Printed circuit boards (PCBs) find application in many domestic, industrial, automotive and military devices. Reliability is a key requirement in all these applications, especially when the devices are subject to harsh conditions such as high humidity, corrosive atmospheres or high levels of dust and contamination.
Conformal coatings are typically used to protect the PCBs from such environments, ensuring optimum performance under extreme conditions.
Applied as thin films, typically in the 25-75µm range, conformal coatings conform to the contours of the board, providing maximum protection with minimum weight or dimensional change to the PCB.
Conformal coatings are of many types, each possessing specific characteristics that make them suitable for particular applications. A number of tests must be performed to distinguish between the different types of conformal coatings.
Before establishing the correct test methods, each application must be considered in detail to provide information on both the standard operating conditions and possible excursions outside of these parameters.
While initial tests are generally conducted to evaluate both the electrical and mechanical performance in standard atmospheric conditions, the surrounding environment can be altered to assess the performance of the coating under more severe conditions such as salt mist, high humidity, high temperature and thermal changes.
Following its exposure to such environments, the coating can again be re-tested for its electrical and mechanical properties, determining its suitability for various applications.
Typical standards for the electronics industry have developed over the years from specific military or defence standards to industry wide standards required for different types of applications. Approvals include MIL-1-46058-C, now superseded by IPC-CC-830B, IEC-61086 and UL746, referring to the most common methods of testing to simulate the typical use of conformal coatings.
IPC (The Institute for Interconnecting and Packaging Electronic circuits), IEC (International Electrical Commission) and UL (Underwriters Laboratory) are the three main test bodies associated with coatings for (loaded) printed wiring boards.
Most approvals consist of many stages and refer to specific methods for each individual test. These methods are also commonly associated with certain governing bodies such as ASTM (American Society for Testing and Materials), BSI (EN) (British Standards Institute (European)), DIN (German Institute for Standardisation) as well as various IEC methods.
The testing methods carefully describe the set up parameters required to evaluate a coating and can be used as a basis for all conformal coating testing, allowing for a comprehensive comparison of different chemistries and processes to be conducted.
Environmental testing often consists of elevated levels of humidity or salt mist and general changes in temperature, conditions for which are created in a corrosion testing chamber. The surface insulation resistance (SIR) is measured before, during and immediately after environmental exposure.
Thermal testing is designed to simulate all possible scenarios, particularly those in extreme conditions such as military, aerospace or automotive applications, and include thermal cycling tests and thermal shock tests.
Performance testing of conformal coatings also includes the evaluation of electrical properties, which is essential to all applications of conformal coatings to a printed circuit board, regardless of the environment of exposure.
Tests are also performed for dielectric strength, surface resistivity, dissipation factor and dielectric constant. Testing is also done for specialist properties such as flame retardancy and resistance to moulds and fungi, harsh chemicals, UV light or corrosive gases.
The best coating is therefore selected based on the application, highlighting all possible exposure scenarios and considering all processing factors. For standard atmospheric conditions many conformal coatings available on the market will provide the level of protection required while the various chemistry and coating types should be evaluated thoroughly for harsher environments.
Acrylic coatings such as Electrolube Ltd IPC-CC-830B approved TFA offer good environmental protection with superior clarity and stability following prolonged exposure to UV light.
However, acrylic coatings do not provide the required protection against chemicals or high humidity environments where prolonged condensation or periodic immersion in water is likely.
In this case, a tougher coating such as Electrolube Ltd UL746 approved DCA or environment-friendly non VOC coatings (NVOC) would provide superior protection and optimum performance of the device under these conditions.
Electrolube is a division of the HK Wentworth Group and a global manufacturer of electro‐chemicals.