Investigation On The Reliability Of CNTs Reinforced Sn-58Bi Composite Solder Joints

The lead-free Sn-58Bi solder with low eutectic temperature is seen the alternative solder to meet the requirements for low-temperature packaging due to the advantages of good adaptability,energy saving,low pollution emissions,etc.However,the application of Sn-58Bi in the packing industry is limited by the coarse and irregular IMC layers and brittle Bi phase.A potentially viable and economically affordable approach to improve the mechanical and electrical properties of a solder is to add appropriate second phase particles into the solder.The nono or micro particles is divided into three caterories.The reactive particles such as:Ag,Au,Ni,Fe,Zn,RE,etc,non-reactive particles such as:Al2O3.TiO2?POSS.SnO2,etc,and the partial reactive particles such as:Ni-CNTs.Co-Al2O3 are choosen as the reinforced particles.In this paper,the composite solder is prepared by adding carbon nanotubes(CNTs)(0,0.01,0.03,0.05,0.07 wt.%)into Sn-58Bi eutectic solder.The effects of CNTs on the wettability,microstructure,IMC and mechanical properties(tensile properties,shear properties,fracture morphology)of Sn-58Bi eutectic solder joints were investigated and the optimum amount of CNTs was obtained by the Sn-58Bi-0.01CNTs composite solder.The interfacial morphologies and mechanical properties of Sn-58Bi eutectic solder and Sn-58Bi-0.01CNTs composite solder joints during electromigration were studied on the basis of this.The creep resistance and creep constitutive equations of plain solder and the composite solder joints under multi-field coupling were investigated.Finally,the creep mechanism of Sn-58Bi was discussed.The microstructures,wettability and mechanical properties of Cu/Sn-58Bi-xCNTs/Cu solder joints(x=0-0.07 wt.%)were investigated.The precipitated phases of plain solder and composite sodler joints are Bi-rich phase,Sn-rich phase and a small amount of U-shaped and H-shaped Cu6Sns phase.The microstructure of CNTs reinforced Sn-58Bi composite solders is refined.Scallop-shape interfacial IMC layers is observed by Cu/Sn-58Bi-xCNTs/Cu composite solder joints,and the thickness of the Sn-58Bi solders joints IMC layers is decreased with increasing content of CNTs.The precipitated phases of IMC layer for plain solder and composite solder joints are Cu6Sn5 and Cu3Sn.The CNTs alter the ratio of precipitated phases at the interface,and the amount of Cu6Sn5 phase is increased.CNT-Sn phase may exist in the IMC layers.The wettability of the Sn-58Bi solder is improved by adding 0.03 wt.%CNTs,and the spreading area of composite solder is increased by 18.9%.The tensile strength,elongation and shear strength of the solder joints are improved by adding 0.01 wt.%CNTs.The stepped fracture morphologies are observed in the Sn-58Bi solder joint,indicating that the Sn-58Bi solder joint is brittle fracture.The unsmooth fracture morphologies of composite solders joints are observed in the fracture surface.The effects of electromigration on the interfacial morphologies and properties of Cu/Sn-58Bi-xCNTs/Cu(x=0,0.Olwt.%)are investigated.,The microstructure of plain solder and CNTs reinforced composite solder is coarsened under current stressing and the Bi atoms were moved from the cathode to anode under the electron wind.The microstructure of Cu/Sn-58Bi-0.01CNTs/Cu solder joint has no obvious Bi-rich phase after 100 h current stressing.It means that the CNTs play a role in hindering the electromigration and refining the microstructure of Sn-58Bi solder.Polar effect is observed in the IMC layers of plain solder and composite solder joints.The thickness of the IMC layers for the plain solder in the anodic interface is increased with increasing electromigration time,while tiny change is observed in the cathode interface.The thickness of IMC for composite solder joint in the anodic interface is increased with the increasing Electromigration time,but the increase is lower than that of IMC layer for plain solder joint,indicating that the addition of CNTs effectively inhibit the growth of interfacial IMC layer.The creep resistance of Cu/Sn-58Bi-xCNTs/Cu composite solder j oints deteriorates with increasing electromigration time.The creep resistance of Cu/Sn-58Bi/Cu solder joint is lower than that of Cu/Sn-58Bi-0.01CNTs/Cu solder joint.The creep resistance of Cu/Sn-58Bi-xCNTs/Cu solder joints under multi-field coupling is studied.The creep resistance of Sn-58Bi solder and Sn-58Bi-0.O1CNTs composite solder joints is decreased with increasing temperature and stress.The creep rupture life of plain solder and composite solder joint is reduced gradually and the steady state creep rate is gradually increased with increasing temperature and stress.The creep rupture life of Sn-58Bi solder joints is higher than that of Sn-58Bi solder joint.The fracture patterns of plain solder and composite solder joints are changed from transgranular fracture to intergranular fracture with decreasing temperature and stress,and the morphologies of dimple is observed on the fracture surface of the composite solder joints when the stress and temperature are low.It indicates that the addition of CNTs can improve the the creep resistance of the joints.The creep resistance of Sn-58Bi-0.0ICNTs composite solder joints is decreased with increasing temperature and current density under the stress-temperature-current coupling.It indicates that the service temperature of solder joints is improved by the current.The creep constitutive equations and creep mechanism of Cu/Sn-58Bi-xCNTs/Cu solder joints under stress-temperature coupling were studied.The stress exponent n of 3.35-4.26 and 3.66-5.20 is obtained by plain solder and composite solder joints,respectively.The addition of CNTs in the solder joints could result in a slightly higher stress exponent at each temperature.The activation energy Q,of plain solder joints 72.02-89.74 kJ/mol and composite solder joints 87.95-114.43 kJ/mol is obtained by regression analysis,respectively.The values of A are obtained as 14.94 for plain solder joint and 2.5 for composite solder joint according to In A achieved by binary linear regression,respectively.Therefore,The tensile creep constitutive equations of plain solder and the composite solder joints are written as(?)and(?),respectively.The tensile creep mechanism of the solder joints is the effects of lattice diffusion determined by dislocation climbing.