Effect Of Liquid-liquid Structure Transition On Structure And Performance Of Sn-Ag-Cu-Bi Lead-free Solder

In recent years, people began to pay close attention to the research and development oflead-free solders. Sn-Pb alloys were gradually discarded. However, research on lead-freealloys was mainly concentrated in the component selection and the increase in newelements in solders. On the other hand, pressure or/and temperature inducedliquid-liquid structure transition was found in liquid state. A combination oflead-free alloys and liquid-liquid structure transition will afford a new idea to thestudy of lead-free solders and the development of new materials.In this paper, SnAgCuBi lead-free alloys have been chosen as the research objectfrom the new viewpoint of liquid-liquid structure transition. The effects of meltstructure transition on their microstructure and performance in lead-free solders areinvestigated. At the same time, the effect of Bi addition on SnAgCuBi lead-free solder isaslo investigated. The main contents and conclusions are as follows:(1) The melt structure changes of SnAgCuBi lead-free solders are studied byelectrical resistivity. The anomalous changes can be observed on theresistivity-temperature (ρ-T) curves at certain temperature ranges in twoexperimental cycles, which indicate the melt structure change.The characteristic of the anomalous change during the first heating cycle isdifferent from other ones, which is the combined action of reversible and irreversiblestructural transition. And the anomalous changes are reversible after the first cycleheating. Moreover, Bi can effectively decrease the melting point of the alloys.(2) Based on the results of electrical resistivity testing, the solidificationexperiments are carried out to explore the effect of liquid-liquid structure transitionon solidification in SnAgCuBi alloys. The results indicate that the solidificationstructure of SnAgCuBi lead-free solder became finer when solidifying from the meltafter liquid-liquid structure transition.(3) Wettability experiments are completed on copper substrate to study the influenceliquid-liquid structure transition on wettability of SnAgCuBi lead-free solders. Theresults show that liquid-liquid structure transition could effectively enhance thewettability of SnAgCuBi lead-free solder on the copper substrate. Moreover, Bi couldeffectively increase the wettability of alloys, the higher Bi content is, and the betterwettability is. (4) Soldering samples for shear strength testing are soldered with differentSnAgCuBi lead-free solder, and shear strength tests are carried out on tester. The resultsshow that liquid-liquid structure transition can improve the shear strength of thesoldering joints. Because of brittle phase of Bi, the shear strength of SnAgCuBilead-free alloys joints firstly increases and then decreases with the increase of Bi.The shear strength of the soldering joints reaches the maximum strength at3.1%Biin our experiments.