Indentation vs. Uniaxial Power-law Creep of Sn-based Solder Material: A Numerical Assessment

It is crucial to understand the creep behavior of Pb-free solder alloys in electronic packaging. Typical service environments are between 298 and 373K. The thermal mismatch induced stresses acting on solder joints result in extensive rate-dependent plastic deformation. The solder alloy is potentially the weakest component in the electronic package because normal operating temperatures are already above 0.5 of the melting temperate (in K). Characterization of small-volume materials has been primarily relying on the method of indentation. The topic of concern in this study is the relationship between indentation creep and uniaxial power-law creep. Two stages of numerical simulation were involved, the first a uniaxial creep test and the other an indentation test. Both were based on rate-dependent viscoplastic behavior as the input material model for the pure Sn and Sn alloys. The objective is to establish a connection between uniaxial creep and indentation creep. For conical and pyramidal indenters, the indentation strain rate is usually expressed as (ε_I ) ̇=1/h dh/dt , where h is the instantaneous indentation depth and t is time. By using this definition of indentation strain rate, the four materials studied here were found to have stress exponents similar to the uniaxial creep response. This similar stress exponent gives rise to a near parallel strain rate-flow stress curves (on the logarithmic scale) between uniaxial creep and indentation creep. The separation between the curves thus provides the strain rate relationship between the two forms of creep. Relatively consistent ratios between uniaxial and indentation strain rates, between 0.20 to 0.28 was obtained, and can serve as guidance for obtaining uniaxial creep behavior using the indentation technique.Mechanical EngineeringMastersUniversity of New Mexico. Dept. of Mechanical EngineeringShen, Yu-LinTaha, MahmoudTehrani, Mehra