Liquid-liquid Structure Transitions Of Sn-Cu-Bi Alloys And Their Effect On Solidification And Solderability

In recent years, pressure or temperature-induced liquid-liquid structure transitions (L-LSTs) have been found in liquids, which challenge our conventional picture of liquid as entity with a continuously varying averaged structure. In-depth investigation on the nature and rule of L-LSTs is of significance to understand the structure of liquid substance and to develop new materials. So far, the studies on lead-free solders materials mainly focus upon the selection of components and their optimization of the mixture ratio, and the influence of minor elements on various properties. Little attention has been paid to the effect of the melt heat history on the microstructures and properties of lead-free solders.In this paper, Sn-0.7Cu-xBi (x=0, 3, 5, 7.5, 10 wt %) lead-free solders have been chosen as the investigation object. From the new viewpoint of temperature induced L-LST, the effect of L-LST on microstructures and soldering properties of Sn-0.7Cu-xBi are investigated. Besides, the effect of Bi on the microstructure and solderability of Sn-0.7Cu solders is studied. The main contents and conclusions are as follows:(1) Temperature dependence of electrical resistivities of Sn-0.7Cu-xBi solders has been surveyed by the DC four-probe method. The results show that the addition of Bi in the Sn-0.7Cu solder decreases melting point while increases the melting temperature range. And the anomalous changes can be observed on the resistivity-temperature (ρ-T) curves at certain temperature ranges in two experimental cycles, and the anomalous changes are reversible after the first cycle heating.(2) The effect of Bi and L-LST on the microstructures of Sn-Cu-Bi solders has been investigated by solidification experiments. It is found thatβ-Sn and Cu-rich phase become finer with the increase of Bi. Moreover, the microstructures become more homogeneous and finer after the melts have experienced L-LST.(3) Spreadability tests of the iron-mold solidified sample on Cu substrate have been carried out. The results show that the addition of Bi can improve the wettability of Sn-Cu(Bi) solders, especially the addition of 0～3 wt %Bi. And the effect of L-LST on the wettability of Sn-0.7Cu solder is more obvious.(4) The consequent thermal aging treatment for the solder joint has been performed at the temperature of 150℃for 0, 120, 240 and 480 h. The results show that the addition of Bi increases the size of Cu-Sn IMC in as-soldered joints, but inhibits the development of Cu-Sn IMC layer in thermally aged solder joints; while L-LST restrains the nucleation and development of Cu-Sn IMC.(5) The shear strength of soldered lap-joint specimen has been measured. The results show that the shear strength of Sn-0.7Cu-xBi solders/Cu joints firstly increases and then decreases with the increase of Bi, and reaches the maximum strength at 3wt%Bi. L-LST enhances the shear strength of Sn-Cu(Bi) soldered joints. In general, Sn-0.7Cu-3Bi solder experienced L-LST has the best comprehensive properties.