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Understanding the Importance of the Moisture Vapor Transmission Rate of Conformal Coatings

Posted by Dan Griffin on Mar 25, 2020 10:00:00 AM
Dan Griffin
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Are conformal coatings waterproof? The above is probably the most common question and most misunderstood subject in the area of PCB protection

A critical function of conformal coatings is to protect PCBs and electronics from damage caused by moisture and humidity. In this discussion, we will attempt to answer some of the more common questions we often hear regarding the level of protection you can expect from a conformal coating.

Are conformal coatings waterproof?

The above is probably the most common question and most misunderstood subject in the area of PCB protection

The short and simple answer is NO.

Are PCBs waterproof?For those of us old enough to remember, it is a bit akin to early generation wristwatches that were marketed as “water-resistant” but were not in fact “waterproof.” Your watch at that time would probably withstand a little showering but was not really designed for swimming and diving in the Caribbean all week. Conformal coatings can be considered “resistant” to moisture vapor and water, but they are not designed to be “waterproof.”

Conformal coatings are purposely designed as semi-permeable membrane coatings, which allow certain amounts of mainly gaseous material to flow in and out. This is a necessity in coating PCBs as there will almost always be entrapped liquids and gases liberated from the board over time, which must be allowed to escape to avoid potential failures.

As a result of the necessity for a semi-permeable surface coating, conformal coatings are not waterproof, meaning they are not completely impervious to water and water vapor over extended periods. They are not suitable in most cases for long-term immersion in liquids including water but are acceptable for intermittent exposures such as splashing. They do not completely guard against moisture vapor and humidity, but the use of conformal coatings will greatly extend service life and performance in high-humidity environments.

Related article:Simple Steps to Ensure Proper Wetting & Good Adhesion

What is “moisture vapor transmission rate?”

Moisture vapor transmission rate is generally abbreviated as MVTR. It is also often used interchangeably with “water vapor transmission rate” or WVTR.

A general definition of MVTR and WVTR is the mass of water vapor that passes through a given area of material being tested at a specified temperature and humidity over a given period of time. As a result, it is generally expressed in grams/square meter/24 hours. The higher the number, the more water vapor that has passed through the material in 24 hours. Lower values mean higher resistance and better protection from moisture vapor.

Example:

Plastic Thickness

MVTR (grams/square meter/24 hours)

PET 1 mil (25 microns)

18

OPP 1 mil 10
HDPE 1 mil   

5

In the above example comparing 3 types of plastic packaging films, the HDPE film would provide the best moisture vapor protection as it has by far the lowest MVTR. For an accurate evaluation, it is important that films and coatings be compared at THE SAME THICKNESS. Thicker coatings will almost always improve protection.

How is MVTR measured and tested?

There are many methods to measure the MVTR of a given plastic film or coating. Regardless of their complexity or simplicity, they all involve a process similar to the diagram below.

Moisture Vapor Transmission Measurement-1Image: Moisture Vapor Transmission Measurement

In this fairly simple way, it is possible to allow the coating to be exposed to a certain humidity for 24 hours and then measure by mass the amount of moisture (water) that passes through into the measurement chamber.

Are all conformal coatings the same for MVTR?

No, not all conformal coatings protect equally against moisture vapor. In fact, the chemistry of the conformal coating plays a critical role in the level of protection you can expect.

Conformal coatings are usually specified to be applied at 1 to 3 mils thick or 0.025 to 0.075 microns thickness. Because of this, it is usually a good idea to look at the MVTR for your coating within the range it will ultimately be coated on your PCB.

The chart below shows the dramatic differences in performance in MVTR testing for some of the most commonly available conformal coating chemistries.

Moisture Vapor Permeability

There are many considerations other than just MVTR such as processing speed, rework considerations, and environmental issues. Nonetheless, as a general rule the following is a simple list from best to lowest for MVTR:

  • Synthetic Rubber (Best)
  • UV Curable
  • 2 Part Epoxy
  • Acrylic
  • Polyurethane
  • Silicone (Worst)

What if My Electronics Require Long Term Liquid Immersion?

OK, so my application requires electronics to be protected against continuous or longer term liquid exposure. Now what?

These applications are beyond the capability of conformal coatings in most cases. Your options can include:

  • Hermetic sealed enclosure or boxing
  • Encapsulation and potting

There are a wide range of available encapsulation technologies such as 2 component epoxies, 2 component urethanes, and UV curable materials. They are generally applied in significantly thicker layers and can provide the needed protection against the most severe environments including longer term liquid exposures. Needless to say this comes at an additional cost due to the larger volume of coating required and thus this approach is usually limited to the most extreme protection requirements.

Resin-Designs-Potting-2

Resin Designs Encapsulations

As a supplier of electronics protection polymers and coatings, Chase Corporation and our employees can help you with an unbiased approach to evaluating your application and process. We’ll show you how to maximize efficiency, minimize cost, and improve product reliability. Our outstanding manufacturing and technical support groups can provide your organization with reliable global supply, unmatched quality, and superior technical support.

Please contact us today to discuss your application.

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Topics: Environmental Conditions, Conformal Coating Process, Conformal coating benefits, waterproofing silicones, waterproofing