Characterizing the Natural Temperature and Humidity Environment Severity
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.Abstract
Contributor Notes
ABOUT THE AUTHOR
Dustin Aldridge is a senior principal systems engineer in reliability engineering with Raytheon Missile Systems, Tucson, Arizona, specializing in reliability risk analysis. Aldridge spent more than 25 years in validation, reliability, test and warranty engineering in the automotive industry with General Motors and Delphi Corporation. He is the recipient of the 2011 IEST Reliability Test and Evaluation Award, an IEST Fellow, an editorial advisor for the Journal of the IEST, and represents IEST on the Reliability and Maintainability Symposium (RAMS) Board of Directors. He also served on the Society of Automotive Engineers (SAE) Reliability Standards Committee contributing to the SAE Robustness Validation Sub-Committee publications. He has published many papers encouraging comprehensive integration of statistically analyzed field environment and customer usage data with accelerated test methods for product development and reliability demonstration.
The Institute of Environmental Sciences and Technology (IEST), founded in 1953, is a multidisciplinary, international technical society whose members are internationally recognized for their contributions to the environmental sciences in the areas of contamination control in electronics manufacturing and pharmaceutical processes; design, test, and evaluation of commercial and military equipment; and product reliability issues associated with commercial and military systems. IEST is an ANSI-accredited standards-developing organization. For more information about the many benefits of IEST membership, visit www.iest.org.