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The thermal performance of a thin (0.5 mm), high interconnect density substrate for flip-chip applications (FC-PBGA) has been studied under natural and forced convection conditions per JEDEC standard conditions . Since the effect of a heat sink not considered, heat conduction within the package and substrate is relatively more important to thermal performance. A package size of 42.5 mm is assumed. The effect of die size, core vias in the substrate, and lid assembly conditions is studied. The lid assembly conditions have the greatest effect on theta-ja (up to 38%). The effect on theta-ja of increasing the number of core vias under the die site is also dependent on the lid assembly. When the lid is attached to a stiffener on the substrate, a negligible change (3%) in theta-ja is observed when the number of core vias is increased (i.e. increasing kzz from 0.6 to 3.1 W/m-K). When a floating lid that is not physically coupled to the substrate is used, up to 9% difference is observed. These and other results are discussed in the light of various conduction heat flow paths within the package.