Provided is a multi-piece integrated heat spreader/stiffener assembly which is bonded to the substrate and die in a semiconductor package following electrical bonding of the die to the substrate, a packaging method using the integrated heat spreader/stiffener, and a semiconductor package incorporating the integrated heat spreader/stiffener. In a preferred embodiment, the integrated heat spreader/stiffener assembly has two pieces, both composed of a high modulus, high thermal conductivity material shaped to attach to each other and a die on the surface of a packaging substrate. A first piece of this assembly is bonded to the substrate surface adjacent to an electrically connected die and to the top surface of the die prior to the dispensation and curing of underfill material which provides the mechanical connection between the die and the substrate. With the first piece of the assembly in place, access may still be had to at least one edge of the die to dispense and cure the underfill epoxy. Once the underfill material is cured by heating, the second piece of the assembly is placed and bonded on the substrate either abutting or overlapping with the first piece. A ball grid array (BGA) process may then be used to apply solder balls to the underside of the substrate for subsequent bonding of the package to a circuit board for use.