A Study Of Interfacial Evolution And Inhibitory Mechanism Of Sn58Bi Under Electromigration

Due to the increasing environmental and health concerns,the use of Pb solder has been baned in electronic products,which has led to an extensive study of lead-free solder materials by domestic and foreign researchers.Among the potential alternatives,Sn-58 Bi arouse widespread concerns with the low temperature,cost and coefficient of thermal expansion.The current density increased with the higher packaging density in electronic products,and now the current density in solder joints has beyond 106A/cm2,which usually becomes serious drawbacks caused by electromigration(EM).In this study,on the basis research of Cu/Sn-58Bi/Cu solder joints,we further investigated the interfacial reaction and inhibition mechanism in the composite solder joints Sn-58Bi/Sn-3.0Ag-0.5Cu.On the other hand,the EM behaviors of Sn-58 Bi with Zn content on substrate were also investigated.The solid-solid EM behavior of line-type solder joints Cu/Sn-58Bi/Cu were studied with a current density of 1.0×104A/cm2 at room temperature.Cracks appeared on the surface and Sn whiskers formed at the anode side due to the atomic migration with the stressing time.The Bi atoms migrate from the cathode side to the anode side which formed Bi-rich layers at the anode side in the interior microstructure during EM.The thickness of Bi-rich layer under 50°C is 4.5 times to the room temperature,which shows the critical effect of temperature.In the liquid-solid EM under high current(24A),the rodlike inter metallic compounds(IMC)formed and growth with the stressing time at the anode side,which called "aging" phenomenon.Besides,the Cu atoms at the cathode interface continuously consumed caused the interface uneven.In the liquid-solid EM under high temperature and low current density(150°C,5×103A/cm2),the growth rate of IMC at the cathode and the anode interface were higher comparied with the solid-solid EM.Further more,the thickness of IMC at the cathode side is higher than that at the anode side during solid-solid EM.The structural composite solder joints were with good welding perfoemance in the as-reflowed state.Compared with Cu/Sn-58Bi/Cu solder joints,the Bi-rich layers and the interfacial IMC at both sides were thinner during EM,which indicated the SAC305 blocked the migration of Bi and Cu atoms.The Bi atoms in Sn-58 Bi at the cathode side migrated from the edge of SAC305 to Sn-58 Bi at the anode side due to the "skin effect".The Bi atoms near the anode side in the Sn-58Bi/SAC305 interface gradually migrate away fromSAC305 while the Bi atoms near the cathode side in the Sn-58Bi/SAC305 interface are different during EM.In the two types of componential composite solder joints,the content of Ag content has a great influence on EM.The higher the Ag content is,the more serious impediment to the EM is.In the componential composite solder joints,the thickness of IMC increased with the stressing time but the growth rate is slow,which was due to the hindrance of Ag3 Sn,and there were no successive Bi-rich layers formed at the anode side.For the Cu-xZn/Sn-58Bi/Cu-xZn(x=2.29 wt.% and 4.89 wt.%)solder joints,the addition of Zn on substrates could reduce the formation of Sn-whiskers and cracks.The diffusion of main elements can be changed at room temperature during solid-solid EM with Zn addition on substrates.The difussion rate of Bi atoms can be reduced,which is opposite to Cu atoms.In the process of liquid-solid EM,the "aging" phenomenon is obvious induced by high current.The thickness of the thickness of anode IMC is higher than that of the cathode IMC during thermo-electric coupling.