|Ask the Experts|
June 10, 2021
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
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:
Conformal coating is certainly one component to an environmental protection strategy.Some cautions here:
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.
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
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.
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
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.
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.
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
For a high humidity environment you'll want to select a coating with low moisture permeability.Robert Gelosa, AI Technology, Inc.
Moisture penetration properties are not the same as hydrophobicity - while silicone may be one of the most hydrophobic, it is actually the worst in preventing moisture penetration and thus worst in preventing moisture induced failures. Operating outdoors, especially in a salt-fog or industrial gas laden environment will exacerbate moisture issues like corrosion.
A coating that blocks moisture and corrosive gas, not just repels liquid water, is key to reliability in a setting like this.
For more details you can check out our webinar discussion: https://www.youtube.com/watch?v=XehszOq7PMs&t=161s
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