Hygro-thermal Mechanical Reliability Study Of Stacked Die Package

The stacked die chip scale package (SCSP) is one of the3D packages, which is widely used in the area of portable electronics, such as cell phone, digital camera. Although SCSP has many merits, the reliability problems are needed to deeply study. Reliability issues associated with moisture and thermal stress is one of the major reliability concerns for SCSP. Mechanical reliability of epoxy molding compounds in plastic packages of integrated circuits (IC) is greatly affected by the compound ability to absorb moisture. Hygroscopic stress was generated by the compound ability to absorb moisture and thermal stress was developed during the reflow process with a high peak temperature play an important role in the integrity and reliability of plastic packages. Therefore, it is important to understand moisture/thermal distribution and its effect on the reliability of stacked die package. In this paper, the reliability of SCSP under damp&hot environment was researched by finite element analysis (FEA) and experiment.In this thesis, moisture absorption, moisture desorption and solder reflow tests of SCSP were implemented. Moisture diffusion characteristics of SCSP available by experiments have been analyzed and typical moisture diffusion characteristics are calculated. It is found that moisture transport deviations from ideal Fickian behavior in SCSP during the moisture absorption, which is the so-called Non-Fickian diffusion. It was attributed to the "two-stage" sorption. A variety of experimental works were design to study the hygro-thermo-mechanical stress on the interface of die/mold compound and die attachment delamination. The experimental results show that the hygro-thermo-mechanical stress under lead free reflow temperature produce considerable influence to the package integrity after the moisture precondition for168h under85℃/85%RH.By finite element analysis (FEA), the distribution of moisture diffusion, thermo-mechanical stress, hygro-mechanical stress and hygro-thermo-mechanical stress under hygro-thermal environment were simulated and calculated for a2-layer stacked die package. The transient analysis of moisture absorption was performed to estimate the package moisture distribution. After moisture precondition for168h under85℃ /85%RH, it is found that the external mold compound and substrate of the package is almost fully saturated. The simulation results showed that thermo-mechanical stress and hygro-mechanical stress influence on the package. By simulation of hygroscopic swelling stress during reflow process, it was indicated that the critical position for the package reliability located at the die corner of the bottom die and the interface between the bottom die-attach and die. So the reliability of the bottom layers is relatively low under hygro-thermal environment.Finally the hygro-thermal effect of ultra-thin stacked die package was investigated which emphasized on the hygroscopic stress developed by moisture absorption and thermal mismatch stress generated during reflow process by finite element analysis. The results showed that thermo-mechanical stress and hygro-mechanical stress also influence on the package. The optimization design was done for the ultra-thin stacked die package: increasing the chip thickness and reducing the thickness of substrate were helpful to improve the hygro-thermal reliability under certain conditions.