Conformal coatings protect printed circuit boards (PCBs) and similar electronic assemblies used for a wide range of aerospace, automotive, consumer, defense and medical applications. Coatings effectively cover PCBs, shielding them from contaminants, liquid incursions, temperature fluctuations and other conditions potentially hazardous to component performance. However, problems can develop if their preparation, application, and drying processes are inappropriately managed.
Consequent defects can compromise the conformally coated surface, leading to poor component operation. Cracking is a major failure mechanism of conformal coating:
Cracking in conformal coating occurs when the smooth surface fractures into sections cracks in the coating leave the area below exposed to potential contaminants.
In these cases, the smooth conformal surface separates into detached segments, causing fractures (cracks) in the coating that uncover the formerly protected regions. Component malfunction can result from prolonged interaction to elements breeching the previously conformal film. Cracking has many causes, but frequently result from temperature inconsistency, particularly when it is too high during the curing or drying processes. Yet, very low temperatures can also stimulate cracking, especially if they are immediately preceded by extreme heat.
Factors Influencing Cracking of Liquid Coating Materials
Liquid coating materials represent the majority of conformal coating. Comprised of acrylic, epoxy, silicone or urethane films, they are applied by brushing, dipping or spray methods, and require curing for successful finish. Several factors induce cracking of liquid conformal coating:
- Excessively high cure-temperature can develop if coating-cure is too quickly implemented. The outer surface of the coating may cure prematurely in relation to its underlying layers, without providing sufficient time for room temperature (RT) drying, a condition that can generate a cracked surface.
- When dry, overly thick coating triggers a mismatch with the film’s coefficient of thermal expansion (CTE), another failure mechanism that leads to cracked surfaces.
- Performance conditions developing because the PCB’s operating temperature is too high or too low in relation to the film’s acceptable range may generate environmental extremes that interfere with coating flexibility, causing the conformal coating to fracture.
Sometimes these conditions can develop in tandem with others, or they may happen independently. Steps can be taken to minimize their occurrence and impact. Most prominent are:
- Reducing the coating’s initial cure temperature.
- Ensuring an adequate period of initial RT drying is complete before exposing the film to higher temperature.
- Avoiding CTE mismatch by verifying the film is not too thick to allow drying appropriate to the coating material and its specific purpose.
- When in doubt, select a more flexible coating material, with a wider temperature range performance to discourage cracking.
This evidence suggests that many of the causes of cracking emerge during the coating’s curing process. Liquid conformal materials – acrylic, epoxy, urethane and silicone – require curing of various durations to reach appropriate film consistency. As such, they are subject to cracking under the conditions cited above. Such is not the case for parylene.
Causes of Parylene Cracking
Unlike wet coatings, parylene is not applied by such liquid methods as brushing, dipping or spraying. Its chemical vapor deposition (CVD) process allows the gaseous parylene to penetrate deep into the substrate’s surface, providing a superior coating in many cases. However, its performance efficacy is not guaranteed, and can be critically affected if an unsuitable parylene type is used for coating, or if coating thicknesses is inappropriate to the particular assembly, its materials or uses.
In this respect, a major parylene failure mechanism, environmental stress cracking (ESC), may develop. As with cracking for wet coatings,
- parylene ESC is a consequence of PCB operating temperatures that are either too low or too high;
- here again, unexpected surface flex cracks the coating.
Cracking can be restricted by maintaining lower range deposition pressure during CVD. CTE control is improved through reducing coating thickness and monitoring temperature levels.
Cracking occurs when a smooth conformal surface fractures into sections. The cracks between the sections leave the area below exposed to potential contaminants. Cracking of liquid coatings is often a result of inappropriately implemented curing procedures. CVD-applied parylene needs no curing, but suffers from similar cracking failure mechanisms. For both wet coatings and parylene, very high temperatures are a common cause of cracking, as are too rapid fluctuations in temperature, during either coating application or assembly operation. This is especially true in cases where operating conditions requiring exceptional heat are followed immediately by very low temperatures. Coating that are too thickly applied is particularly susceptible to cracking from temperature fluctuation. Strategies for avoiding this defect include reducing coating thickness to assure competent CTE-management, and maintaining strict attention to temperature control.
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