Investigation On Composition Optimization Of Sn-Bi Series Low-temperature Lead-free Solder Alloys

Sn-Bi series low-temperature lead-free solder is expected to be the second-generation lead-free solder instead of Sn-Pb series solder because of its low melting temperature,which has attracted wide attention.However,due to the brittleness of the solder,the popularization and application of Sn-Bi solder in the field of electronic packaging are limited.In order to reduce the brittleness of Sn-Bi eutectic filler metal?Bi content 58wt.%?,this paper selected Sn-Bi subeutectic filler metal as alloy matrix?Bi content 40wt.%?,and studied the influence of alloy elements Zn,Ag and Cu on the melting characteristics,wetting spread,solder joint microstructure and mechanical properties of the alloy.Firstly,Zn?0.5-2wt.%?was added into Sn-40Bi solder alloy to suppress the Bias of Bi phase in solder joint and make Bi phase distribution more uniform.Solder joint interface observation showed the following results:the intermetallic compound composition of Sn40Bi/Cu and Sn40Bi-0.5Zn/Cu within 100⁸00 hours of aging was mainly Cu₆Sn₅;The composition of intermetallic compounds between Sn40Bi-1.5 Zn/Cu and Sn40Bi-2Zn/Cu solder joints gradually changed from Cu₆?Sn,Zn?₅ to Cu₅Zn₈ after 100-800 hours of aging.With the increase of Zn content,the melting temperature of solder alloy decreases?that is,the temperature of solid and liquid phase line decreases,the melting process decreases?,and the wettability of solder alloy decreases.The mechanical properties test show that the microhardness of solder increases slowly with the increase of Zn content,and the shear strength of solder joint decreases gradually,all fractures are brittle fractures with cleavage fracture as the main fracture.Secondly,the effect of adding Ag?0.1¹.5wt.%?to Sn-40Bi filler metal alloy was studied.The results were as follows:when Ag content was 0.1¹wt.%,the Bi phase volume fraction near the solder joint interface decreased,and the grain size decreased and showed a discrete distribution.Sn-40Bi-xAg/Cu solder joint after 100-800 hours of aging time,the morphology of intermetallic compounds at the interface was generally scallop like,and the components were all cu-sn compounds after EDS detection and analysis.Ag can inhibit the growth of intermetallic compounds at solder joint interface during aging.With the increase of Ag content,the melting process of solder alloy decreases first and then increases,and the wetting spread of solder gradually increases.The microhardness of solder increases with Ag content,and the shear strength of solder joint increases first and then decreases.All fractures show microfracture characteristics of microporous aggregate fractures.Finally,the effect of Cu?0.1¹.5wt.%?on Sn-40Bi-0.5Ag filler metal was studied.The experimental results showed that the morphology of intermetallic compounds at the solder joints of Sn-40Bi-0.5 Ag-xCu/Cu was generally scallop like after 100-800 hours of aging,and the components were all cu-sn compounds detected and analyzed by EDS.The melting characteristics of solder alloys did not change obviously,and the wetting spread of solder alloys increased gradually.The microhardness of solder increases with Cu content,and the shear strength of solder joint increases first and then decreases..When Cu content is less than 0.5wt.%,the fracture mode of solder joints is ductile fracture,when Cu content is 1 wt.%,it is a ductile-brittle mixed fracture mode,and when Cu content increases to 1.5wt.%,it is completely transformed into a brittle fracture mode.