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Sensible Design: Important Considerations for Conformal Coating Selection and Performance

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 various 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 temperature 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 consideration, it could lead to cracking, severely compromising a coating’s protective capabilities.

Second, what degree of chemical resistance is required? Acrylic materials, while easily removed for rework, are usually highly susceptible to attack by solvents. Meanwhile, polyurethane materials provide more chemical resistance but are generally 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 protection 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 corrosive 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 emissions will be drastically reduced (if not eliminated), and workforce health will be better protected, ensuring easier compliance with local legislative requirements. Moreover, the energy required for curing these materials is significantly lower than that needed for solvent-based materials, resulting in reduced energy bills and reduced CO₂ emissions.

From a performance point of view, solvent-free materials can be applied slightly more thickly, improving coverage and protection. Because they’re easier to process and more readily compatible with rapid throughput manufacturing operations, solvent-free formulations are often technically superior and can meet the demands of challenging applications in the automotive and aerospace sectors where increased condensation and thermal shock resistance is required.

To read this entire column, which appeared in the March 2019 issue of Design007 Magazine, click here.