This paper describes a new diagnostic technique for analyzing microstructural changes occurring to flip chip joints after accelerated thermal tests. Flip chip reliability was assessed at high temperatures, with and without the application of electrical bias. A combination of standard metallurgical polishing techniques and the use of a focused ion beam (FIB) lift out technique was employed to make site-specific samples for transmission electron microscopy (TEM) cross-sections. We studied evaporated 95Pb/5Sn bumps, on sputtered Cr/CrCu/Cu/Au as the under bump metallization (UBM). Thermally stressed samples were tested for electrical continuity and evaluated using 50 MHz C-mode scanning acoustic microscopy (C-SAM). Failed samples were crosssectioned and large voids at the UBM were observed optically. TEM specimens taken from the predefined UBM region of degraded flip chip devices provided critical microstructural information, which led to a better understanding of a cause of degradation occurring in the flip chip joints.