Modelling of the mechanical behaviour of copper in 2nd level interconnection structures

The paper presents an approach to model the mechanical behaviour of copper in 2nd level interconnect structures in electronic assemblies. The discussed mechanical models were analysed in ANSYS and LS-DYNA FEM-Software in order to simulate the performance of typical structural elements in electronic assemblies, such as copper traces or solder pads. Loading conditions span a wide range from low rate deformation of thermal cycles to high rate deformation during drop testing. This study investigated the effect of different mechanical properties of copper, with respect to the stress-strain relationship in 2nd level interconnects. The effect of the anisotropy of Young's modulus, in addition to the effect of isotropic cyclic hardening on the resulting deformation and stresses in the copper structures, were analysed. Furthermore, the resulting contact forces at the copper pad to the solder and to the PCB epoxy material, were investigated. This paper presents specific observations made during the three-dimensional finite element simulations of typical interconnect structures. Microstructural investigations were also carried out, such as to be able to correlate particular mechanical behaviour with established knowledge about copper as an FCC material. Grain sizes and texture of real copper traces are estimated. This study relates these particular features of real structures in electronic assemblies to published properties of copper mono- and polycrystalline materials. The importance of microstructural properties, such as grain size and orientation in terms of their respective influence on the results, are also discussed.