Right conformal coatings reduce field failures and costs

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Protecting your printed circuit boards well in harsh environments can extend their operating life by many years. One of the most popular, cost effective ways to do this is by using a conformal coating. This is a protective chemical coating or polymer film, typically 50 µm thick, but can range between 25 to 75 µm. It acts as a breathing membrane that filters out moisture and dirt.

By Jalaja Ramanunni

Tuesday, December 13, 2011: TBenefits of using conformal coating

The right coating will help protect the printed circuit boards (PCBs) from moisture, dirt and chemical contaminants. Even electrochemical corrosion of conductors and solder joints can be prevented by protecting it from moisture. “By being electrically insulated, it maintains long term surface insulation resistance (SIR) levels, and thus ensures the operational integrity of the assembly,” reveals Chris Palin, European region sales manager, HumiSeal, a company that claims to be the creator of conformal coatings. The protective nature of a conformal coating also means that it enhances product reliability, and thereby reduces the potential cost and damaging effects of early field failures. It also provides a barrier against airborne contaminants from the operating environment, such as salt spray, thus preventing corrosion.

Chris Palin, European region sales manager, HumiSeal

How does it save costs?

Customers can decide upon the most important factors likely to adversely impact their PCBs and, accordingly, select the right coating for their application areas. “This helps to reduce costs and the time spent on conducting individual evaluations of circuit boards. Further, it also reduces field failures due to corrosion. It has been found that 25 per cent of field failures in electronics are attributed to corrosion and its effects. Conformal coatings can even reduce costs on repair and support, and extend a PCB’s life,” states Prasad Gujral, managing partner, Drive Technologies, official distributors for Humiseal in India.

Application areas

The high reliability defence, automotive, industrial and aerospace industries are where coatings are used to protect circuits against a combination of moisture, aggressive chemicals and vapours, salt spray, mechanical vibration, and even organic attacks from fungus. “We have over 60 customers in India, mainly in areas like LED drivers and controls, fire alarm systems, power converters, automotive, military, and solar and wind energy. Conformal coatings are also commonly used in consumer and domestic applications susceptible to environmentally induced field failure, such as mobile phones and washing machines,” adds Palin.

Prasad Gujral, managing partner, Drive Technologies

How to choose a coating

There are many types of coatings available and one should choose from them based on the requirement. The most common types are acrylic and polyurethane-based. The selection of conformal coating material and the application method are crucial. A wrong selection can affect the long term reliability of the circuit board, and can even cause major difficulties with both processing and costs. Palin shares, “The common standards we prescribe are MIL-I-46058 C, IPC-CC-830B and UL Underwriters Labs 746E. Using parts of these standards, you can find out the ideal conformal coating based on factors like ease of application, abrasion resistance, humidity resistance, temperature resistance and ease of rework removal.” These standard datasheets are available online. All gel, UV cure and water-based coatings are extremely eco-friendly since they have little or no volatile organic compounds (VOC).

Types of coatings

Palin states that acrylic conformal coating currently makes up 80 per cent of the global market and 90 per cent of the Indian market, mainly because it can be soldered with the release of minimal or no odour. “Its simple drying process, ease of rework, strong moisture resistance, high florescence level and ease of viscosity adjustment are some of the reasons why acrylic is popular,” reveals Dr Lee Hitchens, technical director, SCH Technologies, a conformal coating specialist firm. “Additionally, acrylics give off little or no heat while curing, eliminating potential damage to heat sensitive components. They have good humidity resistance and exhibit low glass transition temperatures, and do not shrink during cure. Acrylic conformal coatings are suitable when moisture is the main threat,” he adds. Acrylics are not considered in cases where chemical and temperature resistance is high, and when the curing period needs to be quick.

Epoxy conformal coatings are usually hard and opaque. They possess good moisture resistance and chemical resistance, but one has to apply a very thick film to achieve this advantage. Their mechanical properties and abrasion resistance are excellent but they have a poorer moisture and dielectric performance. The cure time is average, but due to other epoxy based materials on the board, epoxies are impossible to remove for rework.

Polyurethane conformal coatings are available in single- or two component formulations. Both provide more humidity resistance and chemical resistance than acrylic coatings. “Single component polyurethanes are easy to apply and enjoy a long life but are known to require slightly longer cure times to achieve a full or optimum cure, whereas two component formulations can reach optimum cure properties in one to three hours with the assistance of heat. However, when compared to one component formulations, they can have a relatively short pot life,” explains Dr Hitchens. In these cases, the removal of polyurethane coatings is relatively difficult, particularly after the cross linking of polymers.

The strength of silicone conformal coatings lies in their ability to withstand prolonged exposure to higher temperatures than most other conformal coating chemistries. Hence, they are the primary choice for under-hood automotive applications. They can also be applied in thicker films, making them useful as a vibration dampening or isolation tool when the assembly is placed in a high vibration environment. “Rework of silicone coated assemblies can prove difficult due to their chemical resistance and, unlike acrylics and polyurethanes, they do not vapourise with the application of heat. There is no advantage if you use it below a temperature of 125 degrees celsius, so people do not use it unless they have to,” Dr Hitchens reveals.

Technological advancements

The latest buzz in this area is about ultra violet (UV) conformal coatings, which have been around for a few years but there has been only recent increase in their use. “These coatings dry within seconds, as opposed to waiting for 30 minutes, in the case of other coatings. They survive harsh environments, can withstand moisture, and offer good dielectric and temperature resistance, apart from having high chemical resistance. UV cured materials are ideal for use in high volume applications where curing times could become an issue,” informs Dr Hitchens. Low viscosity silicon and UV curable coatings are also getting popular because they can be used easily with selective coating machines.

Another new trend that Dr Hitchens points out is the use of almost all coatings in gel form, which can be selectively applied around and on critical components. These can be as thick as 1mm, nearly 10 times the normal thickness of a conformal coating, and are available in easy to use syringes. “A lightweight alternative to hermetically sealed housings is required for the electronic circuitry, which has to be robust enough to withstand rigorous UV exposure, lightning strikes and moisture ingress from rain. Humiseal has developed gel versions of some of its existing coatings. The gel versions can be used during selective conformal coating applications as well as for sealing connectors to prevent the ingress of coating material. One such application is the huge LED displays that can often be seen above stadiums during sporting events. The displays are carried on hot air balloons or blimps where exposure to the elements is severe and yet weight remains an issue,” Dr Hitchens concludes.

Solventless or water based coatings are also being used extensively due to tighter environmental regulations prohibiting the use of solvents as carriers or diluents. This has forced manufacturers to seek substitutes for traditional conformal coatings that use toxic organic solvents. Water based conformal coatings have been developed to enable customers to meet stricter emission targets, not to mention the various health and safety requirements placed on industry. They enable enormous reductions in VOC emissions (especially in the automotive sector), are extremely safe, and produce low-odour and low VOC products. Water based coatings deliver physical protection, regulatory compliance and consistent material supply, worldwide.

Sector Protection form

Automotive sector

Moisture, salt spray, detergents

Avionics

Moisture, hydraulic fluid, jet fuel, deicing (fluids used to remove ice collected on airplanes), other fluids

Motor control drives

Salt spray, noxious gases, chemical fumes

Domestic appliances

Moisture, dust, bleach

Application techniques

  • Dipping: This is limited to materials that do not get affected easily by moisture, oxidation or light.
  • Selective robotic coating: Such as Asymtek, PVA, ACR or DIMA. All coating types can be used if the correct dispenser head is selected.
  • Spraying: This involves hand spray using a spray booth or aerosol can. All coatings can be applied in this way.
  • Brushing: Requires extremely proficient and skilled operators in order to be suitable for production purposes.

Electronics Bazaar, South Asia’s No.1 Electronics B2B magazine

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