A microelectronic assembly can include first, second and third stacked substantially planar elements, e.g., of dielectric or semiconductor material, and which may have a CTE of less than 10 ppm/° C. The assembly may be a microelectronic package and may incorporate active semiconductor devices in one, two or more of the first, second or third elements to function cooperatively as a system-in-a-package. In one example, an electrically conductive element having at least a portion having a thickness less than 10 microns, may be formed by plating, and may electrically connect two or more of the first, second or third elements. The conductive element may entirely underlie a surface of another one of the substantially planar elements.