SMT007 Magazine

SMT-July2016

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38 SMT Magazine • July 2016 by Chris Hunt and Ling Zou, NATIONAL PHYSICAL LABORATORY, AND Phil Kinner, ELECTROLUBE LTD Conformal coatings are applied to protect electronic assemblies from adventitious envi- ronmental factors, which include, corrosive gases and fluids and high humidity. Whenever there is a significant level of humidity, there is always the opportunity for parts of the as- sembly to drop beneath the dew point, thus resulting in the formation of condensed water on the surface of the assembly, which can sig- nificantly reduce the insulation resistance of the boards surface, resulting in malfunction- ing electronics. While the characterization of coating per- formance under high humidity conditions is detailed, in well accepted IPC and IEC stan- dards, the performance and testing under con- densing conditions is not so well developed. This situation largely reflects the hardware challenge. Most humidity chambers are de- signed to achieve stable, well controlled hu- midity and temperature conditions, but none of these offer condensing options. Therefore, the user has to improvise. A common approach to attempt to achieve condensing conditions is to ramp at a fast enough rate to cause conden- sation, a feature the humidity chamber design- ers have by and large, successfully managed to remove. Alternatively chambers run very close to 100% relative humidity and hence at this condition condensation will occur in various parts of the chamber. An immediate drawback of these approaches is that chambers of differ- ent designs will perform differently, and will be sensitive to small drops in cooling performance. There are many alternative approaches to achieving condensation, and these are de- scribed in ISO, IEC, ASTM and others, and com- monly attempt to drive a chamber into produc- ing condensation, against the anticipated use condition, and hence sensors in the chamber detect the additional moisture and will work to reduce the humidity level to the required set- point. Thus, the level of experimental control will be very dependent on the chamber perfor- FEATURE

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