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Design007-Mar2019

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80 DESIGN007 MAGAZINE I MARCH 2019 Having covered the subject of conformal coatings in depth over the past few months, now is an appropriate time to review some of the key pointers I have tried to share in my var- ious columns. I present some of my thoughts on the essentials in this five-point guide. Choosing a Coating: Three Prime Considerations First, determine the expected operational tem- perature range for the circuit board—the highs and the lows. Should this be greater than 150–160°C, for example, it is almost certainly an application for a silicone rather than an acrylic or polyurethane conformal coating. Also, consider the temperature excursions. If thermal shock or cycling is not taken into consid- eration, it could lead to cracking, severely com- promising a coating's pro- tective capabilities. Second, what degree of chemical resistance is re- quired? Acrylic materials, while easily removed for rework, are usu- ally highly susceptible to attack by sol- vents. Meanwhile, polyurethane materials pro- vide more chemical resistance but are gener- ally not amenable to rework. Assess whether immersion or splash resistance is required and whether the coating may be exposed to heated solutions of potential contaminants, which will increase their ability to act as a solvent. Third, consider what level of corrosion pro- tection is required. Humidity typically only becomes problematic when condensation occurs, which would require close attention to the thickness and coverage of the coating. But remember, while a thicker coating might provide superior protection in condensing environments or where salt spray or corro- sive gases are present, anything deeper than the 50-micron target thickness may be prone to cracking under conditions of thermal shock or cycling. Why Should I Consider Solvent-free? Selecting solvent-free technology is a balance of ethics, performance, and process. Ethically, solvent- free materials are a smart choice because solvent emis- sions will be drastically reduced (if not eliminated), and workforce health will be better protected, ensur- ing easier compliance with local legislative requirements. Moreover, the energy required for curing these materials is significantly lower Important Considerations for Conformal Coating Selection and Performance Sensible Design by Phil Kinner, ELECTROLUBE

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