| The Sn-58 Bi lead-free solder is widely used in the electronic packaging industry due to its low melting point,good spreading performance and good storage stability.However,the application of Sn-Bi lead-free solder is limited by the coarse and irregular IMC layers and brittle Bi phase.In order to improve the reliability of Sn-58 Bi solder,it is an effective way to add particles.In this investigation,the composite solder was prepared by adding Mo(0,0.25,0.50,1.00,2.00 wt.%)into Sn-58 Bi eutectic solder.The effects of Mo particles on the wettability,microstructure,IMC layer and mechanical properties(tensile properties,shear properties,fracture morphology)of Sn-58 Bi eutectic solder joint were investigated and the optimum amount of Mo was obtained by the Sn-58Bi-0.25 Mo composite solder.The influence of aging time,aging temperature,thermal cycling times and reflow time on the microstructural evolution,IMC growth behavior and the reliability of Sn-58 Bi solder joint and Sn-58Bi-0.25 Mo solder joint was studied.The growth kinetics equation of the IMCs layer that established during the aging treatment was studied,and the enhancing mechanism of the reliability of Sn-58 Bi solder joint by adding Mo nanoparticles was explored.The microstructure,wettability and mechanical properties of Sn-58Bi-xMo solder joints(x=0-2.00 wt.%)were investigated.The result shows that the wettability of Sn-58 Bi solder was improved by adding Mo nanoparticles,and the most refined microstructure and appropriate IMCs layer thickness were obtained by Sn-58Bi-0.25 Mo solder joints.The tensile properties and shear properties of Sn-58 Bi solder joints were improved by adding 0.25 wt.% Mo particles,and the tensile strength,elongation and shear strength of Sn-58 Bi solder joints by adding Mo showed the trend of first rise and then decline.The tensile fracture of Sn-58 Bi solder joint showed a step-like morphology and the fracture mode was brittle fracture.The tensile fracture of Sn-58Bi-0.25 Mo solder joints surface is not smooth,and a small amount of tear marks was found and the fracture mode is the combination of ductile fracture and brittle fracture.The influence of aging time,aging temperature,thermal cycling time and reflow time on the microstructural evolution and IMC growth behaviorof Sn-58Bi-xMo solder joints(x=0,0.25 wt.%)was investigated.The result showed that the microstructure of Sn-58 Bi and Sn-58Bi-0.25 Mo solder joints was coarsened and the IMC layer thickness of the solder joint was increased with increasing aging time and temperature,and a relatively flat and stable IMCs layer of Sn-58 Bi solder joint was obtained by adding Mo.With increasing times of thermal cycling,the microstructure of the solder joints was coarsened and bulk Bi phase was enriched.The IMCs layer thickness of the two solder joints was increased,but the IMC layer of Sn-58Bi-0.25 Mosolder joint was directly transformed into a relatively continuous and flat structure.The longer the reflow time was,the coarser the microstructure of the solder joints was,and the IMCs layer of Sn-58Bi-0.25 Mo solder jointwas changed to partially acicular and serrated structure.Compared with Sn-58 Bi solder joints under the same aging time,aging temperature,thermal cycling time and reflow time,the microstructure of Sn-58Bi-0.25 Mo solder joints was more refined,amd the trend of microstructural coarseness of Sn-58Bi-0.25 Mo solder joint is smaller.The IMC morphologies of Sn-58Bi-0.25 Mo solder joint is more beneficial and the IMCs layer growed more slowly.The influence of aging time,aging temperature,thermal cycling and reflow time on the reliability of Sn-58Bi-xMo solder joints(x=0,0.25 wt.%)was investigated.The result showed that the tensile strength and shear strength of Sn-58 Bi solder and Sn-58Bi-0.25 Mo solder joint were decreased with increasing aging time,aging temperature,thermal cycling time and reflow time,and under the same conditions the tensile strength and shear strength of Sn-58Bi-0.25 Mo solder joints were higher,and the trend of the decrease of the tensile and shear strength is smaller than that of Sn-58 Bi.Compared with the tensile fracture of Sn-58 Bi solder joint under the same condition,the fracture surface of Sn-58Bi-0.25 Mo solder joint is more rough and uneven,and no cavity werefound from the fracture surface,and the better mechanical propertieswere obtained by Sn-58Bi-0.25 Mo solder joint with increasing aging time,aging temperature,thermal cycling time and reflow time.The IMC growth behavior and the strengthening mechanism of enhanced reliability of Sn-58 Bi solder joint by adding Mo nanoparticles were investigated.The result showed that aging temperature is the key factor that causes the excessive growth of IMCs layer and the excessive IMCs layer growth of Sn-58 Bi solder joint is inhibited by adding Mo during the aging treatment.The activation energy for IMCs growth of Sn-58 Bi and Sn-58Bi-0.25 Mo solder joints is 48.94 k J/mol and 53.79 k J/mol.The reliability of the Sn-58 Bi solder joint isimproved by adding Mo nanoparticles mainly through the mechanisms of fine grain strengthening,Orowan strengthening,thermal mismatch strengthening and IMCs strengthening. |
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