Cracking

Definition

Author

• Dr Lee Hitchens, Nexus

Cracking in conformal coating is where the smooth surface of the coating fractures into sections.

The cracks in the conformal coating leave the surface area below exposed to potential contamination penetration.

This allows moisture, water and debris to reach the board level where other defects could then occur such as corrosion.

Download our PDF guide Solving conformal coating problems in the application process now.

Examples of conformal coating cracking

Examples are shown in the images below.

Causes of cracking in conformal coating

Factors that influence cracking include:

• Cure temperature too high: When the conformal coating is accelerated cured within an oven for example the speed of cure can lead to premature drying of the top surface before the remainder of the coating is dry. Cracking can occur.
• Cure Temperature Too Quick: Heating the coating too quickly without allowing enough time for room temperature (RT) drying can cause cracking.
• Film thickness too great: If the dry conformal coating is too thick then this can cause coefficient of thermal expansion (CTE) mismatch and cracks occurring in the coating
• Environment Extremes: If the operating temperature of the circuit board is too high or too low, then this can cause the conformal coating to flex too much and crack.

How to prevent cracking in conformal coating

It is possible to minimise conformal coating cracking effects as follows:

• Reduced Cure Temperature: Lowering the initial cure temperature of the oven can reduce the cracking effect.
• Slow the cure: Allowing an initial drying time at room temperature before exposure to elevated temperature can help minimise cracking
• Lower Coating Thickness: Reduce the coating thickness to avoid CTE mismatch issues
• Material Selection: Select a coating with a wider temperature range performance. Also, select a more flexible conformal coating.

Download our PDF guide Solving conformal coating problems in the application process now.

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