4.2 TEMPERATURE RANGE
For the majority of PWAs, the following starting values for temperature
range can be used: the low limit of temperature range should not be set below
the highest low-rated temperature part, and the high limit should not be set
above the lowest high-rated temperature part. The required temperature range
(T) is 120°C.
For higher indenture levels, the equipment storage specification minimum
and maximum temperature limits should be used to establish the overall range.
Most parts will withstand temperature cycling with power off through a storage
temperature range of -55 to 100°C. The maximum safe range and the fastest
rate of change of hardware temperature provide the best screening.
One potential problem that must be considered is thermal overstressing
parts. If PWAs contain commercial grade semiconductors rated at 85°C, the
equipment should not be thermal cycled above 85°C. The upper temperature limit
during higher indenture level testing must be below the point of overstressing
However, for most PWAs built using parts that are designed to operate over
the full military temperature range, the recommended temperature range (T) of
120°C presents no problems. Temperature ranges for the next higher assembly
(module or unit) should be as close as possible to the PWA level without
overstressing any internal parts.