Analysis of long term reliability of plated-through holes in multilayer interconnection boards Part A: Stress analyses and material characterization

Abstract

The thermomechanical deformation of plated-through-holes (PTHs) in multilayer interconnection boards (MIBs) has been examined so as to develop a suitable test methodology for evaluating the long term reliability of electroless copper PTH structures. The concern over PTH reliability arises from the large Z-direction (through-thickness) thermal expansion coefficient mismatch between the epoxy-glass and copper which generates stresses when the MIB is subjected to a temperature change. Finite element modeling of the PTH is used to calculate the stresses and strains in the copper barrel section of the PTH. By comparing these results for two different PTH idealizations, it was found that the incorporation of an internal copper land into the structure, significantly increases the PTH barrel stress. The maximum stress for both idealizations was found to exist at the midplane of the MIB along the PTH barrel inside diameter. Monotonic and fatigue tests conducted on thin electroless copper foils, coupled with electron microscopy, revealed that electroless copper is an inherently ductile material; the previously reported low ductility of electroless copper is believed to reflect the influence of topographical irregularities in plated foils.