Study Of Microstructural Evolution Of Narrow Gap Cu/Sn-Cu-Ni-xRE/Cu Solder Joints And The Size Effect On Their Mechanical Properties

With the rapid development of electronic packaging technology, the reliability issue ofmicro-interconnects in many electronic products has attracted more and more attentions. Inthis thesis study, Sn-0.7Cu-0.05Ni （SCN） based solder was modified by adding La-Ce mixedrare earth （MRE） of different amounts and the Sn-0.7Cu-0.05Ni-xMRE solders were used toprepare Cu/Sn-0.7Cu-0.05Ni-xMRE/Cu sandwich structure micro-interconnects of variousnarrow gaps in the range of10-100μm. Further, the influence of the solder gap and thermalaging treatment on microstructural evolution and the reliability of joints were studied, and thesize effect on mechanical properties and fracture behavior of joints were characterizedsystematically by combining both experiment and finite element simulation.The results show that the addition of0.05wt%MRE to the SCN solder can improve themicrostructure of the solder for the best, and further refine the IMC particles in solder’smatrix and depress the growth of interfacial IMC layer in the joint, as well as bring about thethinnest IMC layer at the interface. Ni element was found in the interfacial IMC layer andhardly in the solder matrix. During the formation stage of IMC, i.e. the liquid reaction stage,Ni atoms in SCN solders took part in the interfacial reaction to form （Cu,Ni）₆Sn₅and wereconsumed totally. The solid solution type （Cu,Ni）₆Sn₅IMC layer existed near the Cu3Sn layerin the form of Cu atom replaced by Ni atom in Cu6Sn5.Furthermore, after the joints of different heights undergoing isothermal aging at125℃, ithas been shown that along with the prolongation of the aging time, Cu6Sn5particles in thejoints became coarsening and broken, and the total thickness of interfacial IMC layers（including both Cu₃Sn and Cu₆Sn₅layers） increased steadily and many Kirkendall voidsformed in the interfacial IMC layer. For the joints with the same height, the thickness ofinterfacial IMC layer increased with the aging time and a parabolic formula was applied todescribe this phenomenon as the formation of IMC layer was controlled by bulk diffusion.However, the thickness of the IMC layer changed little with the different heights of solderjoints. Moreover, it can also been seen that the growth rate coefficient of IMC layer getshigher in a narrower gap, especially can be as high as9.24×10^-9m/s^1/2for the joint with aheight of25μm. This may mean that the joint with a narrower gap can be more easily influenced by isothermal aging process and consequently failure may be more likely to occur.Moreover, the results manifest that with decreasing the height of solder joints, theabsolute thickness of the interfacial IMC layer in joints changed little while the ratio of IMCthickness to the height of the joint increased correspondingly, and the tensile strength of jointsincreased either. The decreasing height of joints can change the fracture mode of the jointsfrom quasi-brittle fracture at a large joint height to brittle fracture at a small joint height, andthe experimental results were also confirmed by the finite element analysis.