(Dr. Iwona Jasiuk, advisor)
"Micromechancs-Based Interfacial Stress Analysis and Fracture in Electronic Packaging Assemblies with Heterogeneous Underfill"
The flip-chip technology is of interest in high performance packaging technology. It is a first-level chip to package connection technology, and it is made of three components: chip, encapsulant (underfill and solder bumps) and substrate. The flip chip device undergoes thermal loading during a curing process and its operational life. In this research, we focus on the delamination of the underfill from the passivation layer of a chip. We assumed that all components of the flip chip assembly were linear elastic and isotropic, and that their properties were temperature independent. Our analysis was conducted in the context of the uncoupled plane thermo-elasticity under a plane strain assumption.
We calculated interfacial stresses by the finite element method. For the fracture, we used the J-integral method and this method gave us very good matching results with interfacial stress analysis. Also, we calculated the stress intensity factors using crack surface displacements. Again, the finite element method is used to determine J-integral and crack surface displacements. The underfill is usually a composite material consisting of the polymer matrix and silica particles. We assume that the underfill is a homogeneous material having effective properties of a composite. In the parametric study we consider a range of volume fractions of particles in the underfill. We found that the same trend that the higher volume fraction of particles in the underfill leads to lower interfacial stresses was obtained for bimaterial strip and several three-layer models In fracture analysis, we found that the J-integral and stress intensity factors decreased as volume fraction increased. The interfacial stress analysis was consistent with the fracture in all cases. Then, we modeled the underfill as a composite material consisting of the polymer matrix and silica particles. Interfacial stresses and fracture were studied for several particle arrangements and found the effect of inclusion arrangement in the underfill on stresses and fracture at the chip-underfill interface. We found that both interfacial stress analysis and fracture gave the same trends for all cases including random particle arrangements. The statistics of random particle arrangements were studied.
Finally, we did the experiments to prove the results from the finite element method. The three-layer samples consisting of chip, underfill and substrate were made and we applied thermal loading and measured displacements using the micro-DAC. The results were reasonable comparing the results from the finite element method.