Structure Transition Of Lead-free Sn-Sb Base Solder And Its Effects On Solidification And Wettability

Lead is a toxic metal, lead-free solder has been a trend in Electronic packagingand materials. Some countries, such as Europe and America, Japan, input plenty ofenergy into it, it is urgent for our country to develop soldering technology withoutlead.As shown in domestic and abroad information, the studies of lead-free soldermainly focus on the optimization and choice of alloy composition ratio, theinfluence of microelement (such as RE) on the performance of alloy, and the workson technology and equipment of lead-free solder. However, less attention has beenpaid to the lead-free solder preparation conditions, such as the melt thermal historycondition of solder. In recent years, more and more liquid-liquid structuraltransitions induced by temperature in metals and alloys have been found in the areaof materials production and scientific research, and it is noticed that liquid–solidheredity does exist between liquid-liquid structural transition and materialperformance, which may offer a valuable method to prepare lead-free solder.Sn-5Sb-xBi (the mass fraction of Bi is0,1%,3%and5%) is chosen as studyobject assisted by Bi-adding, the effect of melt structural state on solidificationstructure, wettability and shear strength of solder will be explored. Meanwhile, theinfluence of Bi-adding on the structure transition and solder ability of Sn-Sb5solder will also be studied.The main studies are as follows:(1)By investigating resistivity-temperature relationship of differentingredients of Sn-5Sb-xBi alloy by DC resistivity method, the result shows thereare some abnormal in the-T curves of Sn-5Sb-xBi alloy, and the changes ofdifferent ingredient alloys are similar, reversible features are shown after the firstheating and cooling cycle. The melting point decreases and the melting rangeincreases with the addition of Bi.(2)The solidification microstructure from the melt experienced structuraltransition is refined and more uniform.According to the analysis, that the solidclusters or short-range ordered structures are broken or resolved into smallerstructures result in the increase of crystallization nucleation number, the increase ofundercooling degree, the increase of nucleation rate, finally the microstructures arefiner and more uniform.(3) Spreading experiment is taken by soldering the solder sample (prepared bymetal mold solidification experiments) on the copper plate.The result shows that the wettability of Sn-Sb5-xBi solder is improved by the addition of Bi;meanwhile,liquid-liquid structural transitions can imprpve the wettability ofSn-Sb5-xBi solder.(4) The shear strength of Sn-5Sb-xBi soldered-Cu joint is tested, and the resultshows that the joint shear strength is influenced significantly by the addition of Biand the liquid-liquid structural transition. And the joint shear strength firstincreases then decreases with the addition of Bi.When Bi is in the same content, theliquid-liquid structural transition improves the shear strength of soldered joint.