Protecting critical electronic components

Security and reliability benefits of coating systems that extend the working life of industrial networking products in harsh environments. But not all systems are created equal, explains David Moss:

Gosport, UK -- Many chemicals found in the oil, gas, and petrochemical industries can consign electronic components to a short life, creating high failure rates and low reliability. Corrosive agents such as hydrogen sulfide (H2S) gas, hydrocarbons, chlorobenzene, and chemical by-products play havoc with components that control and manage processes, or transport vital data between control centres and remote sites. What is required is a protective coating that can isolate the components from the harsh reality of industrial settings.

Conformal coatings, as the name suggests, do just that by conforming to the contours of elements that populate printed circuit boards (PCBs) and other components present in electronic assemblies and systems. Conformal coatings are thin layers of synthetic resins or organic polymers applied to PCBs and electronic components for protection against environmental, mechanical, electrical, and chemical problems including contaminants such as dust, dirt, fungus, moisture, chemicals, thermo mechanical stress, mechanical shock, and vibration. Conformal coating types include urethane, silicone, acrylics, epoxies, and parylene.

Studies have shown that hydrogen sulfide contaminants in concentrations as low as 10 ppm may attack surface mount (SMT) electronic components. Long filaments of silver sulfide known as "silver whiskers" can form on the surface of the silver electrical contacts of these electronic components, when exposed to environments containing low levels of H2S. The presence of heat, chemicals and moisture can accelerate these formations. These deposits can potentially create short or open circuits that can cause networking products such as Ethernet switches or other electronic products to ultimately malfunction or fail.

Hardened networking boxes are extremely robust, meeting and exceeding IPC/IEC/MIL standards for survivability in power utility and heavy industrial environments. However, specific environmental hazards such as the effects of H2S gas and other corrosive agents on surface mount electronic components are not specifically addressed by normally hardened networking products. This is where conformal coatings come into their own.

Selecting the right conformal coating

There are several parameters to keep in mind when selecting the right conformal coating. The product's application environment is obviously one of the keys, but so are the physical characteristics of component assembly. To what types of contaminants will the assembly be subjected? What is the severity and duration of contact? Is mechanical stress or heat a factor? How delicate are the mounted component leads and connections on a PCB?

Selecting conformal coatings requires an analysis of physical, mechanical, thermal, electrical, and optical properties. Physical properties include viscosity and gap fill, the space between the material and substrate. Mechanical properties include tensile strength, tensile modulus, and elongation. Thermal properties such as temperature range, thermal conductivity, and coefficient of thermal expansion (CTE) are important considerations also. Electrical properties for conformal coatings include electrical resistivity, dielectric strength, and dielectric constant.

Conformal coatings are not restricted to the environments described here. Many special purpose applications make use of conformal coatings, such as electrical power and high voltage products including generators, transformers, circuit breakers, and motor assemblies. Specialised conformal coatings meet military specifications (MIL-SPEC) MIL-I-46058, IPC-CC-830, IPC-4101, MIL-STD-1188, and are suitable for many applications. Flame retardant materials resist ignition or reduce the spread of flames when exposed to high temperatures. Flexible or dampening materials form layers that can bend without cracking or de-laminating.

In fact, flexibility is of primary concern where PCBs are concerned and where PCB real estate is critical. The differences in coefficient of thermal expansion (CTE) between a non-flexible conformal coating and the PC board and its mounted components may lead to damage of light-gauge leads and connections. This effect is particularly a problem on boards that experience repeated temperature cycling.

The choice of the right coating requires knowing what threats the assembly will be subjected to and its vulnerability to the coating characteristics. As mentioned above, an inflexible coating can stress mounted components to the point of failure. The ease of application and its removal when components fail are of prime importance.

IPC/MIL specifications classify conformal coatings into types by the cured chemistry of the coating, and include acrylic, silicone, urethane, epoxies and paraxylylene.

Acrylic is generally the easiest of the conformal coatings to handle. The thermoplastic lacquer base means the coating is easy to apply and easy to remove and repair. Moisture resistance is comparable to urethane and silicone, but it has poor resistance to petroleum solvents and alcohol. Dielectric withstand is greater than1500 volts, and its effective temperature range is from -59C to +132C.

Silicone offers good thermal shock resistance due to flexibility. Like acrylic, it is easy to apply and repair, although overall removal may be challenging. Moisture resistance is similar to urethane and acrylic. Dielectric withstand may be somewhat lower than for the other coatings (1100 volts/mil), but the flexibility of silicone coating allows for much thicker film build than comparable acrylic or urethane coating. Temperature range is from -65C to +200C.

Urethane is a hard, durable coating that offers excellent abrasion and solvent resistance. It offers similar moisture resistance to acrylic and silicone, but significant shrinkage during curing and extremely hard film may stress components. Urethane is a difficult coating to apply and nearly impossible to remove. Rework may be accomplished by burning through coating with a soldering iron on local areas, but the stripping of large areas or whole boards is nearly impossible. Temperature range is same as acrylic.

Epoxies are usually applied as a two-part thermosetting coasting, and offer excellent resistance to moisture and solvents. The coating shrinks during curing, leaving a hard, difficult-to-repair film, which may stress components. Due to the extreme solvent resistance of the film, the coating is virtually impossible to strip.

Paraxylylene is also usually applied as a two-part coating, and is very uniform, yielding excellent pin coverage. Limitations include high cost, sensitivity to contaminants and the need for vacuum application techniques.

Of these various coating types, silicone has proven the best choice of the oil, gas and petrochemical industry. Silicone has the ability to function over a wide temperature and humidity range, provides a durable dielectric insulation, acts as a barrier against environmental contaminants such as hydrocarbon and benzene, is effective against gas permeation, and is also a stress-relieving shock and vibration absorber.

In addition to sustaining their physical and electrical properties over a broad range of operating conditions, silicones are resistant to ozone and ultraviolet degradation and have good chemical stability. Most silicones contain significantly less solvent than organic coating and are available in a wide variety of cure systems. Silicones also offer better repair ability and, in the manufacture of electronic devices, allow the option to salvage or reclaim damaged or defective units as they can be removed from substrates and circuitry by scraping or cutting, or by using solvents or stripping agents.

Conformance to industry standards

Emerging standards for conformal coating thickness are being considered by the IEEE. The new standard will define the appropriate thickness range for each coating type, indicating the recommended values for harsh environments.

Under the IPC-CC-830 standard, silicone (SR) coating thickness was defined in the 1.97-7.87mm range. Accordingly, For its part, GarrettCom offered thin film silicone coatings consistent with this standard for protection against moisture. But after much investigation, the company determined that a thicker silicone coating in the 14mm range was more appropriate for the oil, gas, and petrochemical industry.

Military applications and standards such as MIL-I-46058C demand a heavy-duty silicone conformal coating to protect electronic equipment from highly corrosive gaseous hydrocarbons. This application environment is similar to those frequently found in the oil, gas and petrochemical industry. For this, Type SR (Silicone) requires a thickness of 12.50 to 15.87mm.

In conclusion, silicone coatings provide excellent resistance to industrial chemicals, environmental heat and mechanical shock conditions, low stress on mounted components, ease of use and ease of removal, making this material the choice for hardened industrial networking products such as Ethernet switches and other electronic products.

Using a thicker 14mm silicone coating on PCB assemblies and all of the subassemblies inside of networking products, especially highly configurable products, is more time-consuming and costly than the thin coatings commonly being used. But the better protection and reliability of networking products in harsh environments demand no less than the right coatings for the right reasons.

David Moss is business development manager of GarrettCom Europe, a supplier of industrial networking products for factory floor, control room, power utility and telecommunications markets