A thermal interface assembly and method for forming a thermal interface between a microelectronic component package and a heat sink having a total thermal resistance of no greater than about 0.03.degree. C.-in.sup.2/W comprising interposing a thermal interface assembly between an microelectronic component package and heat sink with the thermal interface assembly comprising a thermal interface material having phase change properties and a sealing member selected from the group consisting of an o-ring and/or shim in an arrangement such that the thermal interface material is shielded from the atmosphere when the microelectronic component package is operational.