Circuits Assembly - July 2008 - (Page 58)

Conformal Coatings enhance mechanical strength and abrasion resistance. The common environment that a coating is subjected to is standard atmospheric conditions. Initial tests generally are conducted to evaluate electrical and mechanical performance. Following these tests, the environment can be altered to assess coating performance 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 then can be retested for thermal and mechanical properties, determining its suitability for various applications. Figure 2. Humidity resistance results of novel coating immediately after testing and after a recovery period. Standard acrylic coating is shown for reference. Volatile organic compounds (VOC) are carbon-based compounds that vaporize easily at room temperature. They are more clearly defined by the EU Solvents Emissions Directive, which states a VOC is “any organic compound having, at 20°C, a vapor pressure of 0.01kPa or more, or having a corresponding volatility under the particular conditions of use.” VOCs can occur naturally; for example, isoprene and monoterpene are two of the most common VOCs emitted by vegetation and are commonly termed Biogenic VOCs (BVOCs). Manmade VOCs come from industrial and domestic sources, including emissions from oil, gas and transportation, and general fuel consumption and solvent use. The latter is of most concern within the coatings industry. The EU Solvents Emissions Directive covers defined operations such as the manufacture of coatings, coating activities (such as PCB conformal coating) and surface cleaning. The threshold limit value for solvent quantity is 5 tonnes per year for such activities as conformal coatings. Manufacturers whose solvent consumption falls below these thresholds fall outside the scope of the directive. Occupational exposure limits (OELs) in the workplace will still be apparent, however. It is therefore clear the Solvents Emissions Directive affects conformal coatings manufacturers and end-users. Why all the fuss? VOC emissions have to be controlled due to their effect on the environment and human health. VOCs contribute to the formation of ground-level ozone, a major component of smog. Such pollution can have many detrimental effects on the environment, in particular, damaging forests and vegetation. When not managed properly, VOCs can also cause health problems. Overexposure causes them to act as irritants and, in worst cases, carcinogens. Therefore, it is fair to say the formation of ground-level ozone is a serious air pollution problem. Ozone is not emitted directly, but is formed from the photochemical interactions of VOCs and nitrogen oxides. The only significant process that forms ozone is the photolysis of NO2; therefore, ozone is in a photostationary state. When VOCs are present in the atmosphere, they react to form radicals that either consume NO or convert to NO2. Increasing the level of VOCs in the atmosphere increases the conversion of NO to NO2 and in turn causes ozone levels to increase. The absence of VOCs would therefore significantly reduce the amount of ozone formed. About VOCs UV cure and water-based technologies have been discussed as possible replacements for solvent-based coatings, and some meet industry standards. Still, solvent-based coatings remain the favored technology on performance and application grounds and therefore provide a good benchmark for the required properties of a conformal coating. Among the various types of coatings used to benchmark were a pair of proprietary materials: DCA, a modified silicone conformal coating and HPA, a transparent acrylic conformal coating, which between them have military, defense standard and UL approvals. Environmental testing often consists of elevated levels of humidity or salt mist and general temperature changes. A humid or salt atmosphere is created in a corrosion testing chamber, typically at around 85% humidity and 5% salt. The atmosphere is kept constant within the chamber for a set time. This can vary from 24 hrs. to a number of weeks, depending on requirements. Surface insulation resistance (SIR) is measured before and immediately after environmental exposure. In order to meet industry standards, a coating’s SIR should be greater than 10 8 Ω following environmental testing. SIR is also measured after a recovery time, typically around 15 min. This is where the coating is removed from the test environment and left to recover in atmospheric conditions before testing. After this time, the coating should have almost completely recovered to its original SIR value. Conformal coatings may also be applied to PCBs forming part of a device subject to changes in temperature. Automotive applications call for extended temperature ranges, especially under-the-hood. Military and aerospace applications can demand even more extreme upper and lower temperatures. Thermal testing is designed to simulate all possible scenarios. Thermal cycling tests are carried out in one chamber, where temperature is changed at a set rate. The highest specification in industry standards is a rate of change of temperature of 12°C/ circuitsassembly.com 58 Circuits Assembly JULY 2008 http://circuitsassembly.com

Table of Contents Feed for the Digital Edition of Circuits Assembly - July 2008

Circuits Assembly - July 2008 Contents Caveat Lector Industry News Market Watch Talking Heads Focus on Business Global Sourcing On the Forefront Screen Printing Tech Tips Will Electronics Follow the Sun? ‘Warm’ Manufacturing Heats Up Reputation Trumps Recession, China ‘Web Circuits’ A Novel Non-VOC Conformal Coating Krypton: Benchmarking Customer Satisfaction 2008-09 CIRCUITS ASSEMBLY Buyers Guide Wave Soldering Pb-Free Lessons Learned Process Doctor Materials World SMTA Int'l Product Preview Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - July 2008

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.