Two-Phase Thermosiphon Cooling Using Integrated Heat Spreaders With Copper Microstructures

High performance of a computationally dense microprocessor can be achieved using two-phase heat transfer closed-loop thermosiphons with relatively simple microstructure additions. Six total microstructure configurations were laser-etched onto polished and treated copper surfaces. These surfaces acted as integrated heat spreaders which were thermally bonded to a heated element shown to accurately simulate both the steady-state and transient temperature profile of a commercially available microprocessor. For the highly wetting dielectric fluid studied, HFE7000, and the evaporator (chip simulator) oriented vertically, the trends of decreasing distance between etched sites leading to improved heat transfer coefficients and reduced temperature overshoots were found. Due to the increased convection heat transfer gained by bubbles rising against a vertically oriented surface, lower thermal resistances were found for this orientation when compared to the horizontal case. Maximum heat fluxes at the evaporator were found to be 25 W/cm
²
across several etched-site configurations and “junction-to-ambient” thermal resistances were as low as 0.4 K/W. Under a transient test, the control “smooth” surface showed a potentially damaging increase to an operating temperature above 80°C in a short time-frame. However, the laser-etched surface reduced this operating temperature spike by nearly 30°C, mitigating the reliability-robbing transient overshoot associated two-phase cooling approaches.