Application of Nonlinear Fracture Mechanics Parameter Δ T ^* to Estimating Wire-Liftoff Lifetime: Effect of Creep Constitutive Equations

At elevated temperatures, creep strain plays an important role in the wire-liftoff lifetime of power modules. In the present study, we examine the effect of creep constitutive equations on the wire-liftoff lifetime N_f . For this purpose, the Norton-Bailey type constitutive equation is used for transient creep. The wire-liftoff is caused by the repeated time varying thermal stresses which result from time variations of junction temperature. Under such a varying stress condition, the strain hardening rule (SHR) is usually utilized in the stress analysis considering creep strain. The modified strain hardening rule (MSHR) was proposed to predict creep deformation behavior precisely under stress reversal conditions. After inelastic stress analysis considering creep strain by using the SHR or MSHR, a driving force for wire-liftoff, a nonlinear fracture mechanics parameter T*-integral range ΔT*-integral, is calculated from the results of the inelastic stress analysis. The ΔT*-integral values calculated using the MSHR are smaller than those calculated using the SHR. Thus the MSHR provides longer wire-liftoff lifetime than the SHR, because a smaller ΔT*-integral value provides a longer wire-liftoff lifetime. Moreover the Coffin-Manson law based on ΔT* is superior to that based on the inelastic strain range Δεⁱⁿ, because the former can be applied to predicting N_f in wider temperature ranges from the room temperature to elevated temperatures than the latter.