A high density electronic package includes a low-modulus-of-elasticity flexible adhesive interposer substrate to which an electronic device, such as a semiconductor chip or die or other component, is attached. The flexible adhesive interposer substrate includes a sheet or layer of a molecularly flexible adhesive having via holes therein in which are built up conductive vias to which contacts of the electronic device connect. A thin layer of metal foil on one surface of the flexible adhesive sheet is patterned to provide contacts and to connect electrically to the conductive vias. The electronic device may be covered by a lid or by an encapsulant attached to the flexible adhesive interposer substrate and/or the electronic device. An electronic package may include a plurality of electronic devices and respective flexible adhesive interposers that are electrically interconnected.