We have developed a solder joint material with a plastic core. In this paper, we report on the thermal conductivity of this material. We evaluated the thermal resistanceθ j-awith 352 pin EBGA and 352 pin PBGA, and also carried out a thermal conductivity analysis using computer simulation. The result show that the θj-aof the 352 pin EBGA was 12°C/W, and the θj-aof the 352 pin PBGA was 20°C/W. So this material is applicable to high frequency and power packages (CPU, MCM, MCP, stacked package and so on) .