Ask the Experts
May 31, 2019
Conformal Coating Recommendation
We are developing an outdoor LED module. This module will needconformal coating to prevent humidity issues and protect the circuit. The product needs a 5 year service life.
What general type of conformal coating do you suggest that offer low cost, and will meet the 5 year minimum target.
Expert Panel Responses
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 LIQUID water flowing across your electronics, then you DON'T want an acrylic.
For really wet situations, I would recommend a silicone conformal coating. If you are talking about a high volume production, then a UV cure silicone. A decent polyurethane may also fit the bill, but some polyurethanes don't like running water very well either.
If the electronics will 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 see 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.
Principal Materials and Process Engineer
Doug Pauls has a bachelors in Chemistry & Physics, Carthage College, BSEE, Univ of Wisc Madison. He has 9 years working experience for US Navy - Materials Lab, Naval Avionics Center Indianapolis. 8 years Technical Director, Contamination Studies Laboratories. 11 years Rockwell Collins Advanced Operations Engineering.
Regarding conformal coatings, here at MicroCare we don't sell conformal coatings but work with many, many different types and brands. Usually our involvement is in the process of rework and repair, where the coating must be removed prior to a repair.
This also allows the MicroCare team to see many different types of coatings, and judge how they have survived over the years of service. My general observations would be as follows:
Overall,I would recommend silicone coatings. They offer the optimum mix of protection,durability, ease of rework and affordability.
- 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. They also are the least durable coating. 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 were still very well protected after twenty 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).
- Paralyene and epoxy coatings are the hardest, most durable, most expensive and impossible to rework. I would not suggest using these expect for the most specialized requirements.
Mr. Jones is an electronics cleaning and stencil printing specialist. Averaging over one hundred days a year on the road, Mike visits SMT production sites and circuit board repair facilities in every corner of the globe, helping engineers and technicians work through the complex trade-offs today's demanding electronics require.
Conformal coating is certainly one component to an environmental protection strategy.Some cautions here:
Determining the best coating type for an application requires looking at a lot of variables, and is beyond the scope of what it is possible to answer without a lot of additional information. Achieving the 5-year life will require sound coating material selection, assembly and coating process development and enclosure design.
- Remember that conformal coating will not protect from liquid water, including more than very temporary condensation
- Conformal coating does not go everywhere, so some areas may remain unprotected, e.g. under BGAs (and you don't want conformal coating under BGAs for reliability reasons)
- You will not want to coat over LEDs if they are high-power LEDsfor illumination
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
A stock answer for these type 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 LED's.
Silicones are capable of both glossy and matte finishes which can dramatically change the LED light from a viewer's perception. However, consideration may also be given other resin types depending on the service criteria.
I would be most happy to discuss your specific requirements and help you bring this issue to resolution. There are just too many variables and test methods to discuss in this forum without further information.
Pierce Pillon is the Laboratory Manager and lead formulations chemist at Techspray, a division of Illinois Tool Works (ITW) and a leading manufacturer of chemical products for the electronics industry.
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 assemble process.
5 years is a short lifetime period for a conformal coating as most are designed for 25 year warranty. We would be happy to help you in the coating selection process, www.humiseal.com
Chris Palin is currently managing European sales and support for HumiSeal Conformal Coatings. His expertise is in test & reliability, solder technology, power die attach and conformal coating.
This is a very interesting area right now in "protective coatings." I put this rather than conformal coatings since there are several ways to solve the issue.
It is possible to use conformal coatings. You just have to 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 yellow. What you have to be careful of is the cost of processing and ensure the LED board is designed to be coated efficiently.
Also, encapsulates are very successful. This is also a very good waterproofing 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 really effective for LEDs. There are now new products recently available that are claimed do not require masking but moisture proof / waterproof the circuit board. This is an area we are investigating strongly and it could revolutionize conformal coatings.
Contact me offline for more info on any of the points above.
Lee has worked within the conformal coating and electronics industry for over 18 years. His work includes scientific research into long term reliability of electronics, technical sales of conformal coating materials and equipment, owner of SCH Technologies, a conformal coating service in the UK, a member of the Diamond Coating Solutions Group, a global liquid conformal coating and Parylene coating service solutions provider, a founding member of Nexus3c, Conformal Coating Centre and a partner of Thin Film Partners.
Conformal coatings come in 4 primary liquid types and 1 polymer type and are generally only single part of a protection system. How your PCBA is packaged and what elements it is exposed to, mechanical and performance attributes will all need to be considered. Also, consider design elements that will make conformal coating application robust like solder mask finish, cleanliness, solder systems and component selection considerations.
The least cost, base materials available are usually liquid Acrylic type, and if applied as recommended in a indoor ambient environment will last beyond 5 yrs. 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 and/or the LED will need to be protected with minimal affect to light wave, Parylene can be applied with the least thickness of all liquid coatings. Parylene provides significant protection at thickness levels are generally undetected on the final product.
Coatings whetherliquid or polymer, have pros/cons and increment up in cost per the product application requirements.
Additional design considerations include substrate, volume, local regulatory requirements and ability to rework the product.
When you weigh all these items above one or two types will stand above the rest.
Engineering is generally a compromise, between what you need and what you can afford. We offer help to our customers the complete spectrum of application options. We look to provide industry leading technology and most often, least cost solution for their application across the life of their product.
Capital Equipment Operations Manager
Specialty Coating Systems
Rodney is currently Operations manager at SCS coatings, Global Leader in Parylene and Liquid Coating equipment. Rodney applies his BS in Computer Integrated Manufacturing from Purdue University, along with 20+ years of Electronic manufacturing and Equipment Assembly, to direct the Equipment business at SCS Coatings. "We provide unique, value added coating equipment solutions for our customers". Including conformal, spin and Parylene coating expertise.
There are two best options for an outdoor conformal coating.
One is a silcone-based product that can also withstand high temperatures that may be associated with the operation of the unit, and it is the most flexible. A one-part silicone would offer the ease of application; just be sure to allow the one-part silicone to cure for a minimum of 3 days at a humidity greater than 50% 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. Not all polyurethane conformal coatings are equal. The benefits of the urethane coating are faster cure time and varying durometers; the drawback of urethane is that they usually require an isocyanate catalyst that needs to be handled with care.
Rick Perkins is a chemical engineer with more than 33 years of Materials & Processes experience. He has worked with Honeywell Aerospace in high-reliability manufacturing, as well as with several oil-field manufacturing companies. He also has a good understanding of environmental, health, and safety regulations.
The very first aspect to review is if the CC will be in contact or over the LED.This is because most CC's will have a UV dye that has the purpose of being able to inspect the boards, however since the LED has a dome that protects the die,this dome if made of silicone can absorb the UV tracer from the CC (Any chemistry of CC) and that will change a bit the color of the light of the lamp 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 you require a UV-less CC and a only a few companies have one. The lack of UV tracer minimizes 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 of good weather-ability 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 is not aged by oxygen or does not delaminate due to strong thermal cycling.
Most outdoor applications you see(from smart meters to Outdoor LED signs to traffic banners) lean towards long lasting CC's that can handle wide ranges of temperatures and longevity. Given the proven performance of certain chemistry's in buildings where joints have to withstand environments for 25+ years it is easy to extrapolate the long lasting resistance to an outdoor application like this one.
Most CC 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.
Wayne Wagner has over 25 years in the conformal coating industry and is the president of Krayden Inc., a leading distributor of engineered materials.
All of the above comments are important. If in doubt you can and should test several types. The overall environment would be a consideration, would the area be near chemical production, or in a salt fog area with high humidity.
Maurice LeBlon, A.I.Technology, USA