Effect Of ND On The Property Of Low-Silver Sn-Ag-Cu Solder

Most of the recent studies of SnAgCu lead-free solder alloys belong to high-silver scope. Itshigh cost hinder the promotion and application of the alloy. In order to solve these problems, oneway is to reduce the silver content to control costs, the other way is to improve the properties ofSn-Ag-Cu low-silver solder by alloying.In this paper, micro alloying, which is one of the mosteffective methods to improve the performance of solder alloy, is used to improve the microstructureand soldering properties of Sn-0.3Ag-0.7Cu solder with trace amount of RE Nd addition in order tostudy the impact of different content (0~1wt%) of Nd elements on the microstructure and propertiesof low-silver lead-free solder alloy.In this paper, we use the wetting balance method to explore the effects on wettability of differentamounts of Nd element in Sn-0.3Ag-0.7Cu lead-free solder.The experimental results show thatappropriate amount of rare earth elements Nd can well improve low-silver Sn-Ag-Cu lead-freesolder’s wetting properties because Nd accumulate on the surface of the liquid solder, this wouldlower the surface tension of the solder. The optimal Nd content for wettability in this test is about0.05wt.%.It was found that the microstructure of the solder matrix could be effectively refined by0.025wt.%~0.1wt.%Nd addition through its being absorbed on grain and phase boundaries. Inaddition, the exist of Nd element also reduces the solidification of the alloy and promote itsperformance.With the addition of0.05wt.%Nd, IMC particles were greatly refined and uniformly distributedin the Sn-0.3Ag-0.7Cu-xNd/Cu solder joints. Grain growth could be greatly inhibited by IMCparticles that formed on grain boundaries through its pinning effect on the migration of grainboundaries. Hence, the mechanical properties of solder joints could be greatly improved by fine-grainand second-phase strengthening. The results of isothermal aging test showed that interface layerformed in the Sn-0.3Ag-0.7Cu-xNd/Cu(x=0,0.05,1) solder joints got thicker with aging time. Thecorrelation coefficients of IMC thickness and aging time were1.06,1.02and1.10. It also showed thatadding0.05wt.%Nd could suppress the interface reaction and reduce the growth rate of interfacelayer in the test conditions. For the process of the interface layer growth, it can be divided into threeparts. First, solder react with Cu and produce scallop-like interfacial compounds, which is called thelongitudinal growth stage; the second part is the lateral growth of the scallop-like interfacial compounds drived by surface tension and curvature; IMC particles get merged beside interface layerand finally fuse with the interfacial compounds together is the last part. Atom diffusion betweendifferent phases is the fundamental cause for the interfacial compounds growth.