Study On Dissolution Of Substrate In Lead-free Solder Joints Upon Electron Current Stressing

Nowadays, with electronic products tending to be shorter, smaller, lighter, and thinner, the electronic packaging density will become much higher and the size of solder joints trends to be much smaller which means the current density through the joints will increase rapidly, and the high current density will lead to serious electromigration (EM) damage. As well known that electromigration is closely related to the reliability of joints, therefore the study of electromigration exists the instructive significance of the reliability research of electronic packaging. In this study, we focuse on the effect of electromigration on substrates dissolution at150℃with a current density of2.0×104A/cm2for50h,100h and200h in Cu/Sn/Cu and Cu/Sn/Ni line-type interconnects; as comparison, the aging experiment is also conducted.The study of Cu/Sn/Cu line-type interconnects shows that:during EM, Cu6Sn5and Cu3Sn IMCs are formed at the Sn/Cu interfaces; the growth of these IMCs satisfies a polarity effect, i.e., the IMCs at the anode side are thicker than those at the cathode side. Controlled by diffusion, the growth of the IMCs at the Sn/Cu interfaces follows parabolic law with time during aging, while Cu serves as the anode during EM, the growth of the interfacial IMCs at the anode side follows parabolic law with time as well. At the cathode side, controlled by the Cu flux of the IMCs dissolution, the growth of the IMCs first increases and then decteases. As the cathode, the dissolution of Cu substrates is in linear relationship with time.The study of Cu/Sn/Ni line-type interconnects shows that:during EM,(Cu, Ni)6Sn5is formed at the Sn/Ni interfaces and the IMCs at the Sn/Cu interfaces are Cu6Sns and Cu3Sn when electrons flow from Cu side to Ni side. When electrons flow in the opposite direction, Cu6Sns and Cu3Sn are formed at the Sn/Cu interfaces; while the IMC formed at Sn/Ni interfaces is Ni3Sn4; large Ni3Sn4particles precipitate in the Sn matrix near the Sn/Ni interfaces after EM for100h and200h. Controlled by diffusion, the growth of the IMCs at the Sn/Cu and Sn/Ni interfaces follows parabolic law with time during aging, while Cu or Ni serves as the anode during EM, the growth of the interfacial IMCs at the anode side follows parabolic law with time as well. At the cathode side, controlled by the Cu flux of the IMCs dissolution, the growth of the IMCs at the Sn/Cu interfaces first increases and then decteases; because the existing of large Ni3Sn4particles can prevent Ni atoms diffusing to Cu side, the thickness of the IMCs at the Sn/Ni interfaces increases continuously with the time increasing. As the cathode, the dissolution of Cu substrates is in linear relationship with time, so is that of Ni substrates.The comparison between Cu/Sn/Cu and Cu/Sn/Ni line-type interconnects shows that: during EM, the growth rate of the IMCs at the Cu anode side in Cu/Sn/Cu interconnects is larger than that in Cu/Sn/Ni interconnects, i.e., the cathode material has an obvious effect on the growth of the IMCs at the anode side; the dissolution rate of the Cu cathode in Cu/Sn/Cu interconnects approximately equals to that in Cu/Sn/Ni interconnects, i.e., the anode material has little effect on the dissolution of the cathode.