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The technology of new packages with high thermal conduction performance, simplified structure, and also high reliability for flip chip devices is described. In order to obtain high thermal conduction, a thermal conduction plate is individually bonded to the back surface of a large-scale integrated (LSI) chip by soft solder and is arranged in close proximity to the inner surface of the cap, when the chip is assembled together with the cap and substrate. The cavity is then filled with a gas which has a high thermal conductance characteristic. As a result, a large part of the heat is effectively drawn off from the back side of the chip to the air-cooling fin through the plate and across the narrow gap filled with the gas. A series of experiments were conducted on a single chip package and a nine chip multichip module. These tests indicated a junction-to-fin thermal resistance of 3.2°C/W for the single chip package and 4.8°C/W as a worst case in the module. In addition a computer model analysis for thermal conduction was studied using a program named TNET-2. It was found that the calculated values corresponded closely to the measured data. More detailed descriptions of packages and results of studies are presented and discussed here.