Hygro-thermo-mechanical modeling of mixed flip-chip and wire bond stacked die BGA module with molded underfill

Package reliability needs to be considered for the design of mixed flip-chip (FC)-wire bond (WB) stacked die BGA module with molded underfill (MUF). The success of the MUF application depends on its performance in thermal shock (TS) test and pressure cooker test (PCT). Mechanical properties (modulus and adhesion strength) of MUF after post mold cure (PMC), reflow and PCT are measured. Shear strength between die and MUF under various temperature and moisture conditions are also characterized. The results show that reflow process and PCT degrade the material properties and adhesion strength. Hygro-mechanical properties, i.e. coefficient of moisture expansion (CME) and saturated moisture concentration (C_sat), are also measured. Based on the measured mechanical and moisture properties, a combined hygro-mechanical and thermo-mechanical stress modeling is performed on the FC-WB stacked die BGA package to compare three types of MUF materials at various temperatures (-40°C, 25°C, 121°C and 150°C) and PCT condition. It is observed that MUF-D3 material induces the lowest stresses on the die active surface. Die stresses induced by MUF with that of conventional mold compound and underfill materials are also compared. The analysis helps in material selection of MUF to enhance the die and package reliability of BGA module.