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64 DESIGN007 MAGAZINE I FEBRUARY 2018 The application method of coating materials is an important consideration. How can the best method be determined? To be completely honest, there is not necessarily a "best method." The appli- cation method selected for an assembly will be based on a combination of factors such as existing equipment/coating processes and the overall design of the assembly. This includes areas that should be coated and those which must not, such as switches and connectors, for example. The best application method would ensure that every board is coated and further receives coating coverage on all required metal surfaces at a sufficient thickness to afford protection against the environment. These requirements will change depending upon the specific board design and environment to pro- tect against; there is no substitute for testing and verification. What typical combination of elements make up the harshest environments that coatings must protect against? For corrosion to occur on a metal surface, several factors must be present: namely, a potential difference, an electrolyte, and ionic impurities. If a coating is well applied, it pre- vents corrosion by preventing external ionic species and liquids such as water from reach- ing the metal surface. Any environment that can cause degrada- tion of the coating prompting the con- ditions for corrosion can be potentially tough on the coating. An example of this could be an environ- ment cycling between high and low temper- atures thereby putting the coating under s t re s s. C o m b i n e d with the high tem- perature speeding up polymer degradation, cracking of the coating and thus failure points can occur. The cyclical nature of the thermal environment will usu- ally lead to differences between the board and ambient conditions, prompting the formation of condensation and potential failures. Further, if this assembly were also subject to salt-spray, the potential for board failure would increase significantly. Coatings are frequently utilised within LED applications, but just how do they work to extend the life of the luminaire? Modern LED luminaires consist of many discrete surface mount LED chips sol- dered to a support (either rigid or flexible). The role of a conformal coating in extending the lifetime of an LED luminaire is no dif- ferent than with a regular circuit board—the coating must be able to resist the environment and prevent corrosion or signal leakage. In the case of LED applications, the coating must be sufficiently optically clear and remain thus to prevent colour or intensity changes in the LED output. DESIGN007 Phil Kinner is the global business/ technical director for conformal coatings at Electrolube. Q Q Q A A A