Creep life assessment of tin based lead free solders based on the imaginary initial strain rate

The creep properties of tin based lead free solders which are Sn3.0Ag0.5Cu and Sn7.5Zn3.0Bi were investigated at temperatures between 298K and 398K. The creep rupture time decreases with increasing the initial stress and the temperature. The analysis of creep curves of the tin based lead free solders apply the Omega method. The creep rate epsivdot is expressed by following formula : ln epsivdot = ln epsivdot₀ + Omegaepsiv, where epsivdot₀ and Omega, are experimentally determined. The parameter epsivdot₀ which is the imaginary initial strain rate increases with increasing the initial stress and the temperature. The parameter Omega is temperature dependent but less dependent of initial stress. The activation energy for epsivdot₀ is 108 kJ/mol at Sn3.0Ag0.5Cu and 83 kJ/mol at Sn7.5Zn3.0Bi. The energies suggest that the lattice diffusion of tin is dominant at creep deformation. The creep rupture time is calculated using the parameters, epsivdot₀ and Omega. The calculated creep rupture time is good agreement with the measured time