It is important to define what the 'maximum operating temperature' actually refers to. There are generally two definitions: package (or case) temperature, and junction (or device) temperature. The package temperature relates to the surface temperature of the package or the device; the junction temperature, to the actual temperature of the active junction on the device (generally the hottest part of the device).
Conventional electronic components are designed to operate over a specified temperature range with upper limits generally set at 70°C for commercial applications, 85°C for industrial applications, and 125°C for military applications. Recent trends in the use of electronics in cars and aircraft have seen the development of reliable electronics for use at 150°C and higher. Some components can (and are) used at higher temperatures, but must be qualified before use. It is important to note that when considering the temperature that electronic packages fail, time at temperature and operating environment are important factors. For example, some devices/packages can survive temperatures of 225°C for relatively short times (eg 1000 hours), but not the >10,000 hours required for longer life (eg aerospace) products.
The maximum operating temperature of a component is limited not only to the performance of the active device, but also by the packaging material and joining technologies used to house the device. It is important, therefore, to consider all of these if a device is to be used above its recommended operating temperature. TWI has assessed these packaging and interconnect technologies on a recent Joint Industry Programme and a EU Collaborative Project, both concerned with High Temperature Materials and Packaging for Electronics and Sensors.