Passive PCB-Mounted Thermal Switch

NASA’s Passive PCB-Mounted Thermal Switch uses a heat pipe that extends from the electronics enclosure wall to the center of the electronics board. The switch includes a wax actuator that extends when warm. The extending piston on the actuator pushes the heat pipe against the anvil of the mechanism, which then provides a low-resistance heat path to the wall of the enclosure. When the wax actuator drops below a certain temperature, the piston retracts. A spring then pushes the heat pipe away from the anvil, breaking thermal contact and conserving heat. A series of insulating materials is used to reduce unwanted heat transfer through the springs. The mechanism is mounted to the board with a thermal interface material and screws to provide high contact pressure and thermal conductivity between the board and the mechanism. Additional heat straps are used to carry heat directly from particularly hot components. A key advantage of this NASA invention is that it does not require any energy input for operations (i.e., it is completely passive). In spaceflight applications, this enables significant mass savings as heaters can represent up to 50% of electronics systems’ power consumption. Given that typical battery chemistries stop functioning at approximately 0C, additional power is required to keep the batteries themselves warm. Thus, reducing heater power requirements by 50% could reduce overall energy storage requirements by approximately 70% – leaving more capacity for sensors, fuel, or other priorities. NASA’s switch is particularly useful for spaceflight applications where electronics are exposed to long bouts of extreme heat and cold, such as on the Moon (where the day-night cycle lasts 14 days with nighttime lows near -173C and daytime highs near 127C), or in deep space. Lunar landers and lunar infrastructure developers might be ideal end-users of the invention. Other applications where electronics experience extreme temperatures may benefit from this NASA innovation. Effective heat dissipation and thermal protection are critical for printed circuit boards (PCBs) used in spaceflight applications, where the boards must withstand extreme temperature fluctuations, especially during deep space or lunar surface operations. PCBs generate heat while functioning. In a vacuum, this heat can only dissipate through conduction and radiation. Ensuring efficient heat rejection requires strong thermal conductivity between the PCB and its enclosure. However, if the enclosure's temperature drops too low – due to environmental conditions, idle processors, or other factors – excessive heat rejection can cause the PCB to fail. Currently, maintaining PCBs above their minimum operating temperature of around -20C demands significant energy and mass (e.g., batteries). In response to this challenge, engineers at NASA’s Marshall Space Flight Center (MSFC) have developed a PCB-mounted thermal switch that allows the PCB to toggle between being “thermally mounted” (heat-dissipating) and “thermally unmounted” (heat-conserving) to the electronics enclosure, depending on temperature conditions. This switch fits easily onto a PCB and is completely passive, eliminating the need for battery power to maintain the PCB within its operating temperature range.