A conformal coating is a thin polymeric film typically applied at 25-250 μm to electronic circuitry to act as protection against moisture, dust, chemicals, and temperature extremes. The coating ‘conforms’ to the contours of a printed circuit board (PCB) to protect the board’s components. When electronics must withstand harsh environments, most circuit board assembly houses coat assemblies with a layer of transparent conformal coating. In addition, conformal coatings are being used as a mitigation to reduce the potential of tin and zinc whisker short circuit risk on electronic assemblies using lead-free finishes and solders.
Conformal coating selection
The selection of conformal coating material needs to be considered in relation to the end-use environment and application method, as well as the life expectancy of the electronics coated. The correct selection ensures long term reliability of the circuit board and the avoidance of difficulties with both processing and costs.
The most common standard for conformal coating is IPC-CC-830B. This standards lists performance criteria as well as providing good references for coating defects as the result of poor surface preparation, coverage, cure, etc. Conformal coating inspection is a critical factor in determining successful coating application and long term reliability of PCBs. Using the IPC standard allows the coating operator to monitor the coating application performance. This can be done manually by the operator in an inspection booth by examining the PCB under white and UVA light or it can be done automatically by a conformal coating inspection system.
Conformal coating chemistries
There are many chemistries employed for conformal coatings out on the market today. While the “Material Considerations” section below is very important to finding the correct conformal coating, it is also important to find a coating chemistry meeting the application needs.
Acrylic Resin (Type AR):
Acrylic conformal coatings are easily applied. They dry to the touch at room temperature in minutes, have desirable electrical and physical properties and are fungus resistant. They have long pot life and low or no exothermic during cure, which prevents damage to heat sensitive components. Also, they do not shrink. The main disadvantage is solvent sensitivity, but this also makes them easier to repair.
Epoxy (Type ER):
Epoxy systems are usually available as “two-component” compounds. These rugged conformal coatings provide good humidity resistance and high abrasion and chemical resistance. They are, however, virtually impossible to remove chemically for rework because any stripper that will attack the coating also dissolves epoxy-coated or epoxy-potted components and the epoxy-glass printed circuit board itself. The only effective way to repair a board or replace a component is to burn through the epoxy coating with a knife or soldering iron.
Polyurethane (Type UR):
Polyurethane conformal coatings are available as single component, two components, UV curable, and water borne systems. As a group, all provide excellent humidity and chemical resistance plus outstanding dielectric properties for extended periods.
Silicone Type (SR):
Silicone conformal coatings are particularly useful for high temperature service, up to about 200° C. They provide high humidity and corrosion resistance along with good thermal endurance, making them desirable for PWA’s that contain high heat dissipating components such as power resistors. Silicone coatings are susceptible to abrasion (low cohesive strength) and have high coefficients of thermal expansion.
Parylene (non-fluorinated or fluorinated)
Parylene is a transparent polymer conformal coating that is deposited from a gas phase in a vacuum. These polymers are polycrystalline and linear in nature, possess superior barrier properties, have extreme chemical inertness, and because of the deposition process can be applied uniformly to virtually any surface and shape. The advantage of parylene coatings is that they cover hidden surfaces and other areas where spray and needle application are not possible. Coating thickness is very uniform, even on irregular surfaces. The three main disadvantages are that (i) any desired contact points such as battery contacts or connectors must be carefully covered with an air-tight mask to prevent the parylene from coating the contacts, (ii) it is a batch process and does not lend itself to high volume processing, and (iii) the cost per PCB can be highly prohibitive due to the capital investment costs and the cost per batch.
Amorpohous Fluoropolymer
An amorphous fluoropolymer can be formulated in to either a solventborne or vacuum deposition coating. If the former, there is low solubility, which results in limited film thickness. Both require special surface preparation to ensure adequate adhesion. Both have poor resistance to some acids and bases.
Below are the chemistries used and properties and strengths for each of the six primary conformal coating chemistries. As noted, approximately 80% of the global conformal coating market utilizes acrylic chemistry.
Type/Property |
Acrylic |
Epoxy |
Polyurethane |
Silicone |
Parylene |
Amorphous Fluoropolymer |
Cost |
Low |
High |
High |
|||
Application Ease |
5 |
3 |
4 |
2 |
1 |
2 |
Dry/cure time |
5 |
5 |
4 |
3 |
5 |
5 |
Chemical resistance |
1 |
2 |
5 |
5 |
5 |
5 |
Solvent resistance |
2 |
3 |
5 |
5 |
5 |
5 |
Moisture resistance |
5 |
5 |
5 |
5 |
5 |
5 |
Ease of field repair |
5 |
1 |
4 |
2 |
1 |
1 |
Heat resistance |
1 |
1 |
3 |
5 |
5 |
5 |
Dielectric properties |
5 |
5 |
5 |
5 |
5 |
5 |
Comments |
80% market |
Up to 150°C |
High chem/solv resistance |
-40°C to 200°C service temp |
Low dielectric, chem. inert |
Low dielectric, inert, High Tg |
1=low property 5=high property
Conformal coating material considerations
Selection of the correct choice of coating resin is dependent upon its service environment as well as its expected life. Acrylics are used extensively due to their lower cost and relatively good overall performance. If a highly corrosive environment or for military and heavy industrial applications, a more robust coating is required. Criteria for selection must be based on answering many questions, which will include:
- What are the physical, electrical, and chemical requirements for the coating material itself?
- Electrical, chemical, and mechanical compatibility with the parts and substances to be coated
- What is being protected against? (e.g., moisture, chemicals)
- What is the service temperature range?
- How fast does the material dry (cure)?
- What type of process and equipment is necessary to achieve the required coating quality (uniformity and repeatability)?
- How easily can the material be reworked once applied?
- How fast can the material be applied and dried (throughput time)?
- Price of the material per unit volume or price per coated part
Conformal coating application methods
The coating material can be applied by various methods, from brushing, spraying and dipping, or, due to the increasing complexities of the electronic boards being designed and with the ‘process window’ becoming smaller and smaller, by selectively coating via robot. Different methods of curing / drying are available depending on the conformal coating material. Nearly all modern conformal coatings contain a fluorescent dye to aid in coating coverage inspection, since any type of defect can result in a catastrophic failure of the board and the system.
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what is the best coating when pcb will be submerged in oil. it has to be oil safe so that over time the coat will not be dissolved or with the oil.
The most chemical resistant coating will be Parylene. If you expect to repair the pcb while in service, that will be problematic as per the article. The second choice would be amorphous fluoropolymer. Oil is one issue. What about temperature? No coating will protect the components from heat.
which potting coating material is IP65 rated ? any recommendation? which can be used on electro-mechanical equipment which used in harsh environment for e.g. Rail.
IP65 refers to dust and forced water blast more or less. So chemical is not an issue. But rail (which I assume is train/rail) would be more harsh. You really should ask your local suppliers of conformal coatings as they are in the best position to answer your question.
What’s your view on nano-coatings?
Any significant disadvantages to choosing the new technology over the more traditional coatings you highlight above?
My application is industrial power electronics (typically 3phase 400V) used in factory environments rather than handheld devices such as mobile phones for which the nano-coating looks to be often applied.
You didn’t specify which part is the nano part. If you’re asking about using nanoscale the were improving the scratch resistance, then you’re into the omocers or solgel coatings. If you’re asking about single wall nanotubes, and you’re talking about moving the charge that something totally different. Generally speaking, I believe in nanoparticles for almost any application. There are a lot of things that you can do with nanoparticles.
In regard to epoxy (Type ER) removal, I have had some success with dremmil tools but to your point it is very labor intensive.
Is there any coating that adheres well with no clean flux residue? Or must all conformal coated boards be cleaned thoroughly first?
Regards
All conformance coated boards must be cleaned thoroughly first. It really goes for any type of coating; it’s not just conformal coatings but any architectural, industrial coating, etc. always specifies thorough cleaning before it is coated. Why would you potentially have failures due to poor cleaning just to save time?
Is acrylic most widely used automotive conformal coating for automotive electronics such as (ECU, PCU, Switches, Sensors, Battery Casing etc) ?
Also would like to know which application is most widely used for above parts (Spray Coating, Dipping, Brush, Selective Coating or Vapor Deposition)
As I pointed out in the article, acrylic coatings are not the most durable coatings, and I can’t really say for certain for all automotive conformal coatings. I can only venture to guess that in either non-vital components, that acrylics could be used. As far as your second question, and really the first, I suggest you invest in a market study of conformal coatings since that would give you a much better perspective, price points and answer.