Thermal Conduction Switch for Thermal Management of Chip Scale Atomic Clocks

Abstract

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 (VCSEL) 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°C to 50°C. A thermal test vehicle has been developed to characterize a sample package with a thermal conduction switch and has been modeled using Finite Element Analysis (FEA). Three cases are presented for the temperature control of the test vehicle under different load placements and environmental conditions: 1) the center resistor in a vacuum package; 2) the center resistor packaged in air; and 3) the side resistor in a vacuum package. At 38°C, the switch snaps upward to reduce the thermal resistance. As a result, the heating power needed to maintain the same temperature is increased from 118 to 200 mW for Case 1. Such a significant change of the thermal resistance demonstrates the effectiveness of the novel thermal switch. However, the switch becomes less effective with air filling the gap, as in Case 2. More interestingly, the switch is not effective at all if the side resistor's temperature is to be controlled as in Case 3.