Reliability Of Sn-Ag-Cu Lead-Free Solder Joints Reinforced With Carbon Nanotubes

In the context of miniaturization of electronic components,electronic packaging solder joints are required to have more reliable performance.A new type of composite lead-free solder is prepared by adding different fractions(0~0.5wt.%)nickel-plated multi-wall carbon nanotubes(Ni-CNTs)to Sn-3.0Ag-0.5Cu lead-free solder by mechanical mixing method.Comparative study of its wettability,melting characteristics,interface IMC layer and mechanical properties of the solder joints during isothermal aging,the following conclusions are obtained:1.The addition of Ni-CNTs significantly improves the wettability of Sn-3.0Ag-0.5Cu solder.The optimum addition amount is 0.05 wt.%,the wetting angle decreases by 49.76%,but when the content increases,the size of the wetting angle will secular rise;at the same time,the addition of Ni-CNTs reduces the melting point of the composite solder.As the content increases,the melting point of composite solder slowly decreases,with the highest decrease of 2.108°C.2.Research of interface IMC laver shows that:During the reflow process,Ni atoms replace part of Cu atoms to participate in the solder/matrix chemical reaction to form(Cu,Ni)6Sn5 intemetallic compound.With the isothermal aging experiment,the thickness of the interface IMC layer increases and the average particle sizes become larger,the shape gradually changes from scallop to continuous layer.On the one hand,Ni-CNTs inhibit the growth of the IMC layer by hindering the inter-atomic diffusion.Among them,both of the thickness of interface IMC layer and diffusion coefficient of Sn-3.0Ag-0.5Cu-0.05Ni-CNTs composite solder is the smallest thickness where the diffusion coefficient is K=3.01×10-17 m2/s;Ni-CNTs inhibit the grain growth of composite solder joint interface(Cu,Ni)6Sn5 by hindering grain boundary migration and reducing the grain surface energy.Moreover the inhibitory effect Sn-3.0Ag-0.5Cu-0.05Ni-CNTs composite solder is the best.It is calculated by fitting curve that with the increase of isothermal time,the average grain size of(Cu,Ni)6Sn5is linearly related to t0.39.3.The addition of Ni-CNTs will increase the microhardness of Sn-3.0Ag-0.5Cu lead-free solder,but with the content of the reinforcing phase increases,the microhardness is slowly decreasing,When the content of Ni-CNTs is 0.05 wt.%,the microhardness value of the new composite solder joints is 15.49% higher than that of the solder without reinforcement phase.It is attributed to the addition of Ni-CNTs that will directly strengthen and initiate dispersion strengthening,fine-grain strengthening and dislocation strengthening,thereby enhancing the resistance of the solder to deformation and reducing the penetration depth of the diamond probe.4.In the isothermal aging experiment,the shear strength of Sn-3.0Ag-0.5Cu lead-free solder with different contents of Ni-CNTs shows a phenomenon of first increasing and then decreasing,Sn-3.0Ag-0.5Cu-0.05Ni-CNTs has the highest shear strength,which is 29.71 MPa.The reason is that the reinforcing phase Ni-CNTs in the composite solder can prevent crack growth,consumed energy,load transfer effect.However,when the content continues to increase,Ni-CNTs will agglomerate and entangle together to form defects.When the content of Ni-CNTs is constant,the shear strength decreases as the isothermal time increases.5.Comprehensive wettability,melting characteristics,behavior change of IMC layer in solder joint and mechanical properties of solder joints,the performance of the new composite solder is the best when the addition of Ni-CNTs is 0.05 wt.%.