Wednesday, September 15, 2010 | Jade Bridges, Electrolube Ltd.
The electronics industry is one of the most rapidly expanding industries to date, with endless research and introduction into new applications. PCBs are found in many domestic, industrial, automotive and military devices, to name but a few. In most of these applications, reliability is essential--even when the device may be subject to harsh conditions, such as high humidity, corrosive atmospheres or high levels of dust or contamination.
Conformal coatings can be utilised to protect PCBs from such environments, ensuring optimum performance under the toughest conditions. The coatings are applied as thin films, typically ranging from 25 to 75µm, and conform to the contours of the board, providing maximum protection with minimum weight or dimensional change to the PCB. This is one of the primary advantages of conformal coatings, particularly with popular trends in miniaturisation and portable devices.
Many types of conformal coating are available on the market, each providing a few characteristic properties which make them suitable for particular applications. To distinguish between the different types available, a number of tests must be performed on the coating. Therefore, to establish 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.
The most common environment that a coating can be subjected to is standard atmospheric conditions. Initial tests are generally conducted to evaluate both electrical and mechanical performance. Following this, the surrounding environment can then be altered to assess the performance of the coating under more severe conditions. Such conditions can include salt mist, high humidity, high temperature and thermal changes--either as a gradual rise or decline in temperature or an immediate thermal shock. After exposure to such environments, the coating can then 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 many different types of applications. Some of the most highly regarded approvals include MIL- 1-46058-C, now superseded by IPC-CC-830B, IEC-61086 and UL746. These refer to the most common methods of testing to simulate the typical use of a conformal coating. Some obsolete standards, such as DEF-STAN 59/47, are also considered important within the industry, but newly-developed coatings can no longer be approved to these standards. IPC, International Electrical Commission (IEC) and Underwriters Laboratory (UL) 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 American Society for Testing and Materials (ASTM), British Standards Institute (European) (BSI (EN)), German Institute for Standardisation (DIN) as well as various IEC methods. These 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.