Electromigration reliability and morphologies of Cu pillar with microbump under high current density stressing

Current-induced failures in fine pitch Sn micro-bump with Cu pillar have been investigated under a high current density of 3×105 A/cm2 and temperature of 150°C. Both joule heating and high current density led to void formation and an abrupt increase of electrical resistance at the onset of the current stressing. The electrical resistance fluctuated for a period of time when Cu and solder continue to transform into intermetallic compounds (IMC). The electrical resistance eventually becomes stable when the entire solder have transformed into IMC. In early stage, the metallurgical reaction started with significant Cu dissolution into solder, followed by void formation between IMC and the remaining solder. The failure was due to the substrate-side Cu trace dissolution into solder driven by electron flow in high current density tests. When the stressing time increased to 160hrs, the Cu pillar and Cu trace continue to consume and solder transform to IMC joint Cu₆Sn₅, then Cu₃Sn IMC became thicker. When stressing time increased to 2145hrs, the solder has fully reacted and transformed into full Cu₃Sn IMC joint. However, the EM and kirkendall voids coalesced into continue cracks at the interface between the IMC joint and Cu pillar and Cu trace due to the current stressing caused the resistance of the micro-bump increased abruptly.