Effects Of Electromigration On Cross-solder Interaction And Interfacial Reaction In Lead-free Solder Joints

With the continuous miniaturization of electronic devices, the solder joints in electronic packaging are downsizing and the current density applied to the solder joints is increasing. When the current density increases to104A/cm2, electromigration (EM) may seriously affect element diffusion behavior and interfacial reaction. Therefore, the effects of EM on element diffusion and interfacial reaction in lead-free solder joints are becoming research hotspots.Due to the asymmetric structure of the actual solder joints, serious current crowding effect and Joule heating effect occur at both the entrance and the exit areas of current. Under this condition, the interfacial reaction in solder joint during EM becomes more complicated. In order to better reveal the mechanisms of EM on element diffusion behavior, cross-solder interaction and interfacial reaction, line-type Cu/Sn/Cu, Cu/Sn/Ni and Ni/Sn/Ni-P interconnects were used in this study. Based on the results of line-type interconnects, the Ni/Sn3.OAgO.5Cu/Cu and Ni/Sn3.0Ag0.5Cu/Ni-P flip chip solder joints were used to further investigate the effects of EM on interfacial reaction and failure mechanism.The main conclusions are as follows:1. There was a polarity effect in Cu/Sn/Cu interconnect during EM, i.e., the interfacial intermetallic compounds (IMCs) at the anode side were thicker than those at the cathode side. Compared with the aging specimens, the growth kinetics of the interfacial IMCs at the anode side were significantly enhanced during EM, but they still followed the t1/2law. The IMCs at the anode side grew faster under higher current density at the same temperature. The temperature was also one of the critical factors that influenced the EM. The effect of EM became more significant at higher temperature under the same current density. The growth behavior of IMCs at the cathode side was complicated. Due to the initial IMCs in the as-soldered state were thin (less than0.5μm), the IMCs grew at the beginning of EM. After the IMCs at the cathode side increased to a critical thickness, they decreased thereafter. An IMC growth model was build to explain the growth behavior of the interfacial IMCs, and the calculation results coincided with experimental results.2. The flowing direction of electrons strongly influenced the Cu-Ni cross-solder interaction in Cu/Sn/Ni interconnects during EM. In the solid-solid state EM, the Cu atoms could diffuse to the opposite Sn/Ni interface and transformed the Sn/Ni interfacial IMCs from Ni3Sn4into (Cu,Ni)6Sn5only when Cu atoms were under downwind diffusion, while no Cu atoms could diffuse to the opposite Sn/Ni interface when Cu atoms were under upwind diffusion. For the Ni atoms, few Ni atoms could diffuse to the opposite Sn/Cu interface regardless of the direction of electrons. The EM effects became more prominent during liquid-solid EM. For the Cu atoms, a large amount of Cu atoms diffused to the opposite Sn/Ni interface and changed the Sn/Ni interfacial IMCs regardless of the direction of electrons. During EM, the Ni atoms could diffuse to the opposite Sn/Cu interface and changed the Sn/Cu interfacial IMC only when Ni atoms were under downwind diffusion, while no Ni atom was observed at the opposite Sn/Cu interface when Ni atoms were under upwind diffusion. Regardless of experimental condition, EM caused a polarity effect.3. The flowing direction of electrons played an important role on the consumption behavior of Ni-P layer in the Ni/Sn/Ni-P interconnect during EM. When the electrons flowed from Ni-P side to Ni side, Ni2SnP formed at the cathode Sn/Ni-P interface. EM accelerated the Ni-P layer consumption and more Ni-P layer was consumed with increasing EM time. After EM for400h, the Ni-P layer was consumed up and transformed into Ni2SnP and Ni3P layers. When the electrons flowed from Ni side to Ni-P side, Ni3Sn4formed at the anode Sn/Ni-P interface and no obvious Ni-P consumption was observed.4. There was only one failure mode in the Ni/Sn3.0Ag0.5Cu/Cu flip chip solder joint during EM. When the electrons flowed from PCB (Cu) side to chip (Ni) side, the current crowding effect induced rapid local dissolution of Cu pad and crack formed at the electron entrance. After EM for1000h under a current density of1.Ox104A/cm2at150℃, the Cu pad was consumed up and the crack extended to the whole interface. High temperature accelerated the failure process, and the solder joint failed after EM for143h at180℃,5. There were two failure modes in the Ni/Sn3.0Ag0.5Cu/Ni-P flip chip solder joint during EM. When the electrons flowed from PCB (Ni-P) side to chip (Ni) side, EM significantly accelerated the consumption of Ni-P layer and crack formed at the cathode interface. After EM for600h under a current density of1.0x104A/cm2at150℃,the Ni-P layer was completely consumed and the crack extended to the whole cathode interface. When the electrons flowed from chip side to PCB side, the Ni UBM and the Cu pad at (he chip side were obviously consumed during EM. After I'M for600h under a current density of1.Ox104A/cm2at150℃, not only the Ni UBM but also the Cu pad under the Ni UBM was completely consumed and large crack formed at the cathode interface. The Ni/Sn3.OAg().5Cu/Ni-P solder joints failed after EM for155h under a current density of1.0xl04A/cm2at180℃...