Manifold Microchannel Cooler for Direct Backside Liquid Cooling of SiC Power Devices

A unique manifold microchannel cooler has been developed based on single phase liquid forced convection flow. It is the first of its kind to directly cool the backside of a device using a manifold microchannel design which minimizes the pressure drop across the channel while allowing the maximum cooling potential and temperature uniformity. This report illustrates the fabrication sequence, packaging procedures and test set-up required to assess the cooler’s functionality. Simulations of heat dissipation and fluid flow in the assembly were conducted using the Floworks® module of Solidworks® modeling software. The cooler has been tested using a SiC diode and initial test results show very low thermal resistivities and low thermal increases of the SiC devices. Increasing power levels and higher packaging densities of today’s microelectronics create a need for improved cooling methods to improve heat transfer from power devices. This cooler is targeted at cooling small, high power SiC devices which are seeing increased use because of their fast switching speed and higher reverse bias breakdown voltage. The manifold design uses larger channels to transport fluid into and out of the cooling region and smaller crossover microchannels that are the active cooling area. This type of design minimizes pressure drop, maximizes heat transfer and allows temperature uniformity across the device area. A 25 mm × 5 mm × 1.5 mm (thick) cooler was fabricated to cool a 4 mm × 4 mm × 0.4 mm (thick) SiC diode. The cooler has been fabricated out of a single 1 mm thick silicon wafer, using deep reactive ion etching (DRIE) to create both the microchannels and the manifold channels. The manifold channels are 200 μm wide with a 250 μm pitch with an 800 μm depth. The microchannels are 20 μm wide with a 40 μm pitch with a 200 μm depth. The dimensions were chosen based on initial modeling results to maximize the ratio of the cross sectional area of the manifold to the microchannels while staying within fabrication limitations. The SiC diode is bonded directly onto the cooler using a gold tin eutectic bond. A 1.6 mm outer diameter stainless steel capillary tube is used to flow the liquid coolant into the manifold channels and is sealed with an epoxy. Experimental results show excellent thermal results with thermal resistances less than 0.1 K/(W/cm²), heat fluxes over 600 W/cm², and small increases in device temperature. All these results come from a cooler which has a volume less than 200 mm³. The cooler design has been shown to be very durable and has a very tight fluidic seal with no leaks throughout the experimentation. It was fabricated using standard MEMS techniques and can be easily repeated. Simulations have been correlated with experimental data for a variety of pressure drops. Simulations have also shown the cooler to be capable of keeping the device under 125 C and the water temperature under 100 C (boiling) for a heat flux of 2500 W/cm².