Cracking in Conformal Coating: Major Causes and Preventative Strategies

Conformal coatings protect printed circuit boards (PCBs) and similar electronic components used in a wide range of aerospace, defense, transportation, electronics and medical devices. 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 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. The 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 and 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 results from temperature inconsistency, particularly when the temperature is too high during the curing or drying processes. However, 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:

• Extremely high cure temperature can occur if coating cure is implemented too quickly. The outer surface of the coating may cure too early in comparison to its underlying layers, without providing adequate time for room temperature (RT) drying. This can lead to a cracked surface.
• When dry, excessively thick coating causes a mismatch with the film’s coefficient of thermal expansion (CTE), which is another failure mechanism that creates cracked surfaces.
• When the PCB’s operating temperature is too low or too high in relation to the film’s acceptable range, conditions may develop that generate environmental extremes. This interferes with coating flexibility, which causes the conformal coating to fracture.

Sometimes these conditions can develop along 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, such as acrylic, epoxy, urethane and silicone, require curing of various durations to reach the appropriate film consistency. As such, they are subject to cracking under the conditions listed above.

Causes of Parylene Cracking    

Unlike wet coatings, Parylene is not applied by brushing, dipping or spraying. Its chemical vapor deposition (CVD) process allows the gaseous Parylene to penetrate deep into the substrate’s surface to create an ultra-thin and pinhole free coating. 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 component, its materials or purpose.

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 which causes cracks in 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.

Summary

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, common causes of cracking include very high temperatures and 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 is 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.