Effect Of RE And Ga On The Microstructure And Properties Of Low Silver Sn-Ag-Cu Solder Alloys

Sn-3.0Ag-0.5Cu and Sn-3.8Ag-0.7Cu lead-free solders are the most commonly used microelectronic interconnection solders,but the high content of precious Ag is resulting in increased costs of the products.With the rapid development of electronic products,the period of product updating has been shortened continuously,thereby improving the competitiveness of electronic products has already become common understanding inside industry.However,due to the decrease of Ag content,the wettability and solder joint reliability of“low silver”Sn-Ag-Cu solder alloy greatly decreased.In this paper,the effects of rare earth elements Nd,Pr and Ga on microstructure,wettability and mechanical properties of Sn-0.3Ag-0.7Cu（SAC0307）and Sn-0.5Ag-0.7Cu（SAC0507）alloys have been systematically and deeply studied.Firstly,the effects of Nd,Pr and Ga addition on the microstructure,melting characteristics and mechanical properties of SAC0307 and SAC0507 solders were researched respectively.It is found that SAC0307 and SACP0507 solder have the best comprehensive properties when the doping amount of Nd reached 0.1wt.%and Ga reached 0.5wt.%respectively.Further studies find that the addition of rare earth element Nd in the range of 0.05⁰.1wt.%can significantly reduce the degree of solidification undercooling of SAC0307 alloy,resulting in△T dropped to about 1/4 of the original value.Secondly,based on the mechanical behavior of the low silver SAC solders,the effects of the strain rate(1?10^-4s^-1,1?10^-3s^-1,1?10^-2s^-1,1?10^-1s^-1、6.7?10^-1s^-1),temperature load（28^oC,80 ^oC,20 ^oC,180 ^oC）on tensile deformation behavior were studied via taking SAC-xNd alloy for importance,the intrinsic mechanism of effects of xNd on mechanical response were further studied.The results have displayed that the strain rate sensitivity exponent of SAC-0.1Nd alloy reached the maximum value,showing the best resistance to necking down during deformation,and exhibits a minimum activation energy,which made the alloy performs higher strength and plasticity.Thirdly,the alloys were placed in atmosphere and heated to 245^oC for liquid oxidation test.The results showed that the addition of trace elements Nd and Pr+Ga could improve the oxidation resistance of the alloy remarkably.This phenomenon is due to the segregation behavior of Ga and Nd on the liquid metal surface,and a dense（Sn,Ga/Nd）O composite oxide film formed on the metal surface.When the addition of Ga reached 0.5 wt.%（Nd to 0.1 wt.%）,best oxidation resistance and the wettability on the copper substrate of solder obtained.Finally,the mechanical properties after aging of micro alloyed joints were studied based on optimum chemical composition of solder alloys.The effects of different additions of rare earth Nd/Pr and Ga on the shear strength of SAC micro solder joints under reflow soldering and aging conditions were investigated.The microstructure evolution law of SAC-xNd and SACP-yGa/Cu solder joints during aging process was also studied,and the evolution rule of mechanical variation and mechanical properties of SAC solder joints after Nd and Ga addition were demonstrated.In conclusion,trace amounts of Nd and Ga addition can improve the shear strength of low silver SAC micro solder joints,as well as the reliability of low silver SAC solder joints.Meanwhile,similar effects have been performed by adding Pr and Ga.The mechanical properties of solder joints show a downward trend with the increase of aging time when aging at 150℃,which is still higher than the original low Ag alloy.In a summary,the optimized Sn-0.3Ag-0.7Cu-0.1Nd,Sn-0.5Ag-0.7Cu-0.05Pr-0.5Ga solders have already met the requirements to replace Sn-3.0Ag-0.7Cu（bearing 3.0wt.%Ag）,Sn-3.8Ag-0.7Cu（bearing 3.8wt.%Ag）in many electronic products soldering procedures.