Multi-layered structures with various material combinations are extensively used in electronic packages such as MCM (Multi Chip Module), CSP (Chip Scale Package), WLP (Wafer Level Package), etc. Since the mechanical failure of the electronic packages is closely related to that of the multi-layered structures in the packages, it is important to evaluate the reliability of the structures. The reliability-related issues in electronic packages are interfacial delamination between two dissimilar materials, crack initiation at the interface corner, chip cracking due to package bending, and so on. In this study, the analytical and the experimental techniques for reliability evaluation are reviewed and improved on the basis of interfacial fracture mechanics and thermo-mechanical analysis of multi-layered structures.
For the analysis of interfacial delamination, the well-known complex potential method for interfacial crack problems is applied. To obtain the analytical solutions for interfacial straight and circular cracks in isotropic bimaterials, a method which is simpler than one using the conventional Plemelj formula is proposed. The proposed method can be also applied to solving interfacial straight crack problems in anisotropic bimaterials. To reveal the usefulness of the proposed method, several examples are considered. Moreover, to evaluate T-stresses for an interfacial crack in anisotropic bimaterial, the relation between the J-based mutual integral and the T-stresses is derived explicitly. Some basic problems for interface cracks in anisotropic bimaterials are solved.
Interfacial cracks have several features which are different from those of cracks in homogeneous materials. Among those, the loading mode dependency of interfacial toughness has been a main obstacle to the widespread utilization of interfacial fracture mechanics to interfacial delamination problems. In this study, plasticity-induced toughening of an interface crack between an elastic-plastic material and an elastic material is studied. A useful relationship between the plastic dissipation and the plastic zone size is derived via an effective crack length model. Non-orthogonal stress modes for interface cracks are proposed on the basis of the plastic dissipation mechanism and a mixed-mode criterion for interfacial crack growth is also proposed using these stress modes. The proposed method using the non-orthogonal stress modes surpasses the conventional method in representing the asymmetric behavior, mode-dependent toughening and ε-dependency of interfacial crack growth.
For a fast and simple analysis of the failure mechanisms of multi-layered structure, an analytical model of multi-layered structure is presented on the basis of composite beam theory and edge singularity analysis. Near the interfacial corner of the multi-layered structure, the stress singularity and the stress intensity factor of the corner are analyzed, and then useful design guidelines for reliable systems are presented. Far away from the corner, the behavior of the multi-layered structure is simplified to that of composite beam. Warpage, stress and energy release rate of the structure under mechanical and thermal loads are evaluated. To analyze the thermo-mechanical behavior of the multi-layered structure, the cut and paste procedure of two-layer structure proposed by Suo and Hutchinson is extended. The proposed model can be applied to chip cracking and interfacial corner cracking as well as the interfacial delamination without requiring heavy numerical computations.
To verify the usefulness of the present study, three examples are selected. First, reversible wafer bonding technique for compound semiconductor process is analyzed. The main reliability issue in the reversible wafer bonding technique is wafer cracking due to thermal stress induced by CTE mismatch. The thermal stress distribution of the bonded wafer structure is calculated via the proposed model and the optimum bonding condition which gives minimum thermal stress in the bonded wafer structure is determined. Second, the thermo-mechanical analysis of PCB-based Flip Chip Package is presented. In this package, silicon chip is attached to PCB via ACF (Anisotropic Conductive Film). Warpage, stress singularity, and stress intensity factor of the Flip Chip Package are evaluated for three types of ACF and the best choice of ACF is selected on the basis of failure mechanism. Third, the delamination problem of copper thin film on silicon substrate is dealt with. Simplified multi-layered structure containing copper thin film is considered and the crack driving force for the delamination crack between copper thin film and silicon substrate are evaluated under thermal residual stress.
For the determination of adhesion strength of multi-layered structure, a design of specimen having 3-layered structure together with a loading device is presented. The specimen design has the advantage of requiring relatively simple preparation procedures and giving an accurate analytical model. Moreover, the specimen can be used to measure the adhesion with varying phase angles by controlling thermal stress. The loading device is of 4 point bending type. The interfacial fracture toughness as a measure of adhesion for Si/Cu/Epoxy system is determined experimentally over a range of phase angle using the proposed method.
