Electromigration Behavior Of Interfacial Cu₆Sn₅Phase Formed In Lead-free Solder Joints

Electromigration means a diffusion controlled mass transport phenomenon dueto the electrical current. It is one of the most dominant reasons to cause the jointfailure in microelectronic packaging. With the miniaturization tendency of devices,the dimension of the bumps has a significant shrink and the current density of thebumps has risen sharply. Nowadays, the value of the average current density hasexceeded the threshold of the solder bumps （104A/cm²）. Therefore, the research onthe electromigration behavior of the solder bumps is of great significance. The studyof the electromigration behavior of Cu₆Sn₅phase, which is one of the most commonintermetallic compounds （IMC） formed on the Sn/Cu interface, can provide theimportant informations on the solder joint reliability. Thus, in this paper,Sn3.0Ag0.5Cu and Sn0.7Cu solder were wetted with the Cu pad to form the specificlap joints and the electromigration behavior of interfacial Cu₆Sn₅phase were deeplystudied via in-situ and ex-situ experiments.According to the observations of the four groups of ex-situ electromigrationexperiments as well as electromigration kinetic model raised by Tu, we summarizedan evolution mechanism of Cu₆Sn₅layer on the cathode/anode interfaces during EM.In cathode, the Cu₆Sn₅layer gradually reduced with the increasing time. When theCu₆Sn₅layer was completely consumed, a new Cu₆Sn₅layer would grow rapidly. Inanode, the Cu₆Sn₅layer would thicken with the increasing time, and with highercurrent density the Cu₆Sn₅layer would be thicker. Based on the morphologies andgrain orientations of Cu₆Sn₅phase at the cathode/anode during EM, we concluded alocal growth behavior of Cu₆Sn₅phase during EM. The phenomena were found thatCu₆Sn₅phase can hinder the consumption of Cu pad, and the protection ability wasassociated with the structures of Cu₆Sn₅phase. A spalling mechanism in cathode anda nucleation and growth mechanism in anode were presented during EM, and weconfirmed the spalling mechanism of Cu₆Sn₅grains is the dominant reason toproduce the interfacial failure of the solder joints. The grain orientation and rotationeffects of Sn grains on Cu₆Sn₅growth were also studied. When the c axes of Sngrains were parallelled to current directions, the velocity of EM is the fastest;when the c axes of Sn grains were perpendicular to the current directions, thevelocity of EM is the slowest. The Sn grain gradually rotated to hinder Cu atomicmigration with increasing time. In addtion, the stress patterns and slip bands on thesurfaces of the samples also confirmed the phenomena of the Sn rotations.