Working with electronics and conformal coatings the question I most often encounter by family and friends is: “Why aren’t all of my electronics waterproof?” My usual answer is: “Why would they need to be?” Do you really need to use a toaster in the rain?
That said, there are many devices that benefit from protection from liquids. Ionized water, the kind of water that flows from your tap or falls as rain, conducts electricity and can cause electrical shorts and electrical overstress (EOS) which can kill electronics and possibly even people when there’s enough electricity. So how do we prevent water damage, when should we and which devices are worth the time and cost?
Enclosures and Gaskets
To prevent a catastrophic failure when a product is exposed to water, any conductive areas on an electronics device need to be protected by some kind of barrier. In the past, devices that have needed waterproofing have used enclosures and gaskets. The enclosures were typically bulky and heavy, had to be carefully aligned and not disturbed to function well.
The disadvantages of enclosures with gaskets as the sole source of protection are that:
- Falls and physical shocks can potentially damage and/or dislodge the gasket which has to be repaired or replaced to ensure the original level of protection
- Sudden variations in temperature can lead to damage of one or both of the enclosure and gasket, since they may have different coefficients of thermal expansion (CTE) which leads to overwhelming stress
- Any enclosure that’s been opened should be tested to ensure the expected performance
- Physical buttons, switches, and other elements external to the enclosure shouldn’t be operated underwater or while it’s not dry, since movement may create openings through which water could enter
So while it was possible to make devices water-resistant or even waterproof, enclosures and gaskets had limitations and tended to lose effectiveness over time under normal usage conditions, making them unreliable.
Rather than depend on bulky and heavy enclosures as the sole source of protection, conformal coatings have proven themselves time and again as an effective means of protecting devices, while also lowering the weight and reducing the size of the product. General information about conformal coatings can be found in IPC-HDBK-830A (August 2013) – Guidelines for Design, Selection and Application of Conformal Coatings.
The different types of conformal coatings include polymers and resins as acrylates, epoxies, polyurethanes, silicones, and parylenes. Each material has it’s own strengths and weaknesses:
- Acrylates are used for general protection and are easy to apply and remove, but typically have weak chemical resistance.
- Epoxies are used for high abrasion resistance and chemical resistance, but extended high humidity or water immersion typically leads to a breakdown of the coating. Epoxies are also difficult to remove.
- Polyurethanes are good for moisture and chemical resistance, as well as abrasion resistance, but they’re very difficult to remove and can release toxic fumes when trying to do so with heat.
- Silicones protect over very wide temperature ranges, have good chemical resistance, and are flexible, but removal can be challenging.
- Parylenes have nearly 50 years of excellent performance in critical applications and industries and are often shown to be superior in terms of uniform coverage, barrier properties, performance at comparably thinner films, less stress on mechanical structures, and very little added weight. It is important to note that parylenes require a batch-style coating process which can affect the cost and processing time. Also, removal can be challenging and requires specialized techniques like micro-abrasion or ablation.
So we can see that there is a wide variety of conformal coatings available to help protect devices from liquids but each has its limitations that make it more suitable for some devices and not well suited to others. Take parylenes for example. They’re well suited for devices that are meant to undergo heavy use in critical fields and require a very thin coating that limits size and weight while maintaining maximum protection. However, the deposition process may limit its use for devices that are cheap, mass-produced, and replaced often.
Parylene Gas & Liquid Permeability
Parylene C has the best barrier properties, including preventing both gas and water vapor penetration. Table 1 below shows the gas permeability and water vapor transmission rate (WVTR) data of the most common parylenes with examples from epoxy, polyurethane, and silicone coatings.
Table 1. Barrier Properties – Gas Permeability and WVTR of Conformal Coatings
|Parylene F (VT-4)||–||16.7||–||–||–||–||–||0.28|
Ref.: Licari, James J. Coating Materials for Electronic Applications – Polymers, Processes, Reliability, Testing. William Andrew Publishing, 2003 and various companies’ literature.
Parylene C also performs well when immersed in solutions of sodium chloride salt in water. Table 2 below shows Parylene C’s performance compared to examples of epoxy, polyurethane, silicone, and TeflonTM coatings.
Table 2. Resistance of Different Polymers to 0.9% Saline Solution
|Polymer||Coating Method||Layer Thickness (microns, μm)||Time Until Total Breakdown|
|Parylene C||CVD||25||> 30 d|
|Epoxy (ER)||Dip Coating||100 ± 25||6 h|
|Polyurethane (UR)||Dip Coating||100 ± 12.5||6 h|
|Silicone (SR)||Dip Coating||75 ± 12.5||58 h|
Ref.: Mordelt, G., Heim, P. High-Tech-Beschichtung der Zukunft, Metalloberfläche 52(5), 368 − 371 (1998).
In general, Parylene C performs extremely well as a barrier to corrosion due to the coating’s ability to minimize the influence of the factors that affect coating lifetime and performance, including the following:
- Oxygen permeability − low oxygen permeability for a polymer coating, meaning parylene is an effective oxygen barrier
- Water vapor permeability − very low WVTR for a polymer coating
- Liquid water uptake − Parylene C has very little water absorption
- Ionic permeability − salts have a difficult time passing through the coating
- Coating porosity − at a thickness of just 5 to 8 microns, Parylene C forms a pinhole/pore-free coating
Parylene Abrasion & Wear Resistance
Parylenes have very good abrasion resistance, but since it’s a polymer film, it will still be susceptible to scratches and wearing away. Usually a parylene-coated assembly is housed within an enclosure or housing which can take the brunt of the abrasion and wear, so that abrasion within the enclosure becomes negligible.
When any coating is sufficiently damaged physically, a path for failure is possible, dependent on where the damage is located. If a parylene-coated product is likely to directly experience an abrasive environment and you still would like to benefit from parylene’s impressive waterproofing and chemical barrier properties, then using a thicker coating should extend the product life.
Markets that Benefit from Parylene Waterproofing
Manufacturers of products that can’t fail- such as the aerospace, defense, medical device, industrial, and telecommunications markets implement various water protection methods, including enclosures and gaskets, hermetic sealing, and conformal coatings for good reasons. When mission-critical products have to be absolutely sure that they’ll work when needed, a “belt and suspenders approach” is used where electronics assemblies have conformal coatings inside gasketed enclosures or even hermetic packages with glass to metal seals.
Considering how vital these products are, many manufacturers set a premium price for their products and can more easily afford the costs of better protection. The minimal weight of parylenes is extremely important in the aerospace and automotive markets where any added weight can affect overall fuel efficiencies. Thus, the added expense is made up for via the added value to the manufacturer and buyer.
Some products that produce or receive radio frequencies (RF) need to be tested with conformal coatings since the coatings may affect the RF data in an undesirable manner. Those products are generally protected in a hermetic package.
Product Planning and the Design Stage
There are multiple ways to protect electronics and to ensure the best performance for products, but they need to be designed in. There are many issues that you need to consider when planning for your product to be waterproofed. A good resource on the importance of considering thin film coatings in the design stage is found in IPC-WP-017 – What Conformal Coaters Wish Designers Knew About Coatings.
First and foremost, does the product even need to be waterproof and how waterproof does it need to be? Can it be just water-repellent or water-resistant? Water-repellent is defined as “not easily penetrated by water, especially as a result of being treated for such a purpose with a surface coating.” Water-resistant is “able to resist the penetration of water to some degree but not entirely.” Waterproof is defined as “impervious to water.” So, going from repellent to resistant to proof essentially means that your product claims to have an increasingly stronger barrier to prevent liquids from getting inside and causing damage. Such claims can be valuable in certain markets and therefore warrant additional protective measures.
Typically, the price of stronger water protection also leads to higher costs to manufacture, so it is important to ask how increased cost adds more value to the product. Some contributing factors to higher costs are:
- Adding a step in the manufacturing process, which in turn adds time, labor, and materials to make the product, as well as testing for protection performance and quality.
- Lessons learned from manufacturing and testing may lead to design changes, which can extend the time until launching the product.
- Making it harder for water and other materials to get inside also makes the products harder to fix either through rework or repair, which can lead to higher scrap.
Some of the parylene coating added values include:
- Reduced warranty repair costs.
- Products can be used in more environments that enable more uses.
- Longer product lifetimes.
- Improved brand reputation from more robust products.
- Simplified and minimized design when other protections can be eliminated. (Can be cost saving.)
- Adds functionality that enables a device to exist.
With just about any product, openings to the outside world, such as connectors, can’t be coated or they won’t work. If connectors aren’t coated, then you have to depend on less reliable ways to waterproof, such as gasketed caps, which can become loose, get scratched, become deformed, etc. until it won’t work as a good barrier.
Selection of the right parts at the beginning will lead to better performance, such as button type, number of buttons, type of connectors (inside and out), number of connectors, etc.
If there are a minimal number of buttons and connectors, then it should be easier to protect them. Minimizing the number of connectors and other areas that are masking points also minimizes the potential for coating failures. For the manufacturer, the easier it is to coat the product, the more affordable the process is.
Wrapping It Up
It’s not necessary to waterproof all devices. Doing so would raise costs for the manufacturer and the consumer, delay release of new products, and for many devices it wouldn’t serve a purpose. The added cost of waterproofing disposable electronics that rarely come into contact with liquids wouldn’t be cost or time effective.
For products that would benefit from being waterproof, the parylenes, especially Parylene C, is a great option for a first or second-line of defense, especially in critical fields such as aerospace, automotive and medical devices, where reliability can be a matter of not only cost and efficiency, but safety.
So, while it’s possible to protect all devices from liquids to varying degrees, it’s not always necessary or even advisable, but when it is, parylenes are the best defense for long-term, reliable protection.