Thermo-mechanical reliability study for 3D package module based on flexible substrate

In this study, a 3D package module based on flexible substrate is proposed. In order to improve the thermo-mechanical reliability of the 3D package module, the simulation of the key manufacture process is more and more important. The key manufacture processes of the 3D substrate package module mainly include 2D chip assembly, under-filling, 3D substrate folding, die-attaching, molding, and ball grid array (BGA) drop and reflow. At the molding process, when molding compound is injected into the 3D package structure, it will contact with other parts of the module. Because the coefficient of thermal expansion (CTE) and Young's modulus of the molding compound vary with temperature during cooling down from molding temperature, and differ from other parts of the package, the effect of molding compound cooling down process on the reliability of 3D package module is necessary to investigate. This paper performs the thermo-mechanical reliability finite element method (FEM) simulation of the molding compound cooling down process from molding temperature 125 °C to room temperature 25 °C to predict the warpage of the 3D package module. In order to conduct the molding compound cooling down simulation correctly, this paper testes Young's modulus of molding compound dependent on time and temperature by using Dynamic Mechanical Analysis (DMA). The simulation result shows that the maximum deformation of the 3D package module is only 6.27 μm, which appears in the corner of the 3D package module. Therefore, the process doesn't lead to warpage of the 3D package module and the molding compound is suitable for the 3D package. Besides, the thermo-mechanical reliability FEM simulation of the 3D package module under thermal cycling (-40/125 °C) is evaluated to find out the stress and strain distribution of the 3D package module, the BGA and to predict the warpage, delamination, die cracking, solder joint cracking, excessive substrate deformation and cracking of the 3D package module. Because the solder is viscoplastic, the Anand constitutive model is applied to represent the nonlinear deformation behavior of solder. The simulation results indicate that the 3D package module has higher thermo-mechanical reliability.