Effect Of Ga And Nd On Microstructure And Properties Of Low-sliver SnAgCu Solder

Sn-Ag-Cu solder is considered to be the best alternative to the traditional Sn-Pb solder due to its superior comprehensive properties.However,the Ag content in widely used Sn-3.8Ag-0.3Cu solder is relatively high,it can not meet the increasing cost control requirements of the electronic industry due to the high price of Ag.Against this background,element Ga and Rare Earth element Nd were added to improve the drawbacks of Sn-0.3Ag-0.7Cu solder caused by the decrease of Ag content,the impact of different Nd content on the microstructure and properties was studied.Moreover,the mechanism for improvement of Ga and Nd on solder properties was also investigated.Wetting balance method were adopted to study the wetting properties of the Sn-0.3Ag-0.7Cu-xGa-xNd solders.The results showed that the addition of Ga/Nd elements can effectively improve the wettability of the solder.Among all solders,The wetting time of Sn-0.3Ag-0.7Cu-0.5Ga-0.1Nd solder was the shortest,which was about 37%shorter than that of Sn-0.3Ag-0.7Cu,its wettability had reach the level of high-silver Sn-Ag-Cu solders.Differential Scanning Calorimeter（DSC）results indicated that the difference between the melting interval of Sn-0.3Ag-0.7Cu and the melting interval of Sn-3.8Ag-0.7Cu decreased from 7.7°C to 3.9°C after alloying Ga/Nd element,and the liquidus temperature of the solder decreased by about 2°C compared with that of Sn-0.3Ag-0.7Cu.Alloying Ga/Nd elements together can also effectively refine the matrix structure of Sn-0.3Ag-0.7Cu solder,when the Nd content exceed 0.1 wt.%,the microstructure was the most uniform and the grain reached the highest degree of refinement as well.The results of mechanical properties test of solder joints indicated that the mechanical properties of solder joints can be significantly improved by adding appropriate amount of Ga/Nd.The shear force of Sn-0.3Ag-0.7Cu-0.5Ga-0.1Nd solder joint was the highest,which was about 36%higher than that of Sn-0.3Ag-0.7Cu solder joint,reaching the 91%of Sn-3.8Ag-0.7Cu solder joint.Microscopic scanning analysis showed that alloying appropriate amount of Ga/Nd elements can effectively refine the microstructure of solder joints,the interfacial structure was uniformed and refined while no coarse intermetallic compounds formed.The interface layer was smooth and thin after adding proper content of Nd,however,no obvious rare earth phases was observed after adding excessive amount of Nd.Instead,a new small block-like phases formed in the solder microstructure,which is different from the previous studies.During the isothermal aging process,the mechanical properties of solder joints decrease with the increase of aging time.This is mainly due to the coarsening of intermetallic compounds in the interfacial,in which Sn-0.3Ag-0.7Cu-0.5Ga solder joint has a highest interfacial layer thickness and stratification was also appear.The microstructure of solder joints with Nd element was still uniform and refine,while no obvious stratification appeared.EDS and XRD results indicated that Ga₂Nd phase appears near the solder joint interface.Thermodynamic calculation results show that Ga₂Nd phase is easier to form and has higher thermal stability than Sn-Nd rare earth phase.The dispersed Ga₂Nd phase effectively avoided the formation of hard brittle rare earth phases and mechanical properties degradation of solder joints.Therefore,the mechanical properties of Sn-0.3Ag-0.7Cu-0.5Ga-0.1Nd solder joint can be maintained,after aging treatment over 720 h,the shear force of its solder joint was still higher than that of Sn-0.3Ag-0.7Cu solder joint.