How to choose conformal coatings that work over the long term

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In an informative panel discussion, global experts dealing in conformal coating solutions share detailed insights on the various considerations for purchasing conformal coatings for LED outdoor modules. This panel discussion was originally published at www.circuitnet.com

EB: How does one select the right conformal coating?
Wayne Wagner, president, Krayden Inc.: The very first aspect of review is to see if the conformal coating (CC) will be in contact with the LED or over it. This is because most CCs will have a UV dye in order to be able to inspect the boards. However, the LED has a dome that protects the die, so if this dome is made of silicone, it can absorb the UV tracer from the CC (any chemistry of CC) and that will change the colour of the lamp’s light a bit—towards the blue side of the spectrum.

It has been found that sometimes even the lateral contact on the LED is enough to get the UV tracer migration in a few weeks.

If the CC is going to be over the LED or in contact with it, you require a UV-less CC and only a few companies have one. The lack of UV tracer minimises the potential effect over the LED.
If the application of the coating is not in contact with the LED, you have more options. However, the need for good weatherability will point you towards longer lasting coating technologies. Even when the lamp is facing down or the board is not exposed to solar light, you require a CC that does not age on exposure to oxygen or does not delaminate due to strong thermal cycling.

Most outdoor applications (from smart meters and outdoor LED signs to traffic banners) lean towards long lasting CCs that can handle a wide range of temperatures. Given the proven performance of certain chemistries used in buildings, where joints have to withstand harsh environments for over 25 years, it is easy to extrapolate the long lasting resistance needed for outdoor applications like those mentioned here.

Most CCs are similar in cost per application these days. But you need to really look at cost per board. Personally, all of our technical sellers use a ‘coating cost calculator’ to estimate the final cost per board.

Rodney Miller, capital equipment operations manager, Specialty Coating Systems: Conformal coatings come in four types of primary liquids and one type of polymer. They are generally only a single part of a protection system. How your PCBA is packaged and what elements it is exposed to, as well as its mechanical and performance attributes will all need to be considered. Also, consider design elements that will make conformal coating applications robust such as solder mask finish, cleanliness, as well as selecting the right solder systems and components.

The least cost, base materials available are usually of the liquid acrylic type, and if applied as recommended in an indoor ambient environment, will last beyond five years. As you increase the mechanical and/or exposure types, other liquid coatings like urethane, silicone and epoxies can be considered as alternatives.

If your design is affected by heat or if the LED needs to be protected with minimal effect to the light wave, parylene, which has the least thickness of all liquid coatings, can be applied. Parylene provides significant protection at thickness levels that are generally undetected on the final product.

Coatings, whether liquid or polymer, have their own pros and cons and the costs vary depending on the product application’s requirements. Additional design considerations include the substrate, the volume, local regulatory requirements and the ability to rework the product. When you evaluate all these considerations, one or two types of coatings will rank above the rest.

EB: What are the environmental protection strategies linked to using conformal coatings?
Fritz Byle, process engineer, Astronautics: A conformal coating is certainly one component of an environmental protection strategy. Some things to be cautious about here are:

  • Remember that conformal coatings do not offer protection against water, including anything other than very temporary condensation.
  • Conformal coatings cannot be used everywhere, so some areas may remain unprotected, e.g., under BGAs (and you don’t want conformal coatings under BGAs for reliability reasons).
  • You will not want to coat over high-power LEDs used for illumination.
    Determining the best coating type for an application requires looking at a lot of variables. Achieving the five-year life will require selecting sound coating material, assembly and coating process development as well as enclosure design.

EB: What are the best conformal coating options if one wants to run an outdoor LED module for at least five years?
Chris Palin, European manager, Humiseal: The basic material type required would be an acrylic conformal coating designed for LED applications such as HumiSeal 1B73LED. You will need to consider how you intend to apply the material, and its compatibility with both the LED lens and solder flux residue if it is a no-clean assembly process. Five years is a short lifetime for a conformal coating, as most are designed for a 25-year warranty.

Rick Perkins, president, Chem Logic: There are two good options for an outdoor conformal coating. One is a silicone-based product that can also withstand the high temperatures that may be associated with the operation of the unit, and it is the most flexible. A one-part silicone CC will offer ease of application; just be sure to allow the coating to cure for a minimum of three days at a humidity greater than 50 per cent or higher. Make sure to segregate the silicone conformal coating process from the electrical soldering process, otherwise cross-contamination could be fatal to the electricals.

The second option is a two-part polyurethane CC. Not all polyurethane conformal coatings are equal. The benefits of the urethane coating are a faster cure time and varying durometers; the drawback is that they usually require an isocyanate catalyst that needs to be handled with care.

EB: What type of coating should be used to protect an outdoor LED module from damages due to humidity or moisture?
Doug Pauls, principal materials and process engineer, Rockwell Collins: It depends on how much access the humidity has to the electronics. If you are talking about humidity, and humidity only, then a good acrylic may be sufficient. If you have the possibility of water flowing across your electronics, then you don’t want an acrylic.

For wet situations, I would recommend a silicone conformal coating. If you are talking about high volume production, then a UV cure silicone is a good choice. A decent polyurethane may also fit the bill, but some polyurethanes don’t react to running water very well either.
If the electronics is going to be exposed to direct sunlight, then you may want a UV inhibitor in the coating, which may rule out the UV curable coatings. If you look at QPL-46058, you can find the companies that provide conformal coatings for electronics that go into the environments you talk about. Any one of them should be able to find the best fit of a conformal coating for your application.

Pierce Pillon, laboratory manager, Techspray: A stock answer for these types of service environments would be a silicone coating, of which there are many to choose from. Another consideration would be the coating potentially scattering the light from the LEDs.
Silicones are capable of both glossy and matte finishes, which can dramatically change the LED light from a viewer’s perception. However, do consider other resin types also, depending on the service criteria.

Lee Hitchens, owner, SCH Technologies: This is a very interesting area right now in ‘protective coatings’. I say this rather than conformal coatings since there are several ways to solve the issue.

You just must make sure the coating selected is suitable to be exposed to UV light over long periods of time. Otherwise it can degrade. For example, the UV trace in the coating can degrade and turn yellow. What you must be careful of is the cost of processing, and ensure the LED board is designed to be coated efficiently.

Also, encapsulates are very successful. They are a very good water-proofing method, especially for devices that could get actual water on the boards. Again, design is critical for cost reduction.

Finally, the new surface modifiers coming out now actually look like they could be effective for LEDs. There are new products that supposedly do not require masking, but moisture-proof/water-proof the circuit board. This is an area we are investigating, and it could revolutionise conformal coatings.

Mike Jones, VP, MicroCare: My general observations would be as follows. Acrylic conformal coatings are inexpensive, easy to work with and provide reasonable coverage for products with modest performance requirements. Many companies make such coatings and so prices are very competitive. These also are the least durable coatings. MicroCare and many other companies make solvents that will remove acrylic coatings.

Silicone coatings are made by several companies, the most prominent being Dow-Corning. These products usually are a little thicker and heavier, but also can handle greater temperature extremes. In particular, since silicone is a poor transmitter of heat, these often are used in high-temperature environments. MicroCare offers two products that remove silicone coatings; I am not aware of any other benchtop product from other companies that can do so.

Urethane coatings generally are the thickest type of coatings and provide exceptionally durable protection. Humiseal is the big brand name here. I have seen urethane coatings on circuit boards used on trains, and they remained very well protected even after 20 years of service. These coatings are very hard to rework, and very few solvents will remove them. They also are the heaviest coatings and tend not to be used in applications where weight is a factor (aviation, for example).

Parylene and epoxy coatings are the hardest, most durable, most expensive and impossible to rework. I would not suggest using these except for the most specialised requirements. Overall, I would recommend silicone coatings. They offer the optimum mix of protection, durability, ease of rework and affordability.

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