advantages and disadvantages in terms of their deposition method, chemical properties, physical properties, reworkability, and affordability.
Acrylic resins (AR) are preformed acrylic polymers dissolved in a solvent. The “curing” process really drives off solvent (which can be either organic or water-based) forming a dried film. Acrylic coatings can be easily dissolved in many organic solvents for repair work and provide only selective chemical resistance. Acrylic coatings dry rapidly, have good fungus resistance, have long pot lives, give off little or no heat during cure, do not shrink during cure, and have good humidity resistance. At elevated temperatures, they soften more readily than other polymers. They also have low abrasion resistance, easily leading to scraping, chipping, and flaking. Acrylic resins can be applied by brush, spray, or dip coating. Figure 3-2 shows an example of acrylic polymerization, where one of the reactive C=C double bonds of the acrylic monomer on the left connects with a neighboring monomer repeatedly to form a polymer chain.
Epoxy resins (ER) are usually available as two part compounds that start curing upon mixing, but single part coatings that can be cured thermally or with UV exposure, are also available. Epoxy resins exhibit good abrasive and chemical resistance, as well as reasonable humidity resistance. The coating is virtually impossible to remove and rework requires burning through with a soldering iron. A buffer is recommended around delicate components, since film shrinkage occurs during polymerization. The shrinkage can be minimized by curing at a low temperature. Epoxy resins can be applied by brush, spray, or dip-coating. Figure 3-3 shows an example of epoxy polymerization, where epichlorohydrin on the left reacts with a dialcohol to form an epoxy resin.
Polyurethane resins (UR) are either single or two-component compounds, which provide good humidity and chemical resistance, with high sustained dielectric properties. Due to their high chemical resistance, removal of the coating requires the use of stripping agents which may leave ionic residues. These need to be thoroughly cleaned to prevent corrosion on the underlying board. Polyurethanes can be soldered through for rework, but usually results in a brownish residue, which affects the appearance of the assembly. Polyurethanes have medium bond strength and tend to peel or flake off in large pieces. Polyurethane resins can be applied by brush, spray, or dip-coating. Figure 3-4 shows an example of urethane polymerization, isocyanates reacting with alcohols to form urethane linkages in polymer chains.
Silicone resins (SR) are usually single component compounds that begin curing upon exposure to moisture in the air, along with temperature. Silicones can endure extreme temperature cycling environments with a useful operating range from -55°C to +200°C. They have high humidity resistance, good thermal endurance, good UV resistance, low dissipation factor (useful for high impedance circuitry), and very good adhesion to most PCB materials. For those low surface energy PCB materials, such as polyimides, adhesion can be improved with primer agents or surface treatments of chemical or plasma etching. Silicon resins can be applied by brush, spray, or dip-coating. Figure 3-5 shows an example of silicone polymerization where water reacts with the silicon-containing monomer to form poly(dimethylsiloxane) (PDMS) chains and acetic acid as a byproduct.
Poly(para-xylylene) (XY), also known as Parylene, is a coating that is vacuum deposited in a process called Chemical Vapor Deposition (CVD). It has consistent thickness with true conformance to the board assembly contour, as well as being pinhole and bubble free. Parylene has a good dielectric, low thermal expansion, good abrasion resistance, and outstanding chemical resistance. It has been used to protect circuits from harsh environments, such as high humidity, intermittent immersion, salt fog, atmospheric pollutants, and aggressive solvents. They have been approved by the FDA in medical device applications. They are very effective in high voltage applications, due to its ability to coat sharp edges. They do not adhere well to boards that have ionic residues, so thorough cleaning should be performed prior to coating. It is an expensive process that exhibits poor repairability in comparison to other coatings. The higher cost associated with Parylene is attributed to the vacuum deposition process. This process is very engineering intensive, requiring control of the deposition rate to assure adequate coverage in the areas of interest, while preventing dielectric contamination with difficult and expensive masking of the board assemblies. Figure 3-6 shows an example of Parylene polymerization, where the starting material dimer is opened up and formed into chains.
Specifications and Standards
MIL-I-46058C – Insulating Compound, Electrical (for Coating Printed Circuit Assemblies)1: MIL-I-46058C is an older military specification that lists the technical criteria for conformal coating characteristics. It also lists the quality assurance tests and how they are to be performed. A companion document, QPL-46058,2 lists coating materials that are in compliance with MIL-I-46058 and is used by the federal government for acquisition purposes. On November 30, 1998, MIL-I-46058C was declared “Inactive for New Design” with no superseding specification.
IPC-CC-830B (with Amendment 1) – Qualification and Performance of Electrical Insulating Compound for Printed Board Assemblies3: IPC-CC-830B was derived from MIL-I-46058C and establishes qualification and performance requirements for conformal coatings. This standard allows manufacturers to qualify conformal coating products and define product performance characteristics to the standard.
IPC-HDBK-830 – Guidelines for Design, Selection and Application of Conformal Coatings4: IPC-HDBK-830 was designed to assist in the selection of a conformal coating. It outlines typical properties of each coating type and how they impact performance considering the intended end use.It also outlines processing steps to assure proper coating application.
Coating Acceptance Criteria
In IPC-CC-830B, conformal coatings fall under two classes: Class A and Class B. Class A is for non-hydrolytically stable conformal coatings, where lower moisture insulation resistance is permitted, and the temperature and humidity aging test is not required. Class B is for hydrolytically stable conformal coatings, where higher moisture insulation resistance is required, and the temperature and humidity aging test is required. These classes do not directly correlate to the Class 1, Class 2, and Class 3 in other IPC documents.
There are many testing requirements in IPC-CC-830B that a conformal coating must undergo to be qualified and accepted for use. These requirements and their respective test methods from ASTM, IPC, and UL, fall under thirteen categories:
- Shelf Life
- Fungus Resistance
- Dielectric Withstanding Voltage
- Thermal Shock
- Moisture and Insulation Resistance
- Fourier Transform Infrared (FTIR) Spectroscopy
- Temperature and Humidity Aging (Hydrolytic Stability)
A board assembly must be cleaned and dried eight hours before conformal coating. Removing any water in the assembly may be accomplished by an oven bake at 93°C +/- 5.5°C, for a minimum of four hours. The coating material is applied using a method that will yield complete coverage without excessive filleting or runs. Common coating methods include spraying, brushing, dipping, or chemical vapor deposition (in the case of Parylene).
The EMPF uses a Gen3 Systems DC-2002 Dip Coater and Gen3 Systems SB-2900 Conformal Coating Spray Booth for applications of conformal coating. The Dip Coater works with the controlled extraction rate of the board assembly from the conformal coating bath. The entire board assembly is dipped into the holding tanks with a controlled removal from the conformal coating to obtain uniform thickness. The Spray Booth works with the operator spraying the board assembly on a turn table, under UV illumination, and a ventilation system to minimize exposure.
The following is a list of considerations to keep in mind when choosing a conformal coating:
- Raw material characteristics: viscosity, VOC free, one-part/two-part, cost
- Final cured material characteristics: dielectric, chemical resistance
- Methods of application: capital equipment cost, speed/throughput
- Cure methods available: heat/thermal, ultraviolet (UV), vacuum deposition (Parylene)
- Cost of curing equipment: in-line heaters, deposition chambers
- Environmental impact: volatile organic compounds (VOCs)
- Cleanliness of board assembly prior to coating
- Ease of rework
- End use application
The EMPF facilities are well equipped to assist with the qualification of conformal coatings. In addition, the EMPF offers different deposition methods and techniques for conformal coating application and board assembly inspection.
1Insulating Compound, Electrical (for Coating Printed Circuit Assemblies) FSC 5970.
2MIL-I-46058 Qualification Information.
3Qualification and Performance of Electrical Insulating Compound for Printed Wiring Assemblies - Includes Amendment 1.
4Guidelines for Design, Selection and Application of Conformal Coatings.