Why use acrylic (AR) conformal coatings?
Acrylic conformal coatings make up a large percentage of the global market. There are many reasons for this including ease of application, cost and easy to repair.
Acrylic conformal coatings have the following features:
- Acrylic coatings provide an excellent barrier to moisture and humidity providing the highest level of protection as possible for liquid applied coatings.
- They are one of the lowest cost coatings to purchase
- They have minimal chemical resistance making it ideal for reworking and removal.
- Acrylics can be soldered through with minimal to no odour release.
- Acrylics dry to the touch in approx. 60 minutes or less and typically cure much faster than most coatings.
- All of these factors make this the most used conformal coating type in the world.
- They can be easily applied with a brush, sprayed by aerosol, spray gun or robot or by dipping.
This makes acrylic conformal coatings the most used conformal coating in the world.
What is an acrylic conformal coating?
Acrylic resins (AR) are single component materials.
More specifically, they are preformed acrylic polymers that are dissolved in a solvent to be applied to surfaces.
They do not cure but dry out. Therefore they can be in theory at least re-dissolved back into a solvent to reform the conformal coating.
Why use polyurethane (UR) conformal coatings?
Polyurethane coatings are one of the second most popular choices for conformal coating applications after acrylic conformal coatings.
Urethane materials are available in many different forms including single component, two component, UV curable, and water borne systems. As a group, all UR coatings provide excellent humidity.
A key feature of UR materials is offering chemical resistance; this makes them one of the more popular coatings when it comes to protection of the critical operation of your printed circuit boards (PCBs) in a chemically aggressive environment.
On the downside, urethanes can have a few more issues such as longer cure schedules, harder to rework due to chemical resistance, higher costs due to higher cost of resins and certain issues with health & safety when soldering through the coatings.
Why use silicone conformal coatings?
Silicone conformal coatings are most widely used in high temperature environments due to their innate ability to withstand prolonged exposure to higher temperatures compared to all other conformal coating chemistries. This characteristic has made them one of the primary choices for under the hood automotive applications where temperature is critical.
They are also capable of being applied in thicker films making them useful as a vibration dampening / isolation tool if the coated assembly is to be placed in a high vibration environment.
Silicone-based conformal coatings can be either tough, abrasion resistant, or soft, stress-relieving materials, depending on the formulation.
They are most recognized for their stability over a wide temperature range and during temperature cycling. These conformal coatings provide good moisture and corrosion resistance and are available as both solvent-based and 100% solids formulations.
The 100% solids materials can be cured with heat, moisture, or UV.
Reworking can be a significant issue but chemical strippers are available that enable this process to varying degrees of success.
As with epoxies and urethanes, there are some handling issues with silicone coatings, specifically if they are moisture cure in nature.
What is parylene conformal coating?
Parylene (XY) is a vapour deposited conformal coating which is applied as a gas in a vacuum chamber.
The final conformal coating easily penetrates everywhere on components, providing complete and uniform coverage.
Why use parylene conformal coating?
The main advantages of using a parylene conformal coating are:
- Parylene coatings are completely conformal to the surface of the Printed Circuit Board (PCB) or product. This means that the coating has a uniform thickness and is pinhole free. As a result, component configurations with sharp edges, points, flat surfaces, crevices or exposed internal surfaces are coated uniformly without voids.
- Parylene coating provides an excellent barrier that exhibits a very low permeability to moisture and gases. This means that products coating in parylene generally are more “waterproof” than the same products coated in a liquid conformal coating.
- Parylene is unaffected by solvents, has low bulk permeability and is hydrophobic. Coatings easily pass a 100hr salt-spray test.
Other advantages include it having excellent electrical properties, parylene is optically transparent, is applied at room temperature and is chemically and biologically inert.
For a more detailed look at parylene click Parylene Conformal Coating FAQs
What type of conformal coating should be used if the board has “press fit” connectors to be pushed through the conformal coating?
The key is to choose a soft coating to enable ease of push-fit and avoid risk of damaging the connector.
Acrylic materials are generally used by most people doing this kind of assembly, although some softer urethane coatings can work with a bit more effort, especially if the connector is fitted as soon as the material is touch dry.
UV cure materials are generally pretty tough after UV curing and so are less suitable (in general) for this kind of assembly.
Is pot and shelf life critical on conformal coatings?
It depends on the conformal coating material type as to how important shelf and pot life need to be monitored.
The most critical type of conformal coating which is affected by pot life is moisture cure materials such as conformal coating silicones. Any premature curing of the moisture cure product can cause production problems and the silicone coatings must be monitored carefully.
Other conformal coatings that may have pot life issues are two part products, which when mixed, will begin to cure within the allotted time period and cannot be reversed.
Correct stock control is crucial to ensure shelf life issues are not introduced.
Is out-gassing an issue with conformal coatings?
Some conformal coating materials out-gas and can cause conformal coating failures or problems to components around the circuit board like optics and sensors.
The effect can occur in a vacuum or when the conformal coating is heated, which can result in conformal coating material volatising and re-depositing on sensitive components.
There are conformal coating materials which have been tested and produce no out-gassing.