Conformal Coating in Low Humidity Environments

Whereas conformal coating is excellent for high humidity protection - is there any performance impact when subjected to very low humidity environments?

B.L.

Expert Panel Responses

Dr. Pauls, could you expand on your answer slightly? We very much respect your knowledge on the subject of conformal coatings, and we are wondering which coating would provide the best protection from Diet Mountain Dew?

Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics

Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.

I will give you my favorite engineering answer - It Depends.

What do you mean by"low humidity"? And what conformal coating are we talking about? Different chemistries respond to the environment differently.

In general, in low humidity environments, there is no humidity challenge to the coating. Any humidity that may have passed through the coating during humid exposures would diffuse out during very dry periods. The only impact I can see if if you are talking about dessicated storage, like in pure nitrogen or 0-1% RH environments.

Conformal coatings are all organic materials and can dry out with time, though I have not ever seen this. Potentially, coating could crack under such situations as the plasticizers leave the material(assuming they are there in the first place). You would be best tocontact the vendor of the coating you are interested and pose the question. Coating vendors test their products in a lot of weird environments.

Doug Pauls
Principal Materials and Process Engineer
Collins Aerospace

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.

Not if it is fully cured.

Mahendra Gandhi
SME - PWB Technologies
Northrop Grumman

Mahendra Gandhi has been working in interconnect industry since 1972.

Conformal coatings provide a breathable membrane protecting from humidity and contamination. They will all vary slightly in resistivity depending on the exposure time to varying humidity levels.

The optimum properties of a conformal coating are measured when it is fully cured and dry therefore there is no performance impact when they are subjected to very low humidity environments. This is possibly the environment when they are protecting from contamination such as dust!

Some polyurethane and silicone coatings rely on moisture in the air to fully cross link the polymer(fully cure).Curing times could be affected if applied in a very dry environment.

Chris Palin
European Manager
HumiSeal

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.

As far as I can tell, there is no downside to using conformal coating in a low humidity environment.

I believe the coating would also act as a barrier to prevent particulate matter from attaching itself to the laminate material and it will also act as a mechanical barrier to protect the surfaces of the board and the components.

Leo Lambert
Vice President, Technical Director
EPTAC Corporation

At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.

Low humidity environments have no impact on the performance of a cured conformal coating.However, in some cases the relative humidity level may impact the cure process itself whereby impacting the post cure performance. To understand this it is important to understand how conformal coatings work.

To begin it is important to note with every polymeric material water absorption will inevitably occur. The design of conformal coatings is unique in that the material is able to maintain adhesion to the respective surfaces coated while it absorbs moisture and ultimately reaches equilibrium. With this in mind, the performance of a conformal coating is only as good as the strength to the surface after cure.

Some conformal coatings utilize UV light to polymerize the reaction. Depending on the configuration of the parts, the material may not see sufficient direct exposure from the source to fully drive the reaction. These are called shadow areas, requiring a secondary cure mechanism to complete the reaction.

Many conformal coatings use heat or oddly enough moisture to drive the secondary cure. Wants sufficient moisture is absorbed by the unreacted parts of the polymer and the material fully sets. At this point moisture penetration is defined by the saturation characteristics of the material and the strength of the conformal coating to the surface. If the relative humidity is low in the surrounding environment during cure is low than there is a chance the material may not reach its optimal adhesion whereby affecting performance.

With a fully cured conformal coating the relative performance in a moisture rich (i.e. wet) versus moisture starved (i.e. dry) environment is negligible. The operational life time achieved in a moisture rich environment, such as south east Asia,would be matched or exceeded in a dryer environment, such as the western United States. The main item to consider is how well was the material cured...

Doug Dixon
Marketing
360-Biz

Douglass Dixon is the Chief Marketing Officer for 360 BC Group, a marketing agency with offices throughout the US. 360 BC specializes in consulting and implementing successful marketing programs that utilize the latest in marketing, sales and technology strategies. As an electronics veteran, Dixon has worked in the industry for over 30 years for companies like Henkel, Universal Instruments, Camelot Systems, and Raytheon. Dixon's electronics industry experience includes a broad skill set that includes engineering, field service, applications, product management and marketing communications expertise.

The only two downsides of coating in low-humidity environments that I can think of are also present in any humidity level, cost of application and a change in dielectric properties of the assembly. Cost cannot be used as a factor if there is any possibility the CCA will in fact see high humidity or even liquid moisture.

The dielectric properties are usually minor. So the only other possibility I can think of is possible cracking/peeling of the coating under extremely dry conditions, but for most coatings that is not very likely.

Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics

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