|M.Sc Student||Strasser Dan|
|Subject||Binding Layers for Delaying Damage in Electronic Boards|
|Department||Department of Mechanical Engineering||Supervisor||Professor David Elata|
This dissertation concerns the efficiency of epoxy glue as a damage delaying underfill for CBGA (Ceramic Ball Grid Array) electronic chips. The work is theoretic and is based on finite elements calculations carried out on different models using ANSYS. The purpose was to prove that epoxy glue used as an underfill between electronic chips and PCB (Printed Circuit Board) will improve the life expectancy of the electronic chip, since it will reduce the stress on the solder paste and will also reduce the plastic strain gained due to CTE (Coefficient of Thermal Expansion) mismatch. Models with large amounts of elements were avoided by referring to the chip on board assembly as a three-layer model in which the medium layer represents the ball, the solder paste and the glue, whether it exists or not. This medium layer was modeled as a composite material and its constants were calculated from the mechanical response of a single CBGA joint to multi-axial loads. The three-layer model was then used to locate the critical joint and to define its displacements. The displacements were then applied to the single CBGA joint as a time-dependent load defining a load cycle. By solving this problem, one can withstand the values of the stress and plastic strain of the solder paste. Substitution of the plastic strain into the DSC (Damage State Concept) damage model obtained the life expectancy of CBGA solders. Comparison of the obtained values from a model with underfill and one without underfill showed that the epoxy glue improves the life expectancy of CBGA interconnections.