Reliability Life Assessment of WLCSP Using Different Creep Models

With the dynamic progress in electronic packaging, possessing a small chip size the Wafer Level Chip Scale Package (WLCSP) is one of the most cost-effective and space-efficient package in the industry. Having dominated the field over a decade, WLCSP still faces a huge issue in developmental phase in form of Solder Joint Reliability as the solder undergoes progressive inelastic deformations over time under constant cyclic Thermo-Mechanical loading. This time dependent deformation is called creep. Constitutive Creep Models have been constantly deployed in finite element modelling in order to eradicate the costly and time consuming experiments such as Accelerated Thermal Cyclic Testing. Various creep models have been proposed so far and Anand Model is considered to be the most prominent one. Also many researchers are concerned about altercations over the negligence of the primary creep in Anand model and the typical prediction of its nine parameters. In this research work, a new hyperbolic Sine form creep model based on the steady state behavior of Anand Model called Modified Anand Model with lesser parameters is developed to have a better characterization of the creep behavior in solders. Assuming the saturated stress condition of Anand Model leads to the derivation of the four parameters meant for the new model. Thus the stress-strain relationship formation of the new model resembles Garofalo-Hyperbolic Sine Model illustrating the steady state creep behavior of the solder. In order to have a better realization of the Hyperbolic Sine Model, simulations are conducted using both the Anand and Hyperbolic Sine Model creep models over a WLCSP under JEDEC-G condition. From the simulation results, it is observed that the resultant strain energy density of the Hyperbolic Sine model has a better agreement than that of Anand Model to the experiment results. The investigation for the cause of variation points out significant findings. It is understood that the hardening effect in solder is due to the stress induced by high temperature creep in Anand Model. Also the presence of a short lived primary creep in Anand model is noticed which occurs as a setback in achieving the steady state creep. By modifying the parameters of the Anand model to neglect the hardening effect, the ability of Hyperbolic Sine Model to predict the lifetime is comparatively better than the Anand model within 10% of the experimental data. Thus we conclude that the uncertainty in determination of the Anand model parameters has brought up the new model to estimate the failure of solders joints and an elegance scheme to compute with precise outcomes.