Study On The Microstructure, Properties And Joint Reliability Of SnCuNi-xPr Lead-free Solder

Among the numerous lead free solder systems exsited, there has not been a kind of lead freesolder yet that can completely replace the traditional Sn-Pb solder. At the same time, in order tosatisfy the acquirements of electronic products with increasing miniaturization and high density, theresearch of joint strength and its reliability in service of a new lead-free solder has become a hot issue.In this paper, micro alloying, which is one of the most effective methods to improve the performanceof solder alloy, is used to improve the microstructure and soldering properties of Sn-0.7Cu-0.05Nisolder with trace amount of RE Pr addtion. Moreover, this article focused on the reliability problemsof SnCuNi-xPr solder joints.It was found that the microstructure of the solder matrix could be effectively refined by0.03wt.%~0.08wt.%Pr addtion through its being absorbed on grain and phase boundaries. In addition,the wettability was improved by the decreased surface tension through alloying Pr.With the addition of0.05wt.%Pr, IMC particles were greatly refined and uniformly distributed inthe SnCuNi-xPr/Cu solder joints. Grain growth could be greatly inhibited by IMC particles thatformed on grain boundaries through its pinning effect on the migration of grain boundaries. Hence,the mechanical properties of solder joints could be greatly improved by fine-grain and second-phasestrengthening.The results of isothermal aging and thermal cycling tests showed that interface layer formed inthe SnCuNi-xPr/Cu(x=0,0.05,0.15) solder joints got thicker with aging time or cycle indexincreasing. An approximate linear relationship beween IMC thickness and cycle index was obtained,while the correlation coefficients of IMC thickness and aging time were0.87,0.73and0.93,respectively. It also showed that adding0.05wt.%Pr could suppress the infacial reaction and reducethe growth rate of interface layer in the both test conditions.For the process of the interface layer growth, it can be divided into three parts. First, solder reactwith Cu and produce scallop-like interfacial compounds, which is called the longitudinal growth stage;the second part is the lateral growth of the scallop-like interfacial compounds drived by surfacetension and curvature; IMC particles get merged beside interface layer and finally fuse with theinterfacial compounds together is the last part. Atom diffusion between different phases is thefoundamental cause for the interfacial compounds growth. By comparing the results of the two test conditions, it was found that the growth rate of interfacelayer for the SnCuNi-xPr/Cu (x=0,0.05) solder joint in the isothermal aging was a little higher thanthat in the-55℃~125℃thermal cycling condition, while mechanical properties got more weakenedin the thermal cycling condition.