Measurement of heat transfer coefficient in spray cooling of a flip chip

With the relentless trend of increasing power and decreasing dimensions, thermal management of electronic packages is becoming a critical issue. Existing cooling techniques are fast becoming inadequate for the powerful electronic components expected in the near future. Researchers all over the world have been experimenting with advanced cooling techniques such as spray and impingement cooling using different liquids such as water and fluroinert fluids. This paper describes an experimental study of spray cooling of a flip chip where the heat transfer coefficient achieved by spray cooling is measured using a thermal transient method. The chip used is an 11 mm x 11 mm test chip containing resistors and diodes for heating of the chip and sensing the chip temperature, respectively. The test chip is mounted onto a printed circuit board (PCB) and water is sprayed onto the chip. Other components in the experimental set-up include a nozzle-pump assembly with essential valves and pressure gauges to regulate the flow as required. A state of the art thermal transient tester (T3ster) is used to measure the transient temperature of the test chip. The transient temperature data collected by T3ster is processed by software to give a plot of change in thermal capacitance versus change in thermal resistances. From this plot the thermal resistance at the surface of the chip due to spray cooling can be obtained and hence the heat transfer coefficient of the spray cooling system is determined. A parametric investigation has also been conducted and the parameters that are considered include chip to nozzle height(H), spraying pressure (P) and heat flux (F) in the chip. A high heat transfer coefficient of 28,500 W/m²K was obtained for H=60 mm, P=1 bar and F=53 W/cm². It was found that the heat transfer coefficient increases dramatically with heat flux, probably due to increased evaporation rate.