| Reliability is certainly the most important concern in modern electronic packaging, and the development of self-adaptive lead-free solder is urgently expected to meet the strict requirements of actual production in recent years. Both controlling the morphology and type of intermetallic compounds (IMCs) in the soldered interface, and the addition of more alloying elements are the promising ways for exploring high performance lead-free solder and enhancing the reliability of solder joint. For an AgZn IMCs exhibiting a shape memory effect may separate out in the microstructure of a Sn-3.7%Ag-0.9%Zn solder alloy, the addition of the fourth alloying element in this basic system is proposed for the promotion of the AgZn phase and further improvement of its properties. Furthermore, the evolution of interfacial structure between Sn-Ag-Zn solder and Cu substrate is investigated.Firstly, the effects of the addition of Al and In components on the microstructural formation of equilibrium and water-cooled Sn-Ag-Zn solder was systematically investigated. It is found that the addition of Al and In adjust the microstructural morphology of the Sn-3.7%Ag-0.9%Zn solder greatly, especially, Al promotes the formation of Ag2Al IMCs and restrains the separation of the AgZn IMCs simultaneously. The effect of In addition relates to the applied cooling rate significantly: there is no difference in the formed phase structures except for morphology at lower cooling rate, and the formation of rod-like mixture (composed of Ag3Sn and AgZn) due to the adsorption of In atoms on the outer surfaces of the Ag3Sn and AgZn IMCs; while only Ag3Sn IMCs was found and the separation of the AgZn IMCs was restrained in the solder solidified at higher cooling rate.Secondly, the microstructural evolution of the investigated solders during actual service was simulated by high temperature aging process. For the equilibrium solders, no phase transformation takes place during aging, except for the successive growth of the formed IMCs and the final refined microstructures. For the water-cooled solder, the aged solders with Al are similar to the equilibrium solders, however, the AgZn IMCs separates out again and the Ag3Sn IMCs keeps on growing in the solders with In during aging, which leads to a refined microstructure (identified even by the disappearance of the coarseβ-Sn phase). Lastly, the soldered interfaces between the Sn-3.7%Ag-0.9%Zn solder and Cu substrate for different soldering periods were explored. The evolution of the IMCs layer in the interface can be divided into three stages. At the early stage, the interface is composed of the Cu5Zn8 IMCs primarily. Then the formed metastable Cu5Zn8 IMCs layer begins to decompose from its bottom where the new Cu6Sn5 IMCs layer forms simultaneously, and this process is controlled by the diffusion of Sn atoms in the initially-formed Cu5Zn8 IMCs layer. Finally, the initial metastable Cu5Zn8 IMCs layer is completely replaced by the newly-formed Cu6Sn5 IMCs layer which will grow during the remaining soldering time and subsequent cooling segment, and this process is controlled by the diffusion of Sn in the Cu6Sn5 IMCs layer. The thus obtained diffusion coefficient of Sn in the Cu6Sn5 IMCs layer is a little larger than that given in other references, which could be attributed to the different controlling factors in the middle and final stages of the evolution of the IMCs layer in the explored interface. |
