Characterizing the Natural Temperature and Humidity Environment Severity

Abstract

Abstract Severe yet common product environmental tests are 1000 hours at 85 °C/85% RH or 95 °C/95% RH for products, circuit card assemblies, and electronic components. Such environments never occur naturally; however, they attempt to simulate the corrosion damage that could be expected in service. To what natural environment do these tests correlate? Further, how do tests based on MIL-HDBK-310, MIL-STD-810, and STANAG 2895 B3 daily environments compare to these standard test environments? The analysis employs the Physics-of-Failure (POF) Peck power law temperature-humidity model with common conservative values for the Arrhenius activation energy and the relative humidity exponent, based on aluminum corrosion, coupled with climatological data from 49 United States weather stations and 19 international locations. The monthly average temperature and humidity extremes were transformed into an hourly diurnal cycle assumed to occur every day of each month. Using the power law model the equivalent time at the test condition was calculated for each day, and summed for each month and location. The 85 °C/85% RH test can be correlated with about 10 years in a hot and moist natural environment, such as Singapore, with 1000 hours of 95 °C/95% RH exposure equivalent to 25 years. Long-term tests based on the worst-case diurnal temperature and humidity cycles in military standards are on the order of 1 in 1,000,000 probability of occurrence in nature.