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Direct bump-on-copper technology provides a direct cost savings, equivalent bumping qualities, and better performance and reliability for products. The bump-on-copper FOC (flexon-cap) technology is modified and derived from the existing high volume, reliable, proven bump-on-aluminum FOC process. A copper compatible adhesion layer, namely, titanium, combined with a NiV barrier layer and a Cu wetting layer for solder are used to construct the tri-metal stack that defines a reliable under bump metallurgy (UBM) for flip chip solder bumps. A proprietary solder deposition technique using solder paste is then applied to form the solder interconnects. Bump quality tests such as bump height and bump shear strength demonstrates an equivalent structure to the existing Al/NiV/Cu based flip chip bump. Wafers constructed as described are tested at both wafer level and in an assembled level. All perform well in the preliminary reliability evaluation including temperature cycle (TC), thermal shock (TS), autoclave test (PCT), high temperature storage (HTS), high temperature operational life (HTOL), and high accelerated stress test (HAST).

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.