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Glass Ceramic Multilayer Substrate for High-end Flip-Chip Packaging
Keywords: electromigration, flip-chip, solder joints
The recent trends of the high-end servers and the supercomputers are progressively growing in process speed. Successively, the backbone of the high speed processing such as high-end LSI (flip-chip type), and relevant substrate's operational circuits are also becoming more dense, miniaturized, and has to withstand a higher current density. However, the recent studies indicate that the higher current density triggers an electromigration (EM) at the solder bumps connecting the under bump metallurgy (UBM) of flip-chip pad (i.e. Ni) and substrate's pads (i.e. Ni/Au). Therefore the voids are caused within the solder joints; which provokes an open circuit. As of result, the life-cycle of the devices are shortened; thus the solution to the EM issue is critically in demand. To respond to such demand, we have studied the mechanism of the void outbreak, by closely examining the differences in diffusion rate amongst the connective metals; within the pads and the solders. To explain further, we have contained the EM occurrence by reducing the differences in diffusion rate by applying the pure Cu for the substrate's metallization pads, which has similar diffusion rate to Sn used in solder bumps. Also, solder-ability was improved by utilizing the solder on pad (SOP). As of result we were able to successfully maintain the life-cycle of the flip-chip solder joints, while tolerating a higher current density. Furthermore, a glass ceramic substrate was used for our study. Since this particular glass ceramic substrate has a coefficient of thermal expansion of 11ppm/K; thus enhancing 1st and 2nd level reliabilities from the heat generated via devices. At the same time, it possesses a dielectric constant of 5; which is capable of maintaining high electrical performances (high speed & high frequency). Henceforth, our glass ceramic substrate is compatible for the increase in current density, while sustaining high reliability.
Hiroshi Matsumoto,
Kyocera Corporation
Kirishima-shi, Kagoshima

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