Effect Of Thermomigration On The Microstructure And Mechanical Properties Of Micro-joints

As the continuous increasing of electronic packaging density,the characteristic size of the interconnecting solder joints is decreasing rapidly,resulting in a large temperature gradient established across the solder joints,which can trigger thermomigration effect(TM).TM can cause the evolution of the microstructure of solder joint,inducing the degradation of mechanical properties.This can bring about a new reliability issue.Therefore,research on the evolution of microstructure and the degradation of mechanical properties of micro-solder joints under TM process is significant in electronic packaging industry.In this paper,a series of sandwich structures of Cu/Sn/Cu,Ni/Sn/Ni,Cu/Sn/Ni solder joints with the bump height of 10 ?m were prepared as samples.Firstly,the microstructure evolution of the micro-solder joints was investigated under 110? with temperature gradient of 1200?/cm for 0?200?400?600?800 h,respectively.Secondly,shear tests were conducted to understand how the microstructure evolution influences the mechanical properties for different solder joints.Finally,the shear creep behaviors of the solder joints were explored under temperature gradient of 1200?/cm at 110?.The important conclusions are followed:For the solder joints with Cu substrate as hot end,like Cu/Sn/Cu and Cu/Sn/Ni solder joints,the thickness of the interfacial intermetallic compound(IMC)at the cold end gradually increased,and decreased at the hot end with TM time.When TM time reached 800 h,the whole solder joints were transformed into IMC due to the IMC rapid growth at the cold end.However,for the solder joints with Ni substrate as hot end,like Ni/Sn/Ni and Ni/Sn/Cu solder joints,the evolution of the interfacial IMC was not obvious with TM time.Both end of the interfacial IMC thicken slowly,and the IMC thickness at cold end was a little larger than at hot end.The shear strength of the solder joints with Cu substrate as hot end like Cu/Sn/Cu and Cu/Sn/Ni solder joints increased with TM time due to the size effect and interfacial constrain.When whole solder joints were fully occupied by IMC,the shear strength of Cu/Sn/Cu and Cu/Sn/Ni decreased due to the weak interconnecting boundary of the interfacial IMC.When Ni substrate acted as hot end,like Ni/Sn/Ni and Ni/Sn/Cu solder joints,the shear strength of the solder joints decreased with TM time and the fracture location was near hot end.It can be explained that the IMC growth was slow not only at the cold end but also at the hot end,and Sn grains near hot end.TM effect accelerated the creep process and shortens the creep life of the solder joints,since the vacancies and interstitial diffusion of Cu and Ni atoms driven by temperature gradient accelerates the creep rate and forms the voids during creep process.Dorn equation was used to calculate the creep stress exponents for different solder joints.The magnitude of the creep stress exponents for different solder joints is showed as Ni/Sn/Ni < Ni(hot)/Sn/Cu < Cu/Sn/Cu < Cu(hot)/Sn/Ni.It is illuminated that the creep life of the solder joints may increase as Ni substrate acts as the hot end,whether the cold end is Cu substrate or Ni.