Study On Liquid-solid Electromigration Behavior In Ni/Sn-9Zn/Ni And Cu/Sn-58Bi/Ni Interconnects

Among the challenges that are brought by the trends of continuous miniaturization and higher performance in electronic packaging technology, electromigration (EM) has become an important and remarkable reliability issue. As the continuous reduction in the size of solder joints, the current density passing through the joints can be as high as104A/cm2, which will lead to a serious temperature rise in the joints due to the effect of Joule heating, and it may be even higher than the melting point of the solder. As a result, liquid-solid electromigration (L-S EM) may occur. Hence, it is necessary and urgent to investigate the L-S EM behavior of lead-free solder joints. In this work, both Ni/Sn-9Zn/Ni and Cu/Sn-58Bi/Ni line-type interconnects were employed to investigate the diffusion behavior of Zn atoms, cross-solder interaction between Cu and Ni atoms and the microstructural evolution of the interconnects during L-S EM under a current density of5.0×103A/cm2at230℃and a current density of5.0×103A/cm2at170℃, respectively.The main conclusions are drawn as follows:(1) A reverse polarity effect was revealed in Ni/Sn-9Zn/Ni interconnects during L-S EM, i.e., the interfacial intermetallic compounds (IMCs) at the cathode side grew continuously and were obviously thicker than those at the anode side. This reverse polarity effect was resulted from the directional migration of the Zn atoms with a positive effective charge number (Z*) towards the cathode under the effect of electron-wind-force. The interfacial IMC at both anode and cathode was identified as Ni5Zn21phase, and no IMC transformation occurred at the interface. Furthermore, the content of Zn in the solder matrix dropped from9wt.%to1.72wt.%.(2) During L-S EM, Cu/Sn-58Bi/Ni showed a polarity effect regardless of the current direction, i.e., the interfacial intermetallic compounds (IMCs) at the anode side grew continuously and were obviously thicker than those at the cathode side. The L-S EM accelerated the interaction between Cu and Ni atoms.(3) During L-S EM, when Ni atoms were under downwind diffusion, both the dissolution and diffusion of Ni atoms were significantly enhanced by electronic wind, and some Ni atoms arrived at the opposite Cu side, resulting in the formation of (Cu,Ni)6Sns at the anode interface after EM for1h. At the cathode, a certain amount of Cu atoms under upwind diffusion reached to the Ni side, resulting in the formation of (Cu,Ni)6Sn5at the solder/Ni interface. The thickness of this (Cu,Ni)6Sn5IMC layer shows a trend of increase at the beginning but decrease thereafter.(4) During L-S EM, when Cu atoms were under downwind diffusion, it was significantly enhanced by electronic wind. However, Ni atoms under upwind diffusion were difficult to reach to the Cu side. Therefore, the interfacial IMC at the Cu side was still Cu6Sn5. At the Ni anode interface, under the combining effect of chemical potential gradient and electronic wind, plenty of Cu atoms diffused to the Ni side and participated in the formation of (Cu,Ni)6Sn5IMC. Furthermore, regardless of the current direction, no segregation of Bi atoms occurred in the solder joints. It is concluded that the diffusion fluxes of Bi atoms caused by chemical potential gradient and electronic wind had opposite directions and similar numerical values during L-S EM.