Narrow Channel Forced Air Heat Sink

The heat transfer between the coolant anti the heat sink contributes a significant fraction of the thermal resistance of a package. It has been widely recognized that the heat transfer can be increased by increasing the area of the heat sink in contact with the coolant, and therefore the use of fins is common. However it has not been generally recognized that for laminar flow in confined channels, the heat transfer coefficient is inversely proportional to the width of the channel. Narrow channel forced air heat sinks have been designed, built, and tested. Three different channel widths, 5, 10, and 25 mils, were investigated. The design of the sinks permit integrated circuits (IC´s) on 1/2 in centers and printed circuit (PC) boards on I in centers. The measured values of individual heat sinks with 30 I/min air flow are 5.5, 3.7, and 3.4°C/W for 25,10, and 5 mil channel widths, respectively. These values are in good agreement with calculated values. A 16 chip package with chips on 1/2 in centers was assembled utilizing 10 mil channel width heat sinks. A plenum fitted over the sinks supplied air to each of the sinks from a single source. The measured values of thermal resistance of the four center dies were between 5.3 and 5.9°C/W. If the chips are mounted directly on the sink instead of on the alumina substrate, the thermal resistance would be reduced by about 2°C/W.