Electromigration Effect on Cu-pillar(Sn) Bumps

Abstract

Increased current density per bump is inevitable with the growing demand for miniaturization. Hence, the electromigration failure mode of bumps is critical to determine the bump current carrying capability. This paper presents the electromigration failure study of the copper pillar (Cu-pillar) bump and compared to that of the solder bumps. Our analysis shows that the Cu-pillar bump could handle higher current which is attributed to its unique structure. Conducted experiment shows that the electromigration failure location of the pillar-bump is away from the high current crowding point where the solder bump failure is. Cu-pillar bump electromigration failure is attributed to the depletion of intermetallic compound (IMC) at the Cu pillar's Cu-Sn interface, which was studied to have a longer failure time than dissolution of under bump metallization (UBM) in most solder bumps (Jang, 2002). In addition, Cu-pillar's excellent thermal dissipation characteristic could facilitate devices to operate at a lower temperature when compared to devices utilizing inferior thermal characteristic bumps. Because reduced temperature slows the electromigration failure, device packaged with Cu-pillar would be more competitive as compared to device which uses solder bumps. With various factors pointing towards a better electromigration performance, Cu-pillar bumps could easily achieve an order improvement in terms of mean-time-to-failure with respect to traditional bumps