Thermal Conduction Switch for Thermal Management of Chip Scale Atomic Clocks (IMECE2006-14540)

We report the first use of a bimetallic buckling disk as a thermal conduction switch. The disk is used to passively alter the thermal resistance of the package of a chip scale atomic clock. A vertical-cavity surface-emitting laser and a cesium vapor cell, contained in the clock, must be maintained at $70±0.1°C$ even under an ambient temperature variation of $−40°Cto50°C$⁠. A thermal test vehicle has been developed to characterize a sample package with a thermal conduction switch. Three cases are presented for the temperature control of the test vehicle under different load placements and environmental conditions: (1) a heating load with a good conduction path to the switch in a vacuum package; (2) the same loading as in Case 1, but packaged in air; and (3) a heating load insulated from the switch, in a vacuum package. At $38°C$⁠, the switch snaps upward to reduce the package’s thermal resistance. As a result, the heating power needed to maintain a constant temperature, $63.9±0.1°C$⁠, is increased from $118to200mW$ for Case 1. Such a significant change of the thermal resistance demonstrates the effectiveness of the thermal switch. However, the switch becomes less effective with air filling the gap, as in Case 2, and the switch is not effective at all if the heating load does not have a good conduction path to the switch as in Case 3. The steady state response of this novel thermal conduction switch has been well characterized through experimentation and finite element analysis.