Conformal Coating In Harsh Environments

Casey H. Cooper
STI Electronics, Inc., Madison, AL, USA

High reliability electronics pose a higher risk of failure when operating in harsh environments such as high temperature and high humidity. Add the possibility of ionic contaminants left on the substrate surface from the electronics manufacturing processes and you have a situation set for failure. The deadly combination of moisture (humidity), electrical potential (voltage bias), and ionic contamination (residue) is enough to create electrochemical failures such as dielectric failure and current leakage - both of which result in degradation in performance, if not complete failure, of the electronics hardware.

One common test standard used to evaluate the effects of assembly materials and manufacturing processes (i.e. residual materials) for hardware operating in hot, humid environments is the Surface Insulation Resistance (SIR) test. Although many test specifications and protocols exist, a more aggressive test protocol is required for high reliability electronics in order to determine if the materials and processes used to manufacture the electronics assembly will result in current leakage and/or dielectric breakdown between conductors.

This paper will study a customized test protocol designed to environmentally stress screen conformal coatings for their ability to withstand moisture ingression and protect the conductors from moisture-related failures such as corrosion and dendritic growth formations. The analysis of data captured from continuous voltage monitoring (for dendritic growth/shorts) in addition to traditional SIR testing will provide the manufacturing engineer the necessary data to evaluate and identify the optimal material set and process parameters to manufacture high reliability electronics for operation in harsh environments.

The Initial SIR data and the SIR data captured after 50°C conditioning with no added humidity showed no signs of degradation in insulation resistance, as expected. Voltage monitoring data collected throughout the first cycle indicated metal migration activity, particularly in the 65°C soak period, and was confirmed with the SIR data capturedThe following summary is for the three test vehicles after completing 1st cycle of temperature/humidity testing.

In summary, the analysis of data captured from both continuous voltage monitoring and traditional SIR testing provides the design engineer with the comprehensive data to evaluate and identify the optimal material set and process parameters to manufacture high reliability electronics for operation in harsh environments.