Comparing Conformal Coatings

Estimated reading time: 10 minutes

Editor's Note: This article originally appeared in the February 2013 issue of SMT Magazine. Silicone and urethane are two of the most popular conformal coating types today; both are used for a variety of reasons, but which is the better coating? What about paraxylene, a transparent polymer conformal coating deposited from a gas phase in a vacuum that can be applied uniformly to virtually any surface and shape including glass, metal, paper, resin, plastics, ceramics, ferrite, and silicon?

While the choice ultimately depends on the application, let’s take a look at the pros and cons of each.

Silicone Conformal Coatings

Many typical silicone coatings have continuous operating temperature ratings of 200°C. This is far higher than most urethanes (125°C). Some silicone coatings are rated as high as 600°C for ultra-high temperature applications. Silicones are in great demand for the automotive industry, where temperatures can reach upwards of 175°C in the engine compartment.

An additional benefit of a silicone conformal coating is its excellent moisture protection. Silicones are used in cases where extreme temperature differences occur--environments that result in excessive moisture. Other conformal coatings fail within hours or days, but silicone, especially when applied thickly, succeeds.

One particular application is for electronically-controlled heaters developed for arctic temperatures. The heaters warm to almost 150°F and then cool to ambient temperature, sometimes as cool as -40°F. This wide temperature swing happens very quickly, resulting in a high-moisture environment. Other coatings, like urethanes, result in board failures. Silicones are the only proven alternative.

Silicone conformal coatings are also among the easiest to apply and rework. Typically low on solvents, they also ensure a smooth coat that cures very quickly--about one hour at room temperature. Apart from this, silicone’s flexibility and softness, as well as its relatively weak resistance to solvents, is suitable for assemblies requiring work after coating. The flexibility keeps labor time down without compromising coating integrity.

When Not to Use

Because silicone conformal coatings must be applied thicker (target thickness of between 0.00197” and 0.00827”) than other conformal coatings, it’s wise to seek other coating options if your application has tight clearance tolerances or if the solder joints cannot support the stresses placed upon them by the thicker layer of coating.

Types of silicone conformal coating include:

• Humiseal 1C49;
• HumiSeal 1C49LV;
• HumiSeal 1C51;
• HumiSeal 1C55;
• Dow Corning 1-2577;
• Dow Corning 3-1753;
• Dow Corning 3-1765;
• Dow Corning 3-1744;
• Dow Corning 3-1953;
• Dow Corning 3-1965;
• Dow Corning 3-1944;
• MG Chemicals 422B;
• Peters DSL 1705 FLZ;
• Peters DSL 1706 FLZ;
• Electrolube SCC3; and
• Electrolube SCC4.

Urethane Conformal Coatings

Type UR conformal coatings are quite resistant to chemical solvents--second only to parylene. As such, applications requiring any prolonged exposure to harsh chemical solvents should consider urethane resins.

Long-term NASA studies [1] have shown that urethane conformal coatings are one of the few ways to successfully mitigate tin whisker growth. Since there’s no known way to completely eliminate tin whisker growth, you must select a proper tin whisker mitigation strategy (see sidebar below). Urethane conformal coatings are a great place to start. Applications that see direct mechanical wear against the coating should consider urethane conformal coatings. Urethane resins are very hard and resistant to mechanical wear. Their hardness is second only to epoxy conformal coating, but rework is much easier with urethanes.Tin Whisker Mitigation & Conformal Coating

Tin whiskers are electrically conductive, crystalline structures of tin that sometimes grow from surfaces where tin (especially electroplated tin) is used as a final finish. Tin whiskers typically grow from lengths of 1 to 2 mm, but have been observed to lengths in excess of 10 mm. Electronic system failures have been attributed to short circuits caused by tin whiskers that bridge closely-spaced circuit elements maintained at different electrical potentials.

People sometimes confuse the term "whiskers" with a more familiar phenomenon known as "dendrites," which are commonly formed by electrochemical migration processes. Therefore, it is important to note here that whiskers and dendrites are two very different phenomena. A whisker generally has the shape of a very thin, single filament or hair-like protrusion that emerges outward (z-axis) from a surface. Dendrites, on the other hand, form in fern-like or snowflake-like patterns growing along a surface (x-y plane) rather than outward from it. The growth mechanism for dendrites is well-understood and requires some type of moisture capable of dissolving the metal (e.g., tin) into a solution of metal ions, which are then redistributed by electromigration in the presence of an electromagnetic field. While the precise mechanism for whisker formation remains unknown, it is known that whisker formation does NOT require either dissolution of the metal NOR the presence of electromagnetic field [2].

According to NASA, the mechanisms by which tin whiskers grow have been studied for many years. A single accepted explanation of the mechanisms has NOT been established. Some theories suggest that tin whiskers may grow in response to a mechanism of stress relief (especially "compressive" stress) within the tin plating. Other theories contend that growth may be attributable to recrystallization and abnormal grain growth processes affecting the tin grain structure which may or may not be affected by residual stress in the tin plated film [2].

Are Tin Whiskers an Issue?

NASA noted that tin whiskers pose a serious reliability risk to electronic assemblies. Several instances have been reported where tin whiskers have caused system failures in both earth and space-based applications. To date, there are reports of at least three tin whisker induced short circuits that resulted in complete failure of on-orbit commercial satellites. There have also been whisker-induced failures in medical devices, weapon systems, power plants, and consumer products [2].

Four main risks should be considered with tin whiskers:

1. Stable short circuits in low-voltage, high-impedance circuits.
2. Transient short circuits.
3. Metal vapor arc.
4. Debris/contamination.

Of these, a metal vapor arc is often the most destructive. A metal vapor arc occurs when a tin whisker initiates a short in an environment possessing high levels of current and voltage.

Unfortunately, there is no known way to eliminate tin whisker growth, only the use of mitigation strategies to limit their affect on product.

Viable Tin Whisker Mitigation Strategy

In 1998, Galaxy IV commercial satellite failed on-orbit due to a metal vapor arc initiated by tin whiskers growing from supposedly tin-lead surfaces. The surfaces were later confirmed as being pure tin, despite certificates of compliance stating otherwise. This failure interrupted pager services for days.

As a response to the Galaxy IV failure, NASA initiated a study to assess the effectiveness of a urethane conformal coating in whisker mitigation. Polyurethane conformal coating (Uralane 5750) was chosen as a coating most common to space applications.

As a result of the NASA study, conformal coating was proven to be a viable tin whisker mitigation strategy. While thinner coats of conformal coating were unsuccessful at preventing tin whisker penetration, Arathane 5750 (a urethane resin) applied at 2 mils thick was found to be strong enough to prevent tin whiskers from penetrating the coating and causing any potential issues.

When Not to Use

For products going into a high-vibration environment, urethane conformal coatings are not a good choice. Because of the mechanical strength and resistance to abrasion characteristics that urethanes typically exhibit, a high-vibration environment could ultimately end up weakening the integrity of this rigid coating. A better choice would be a completely conforming, flexible coating, such as parylene conformal coating. If an application will see a high-heat environment, urethanes will not provide the protection required. Leading urethanes, such as HumiSeal 1A33, offer protection to 125°C. Types of urethane conformal coating include:

• HumiSeal 1A33;
• HumiSeal 1A20;
• Humiseal 1A27;
• Humiseal 2A64;
• HumiSeal 1A34;
• Hysol PC18M;
• CONATHANE CE-1155-35;
• CONAP CE-1170;
• CONATHANE CE-1164;
• Techspray Fine-L-Kote;
• MG Chemicals 4223; and
• Electrolube PUC.

Paraxylene Conformal Coatings

Parylene polymers are polycrystalline and linear in nature, possess superior barrier properties, have extreme chemical inertness, and, because of the deposition process, can be applied uniformly to virtually any surface or shape.

Parylene is unique in that it is created directly on the surface at room temperature. And because there are no liquid phases involved, the coatings are truly conformal, of uniform controllable thickness, and are completely pinhole-free at thicknesses greater than 0.5µ. In fact, a parylene coating completely penetrates spaces as narrow 0.01 mm.

In addition to its excellent electrical properties--low dielectric constant and loss with good high-frequency properties, good dielectric strength, and high bulk and surface resistance--parylene also has good thermal endurance, performs in air without significant loss of physical properties for 10 years at 80°C, and, in the absence of oxygen, to temperatures in excess of 200°C.

Parylene is often applied to substrates or materials where there is no room for any voids in the protective coating. These materials are likely to be placed in harmful chemicals, a moisture packed environment, or even the human body. Such materials are often mission-critical devices that cannot allow environmental factors to alter their performance. When devices need such a stringent level of protection from the elements, parylene is the only logical choice.

Disadvantages of Parylene

Even with all of these benefits, there are still disadvantages to using parylene versus other conformal coatings. One important factor is cost. The cost for parylene is typically higher than other conformal coatings due to many factors: The process itself, the raw materials involved, and the labor required in preparing a device for coating. While this is not necessarily true for all applications, when an item is quoted in parylene and wet chemistry, the parylene pricing will be higher.

The parylene process is a batch process. This means that there is only a finite amount of space available in the chamber for every coating machine run. The goal is to maximize the amount of items to be coated in the chamber. If there is a suboptimal amount of items to be coated available, the difference in price per piece could escalate drastically.

The raw material, parylene dimer, is rather expensive, ranging from $200 to $10,000+ per pound. Because parylene is applied through a vapor deposition process, everything, including items that don’t need to be coated (like the inner diameter of the chamber), are coated. This makes parylene an inherently inefficient process and wasteful with materials, which escalates the end cost to the customer. Masking and otherwise prepping an article for parylene coating can be a labor-intensive affair. Because parylene is applied as a vapor, it literally gets everywhere that air can. Operators and quality inspectors should take this into account prior to coating to ensure that all coating-free areas are just that.

One major issue that often comes up for some high-volume manufacturers is the limited throughput of parylene. Runs of the parylene machine can take anywhere from eight to over 24 hours. As a result of the limited chamber space, a fixed amount of product can be processed during one coating cycle. This, coupled with the high capital cost of new equipment, can wreak havoc with internal and customers’ delivery schedules.

One final disadvantage of parylene to consider is its poor adhesion to many metals. Parylene has always had poor adhesion to gold, silver, stainless steel, and other metals. Many PCB manufacturers use gold in their products because of its conductivity. While there are some adhesion promotion methods that will greatly improve adhesion to these metals, they are either material or labor heavy and can increase costs significantly.

Conclusion

There are many benefits to using silicone, urethane, or paraxylene conformal coating. Certainly either coating is better than none, but the trick is to align your product’s potential issues with the strengths of the conformal coating that can best protect it. Only by addressing and working through these issues will you be able to determine the correct coating for you. If there is some doubt to which coating is best, it may be worth it to have an outside party provide consultation services for your application. References: 1. Lyudmyla Panashchenko, Jay Brusse, Dr. Henning Leidecker, “Long-Term Investigation of Urethane Conformal Coating Against Tin Whisker Growth,” NASA GSFC. 2. Basic information regarding tin whiskers.   Sean Horn is vice president and chief financial officer at Johnstown, Pennsylvania-based Diamond MT, a firm specializing in contract applications of conformal coatings for Department of Defense and commercial electronic systems. Contact Horn at shorn@diamond-mt.com.