Electromigration Reliability of Glass Ceramic Multilayer Substrate with Various Surface Finishes

Abstract

The process speed of high-end servers and supercomputers are steadily increasing. As a result, the backbone of the high speed processing, such as high-end LSI (flip-chip type), and associated substrate circuits is also becoming more dense and miniaturized, while supporting higher current densities. However, recent studies indicate that the higher current density triggers an electromigration (EM) at the solder bumps connecting the under bump metallurgy (UBM) of the flip-chip pad (e.g. Ni) and substrate pads (e.g. Ni/Au). This electromigration leads to voids within the solder joints, which may result in an open circuit. As of result, the life-cycle of the packaged devices is shortened. Thus solution to the EM issue is critical. To respond to such concerns, we have studied the mechanism of the void development, by closely examining differences in diffusion rate among the connective metals - within the pads and the solders. We have mitigated the EM occurrence by reducing the differences in diffusion rate by utilizing high purity Cu for the substrate metallization pads, Cu exhibits a diffusion rate similar to Sn used in solder bumps. Also, solder wettability was improved by utilizing a solder on pad (SOP) construction. As of result we were able to successfully demonstrate an improved life-cycle of the flip-chip solder joints, while accommodating 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 11.8ppm/K, there is an improvement in 1st and 2nd level reliabilities associated with thermal stress from device heat generation. At the same time, it possesses a dielectric constant of 5.8, which is conductive with superior electrical performance (high speed and high frequency). Thus, this glass ceramic substrate is capable of supporting increases in current density, while sustaining high reliability.