A semiconductor stack package includes lower and upper individual packages. When the upper individual package is stacked on the lower individual package, a heat-conducting layer provided under the upper package touches a heat-mediating layer provided on the lower package. Thus, a layer of trapped air found in conventional stack packages is eliminated, and a direct heat-dissipating path is produced through both the heat-conducting layer and the heat-mediating layer. Therefore, the heat dissipation of the stack package is improved. Alternatively, the stack package may have a symmetric configuration in which each IC chip faces away from each other. A memory module has several stack packages mounted on one or both surfaces of a module board. The method includes forming the packages and stacking the packages. The method further includes forming a module board and mounting the stack packages on the module board.