Stacking techniques are illustrated in example embodiments of the present invention wherein semiconductor dies are mounted in a module to become a MCM which serves as the basic building block. A combination of these modules and dies in a substrate creates a package with specific function or a range of memory capacity. Several example system configurations are provided using BGA and PGA to illustrate the stacking technique. Several pin assignment and signal routing techniques are illustrated wherein internal and external signals are routed from main board to various stacked modules. Expansion can be done both on the vertical and horizontal orientations.