Investigation On The Effect Of Alloying Element Addition On Growth Of Sn5Zn/Cu Interfacial Imc And Properties Of Joint

The developing of miniaturization of electronic devices asks for the increasing of the packaging density and the decreasing of the solder joint size,which takes the reliability of the solder joint interface to the core of the reliability of the entire electronic device.At present,the Sn-based lead-free solder is widely used as the main interconnect material in the electronic packaging technology.The formation and growth of interfacial intermetallic compounds(IMC)with the copper interconnects on the substrate are the most important factors affecting the reliability of the package.Sn-Zn lead-free solder has received widespread attention because of its abundant resources,low prices,especially the melting point close to the tin-lead eutectic solder.The phase structure,the phase stability and the atom diffusion energy of Cu₅Zn₈-based IMC that doped with Ag,Al,Ga and Ni are calculated by using the first principles based on the density functional theory in this paper.The calculation results guide the preparation of Sn5Zn-x Ag and Sn5Zn-x Ni solders by metallurgy method.Moreover,the effect of Ag and Ni elements on the formation and growth under the condition of 150?isothermal aging of IMC between Sn5Zn solder and Cu is investigated by theoretical calculation and experiment.Besides,the shear strength changing law of Sn5Zn/Cu joints is found,to which the influence factors are analyzed by solder metallographic observation,fracture morphology and phase component analysis.The results of the first-principle calculations show that doped Ga and Al alloying elements can not form more stable Cu₅Zn₈-based unit cells,while doped Ag and Ni can improve the stability of Cu₅Zn₈-based unit cells,in which Ag and Ni tend to occupy the positions of Cu2(12e)and Cu1(8C)respectively.By calculating the formation energy of void in different Cu and Zn sites and migration energy barrier of neighbor sites of Cu and Zn,the diffusion energy of Cu and Zn is determined,which shows that doped Ag and Ni can improve the diffusion energy of Cu and Zn.It means that doped Ag and,notably,Ni can inhibit the diffusion of Cu and,especially,Zn atoms.After reflow,the interfacial IMC of Sn5Zn/Cu is flat and uniform in thickness with no obvious stratification.According to the energy spectrum analysis and the Cu-Zn phase diagram,the composition is Cu₅Zn₈.The interfacial IMC thickness of Sn5Zn-1Ag/Cu decreases obviously compared with that of Sn5Zn/Cu.Then,with the increase of Ag content in the solder,the interfacial IMC thickness of Sn5Zn-x Ag/Cu increases gradually.The interfacial IMC thickness of Sn5Zn-4Ag/Cu is 5.16?m,which is very close to that of Sn5Zn/Cu.The interfacial IMC of Sn5Zn-4Ag/Cu is divided into two layers,Ag₅Zn₈ near the solder,showing a similar shoot-like morphology,and(Cu,Ag)₅Zn₈ near the Cu.The interfacial IMC thickness of Sn5Zn-1Ni/Cu decreases significantly compared with that of Sn5Zn/Cu.With the increase of Ni content in the solder,the interfacial IMC thickness increases slightly,but it is always smaller than that of Sn5Zn/Cu and also less than that of Sn5Zn-x Ag/Cu.The IMC of Sn5Zn-4Ni/Cu interface has no obvious delamination,the IMC layer is even flat and uniform in thickness,with the composition of(Cu,Ni)₅Zn₈.That is to say,doped Ag and Ni has certain inhibitory effect on the formation of IMC between the Sn5Zn-based solder and the copper interconnect interface,and the inhibitory effect of Ni is more significant than that of Ag.Furthermore,Ag and Ni-doped compound layers were formed at the interface.This is due to the lower formation energy and better stability of Cu₅Zn₈-based cells doped with Ag and Ni by theoretical calculations.As the aging time prolongs,the interfacial IMC thickness of each component solder and copper increases gradually.Alloying elements Ag and Ni can effectively reduce the growth rate of the interfacial IMC during aging,and the growth rate of interfacial IMC between solder doped with a large amount of alloying elements and copper is lower than that of a little amount of alloying elements.The interfacial IMC growth rate of Sn5Zn-x Ni/Cu is slightly lower than that of Sn5Zn-x Ag/Cu.According to the theoretical calculation results,doped Ag and Ni can improve the diffusion energy of Cu and Zn in Cu₅Zn₈-based unit cells,inhibit the diffusion of Cu and Zn atoms,and especially inhibit the diffusion of Zn atoms,which resulting in the decrease of the interfacial IMC growth rate between solder doped with alloying elements and copper;the inhibitory effect of Ni is more pronounced than that of Ag,which resulting in lower interfacial IMC growth rate between Ni-doped solder and copper.The shear strength of solder joints decrease with the increase of isothermal aging time.After a certain aging time,the shear strength of Sn5Zn-x Ag/Cu interconnect joints is significantly higher than that of Sn5Zn/Cu,and the shear strength of Sn5Zn-x Ni/Cu interconnections is slightly higher than that of Sn5Zn/Cu.It means that the addition of alloying elements Ag and Ni can improve the reliability of the solder/copper joints in the aging process,in which Ag plays a more significant role.The location where the interconnect breaks under the shear forces is in the interface IMC and solder matrix near the interface.