Characterizing the humidity and thermal environments of commercial avionics for accelerated test tailoring

The climatic environments, especially thermal cycling and humidity, have long been recognized as key factors in electronics reliability. Most humidity and thermal cycle accelerated tests are somewhat arbitrarily specified and rarely have demonstrated linkage to the in-service environment. This work proposes a methodology to determine realistic humidity and thermal cycle test requirements based on measured environment data.; To illustrate this approach, environment data from commercial airplanes is measured and applied to specific failure mechanisms. Temperature, relative humidity, pressure, and equipment power condition for more than 1100 flights were measured in the key avionics areas of an operating airplane. The local environment at the failure site is derived using transient assumptions. To reduce irregular field data to a form useable by acceleration transforms, counting algorithms are presented that preserve necessary attributes. The time at discrete humidity and temperature intervals is accumulated. For thermal cycle counting, a three-parameter rain flow method is introduced that preserves time-dependent parameters needed for solder fatigue analysis. Tailoring accelerated tests based on average environment parameters is compared with these damage integration approaches.; Surface-mount technology and plastic encapsulated microcircuits are increasingly used in new avionics designs. These technologies are used as example cases of developing realistic accelerated life tests. Existing and computationally simple acceleration transforms are used to estimate the environmentally induced damage. Currently-used accelerated tests are compared with those defined by measured operational data. For plastic encapsulated microcircuits, temperature-humidity-bias tests are used in component qualification. In the case of surface-mount solder joint fatigue, the IPC published a definition of the commercial airplane thermal cycle environment.; Although this is not a sufficient sample to draw general conclusions about the commercial airplane environment, it provides a more detailed view of commercial airplane environments than has ever been available. This methodology will enable equipment designs to be optimized for the real environment and improve accelerated test effectiveness, thereby reducing costs while enhancing reliability.