Non-linear Finite Element Analysis on Stacked Die Package Subjected to Integrated Vapor-Hygro-Thermal-Mechanical Stress

This paper examines the comprehensive effects of vapor pressure, hygroscopic swelling and thermal expansion on the reliability of stacked die package during soldering reflow by the finite element analysis. First, the moisture concentration distributions were obtained for preconditioning at MSL 1 as well as desorption at the reflow process. For the diffusion modeling, existing normalization approaches are only valid under constant temperature/humidity condition and cannot be extended to the reflow process which contains time-varying thermal loading. In this study, the internal source approach was adopted to overcome the concentration discontinuity at material interfaces during solder reflow. A micromechanics based vapor pressure model was used to predict the vapor pressure field based on the moisture concentration results from the diffusion analysis. Second, a comprehensive nonlinear finite element model was proposed to integrate the effects of vapor pressure, thermo-mechanical stress and hygro-mechanical stress with consideration of the non-uniform moisture distribution and the viscoelastic behavior of mold compound. The individual and integrated effects of vapor pressure, hygroscopic swelling and thermal expansion on warpage and stress distribution were investigated. Finally, a fracture mechanics based approach was used to study the thermal-moisture impact on the delamination of the stacked die package. The transient variations of the strain energy release rate during soldering reflow due to the vapor-hygro-thermal-mechanical stress were evaluated and compared under different crack lengths.