Mechanics of Copper Wire Bond Failure due to Thermal Fatigue

Thermal stresses occur on wire Bonds due to mismatch of coefficient of thermal expansion (CTE) between wire and mold compound in addition to the global deformation of the package. This leads to different failure modes (different location of crack in wire bond) based on material and geometrical characteristics of package, ultimately leading to an open circuit. Such failures have been observed and reported widely in literature in the ball bond interface, ball neck region, mid span of wire and at stitch bond; however, little is known about the kind of damage that occurs at these critical regions due to material and geometrical differences in package and wire. Furthermore, different phases of m inter metallic compounds (IMCs) and its thickness that form at the wire bond-bond pad interface alters stress conditions on the wire bond. First part of this study presents a review of existing models to obtain stress in wire Bonds and results of finite element stress analyses from literature. Second part of the work focuses on studying three factors, namely, package type, mold compound material and interfacial changes in wire bond, for understanding types of deformations and stresses that develop on the wire bond. This analysis is aimed at providing vital information about package and materials to aid in optimizing design for extended reliability of wire Bonds.