TAO, the operational ambient temperature, can be referenced in a variety of ways. For example, TAO can be measured at a specific location on a heatsink or chassis, circuit board rail, mounting plate, or it can be an ambient temperature internal or external to a piece of equipment. Since the operational contribution to the individual component failure rates is dependent upon the junction temperature, and the junction temperature is equal to TAO + TR, it is important to make sure that TAO and TR are referenced from the same location. For example, if TAO is referenced at the rail of a circuit board, then TR must be the overall temperature rise from the rail to component junction. This may entail knowing the temperature rises from the rail to the mounting point of the component, from the mounting point to the component case and finally from the component case to the junction.
