(A) study on residual stresses in electroless Ni(P) and Ni(P)/Sn films and peel strengths of Cu/Cr/Polyimide system

Abstract

Electronic packages include various materials such as metals, polymers and ceramics, to perform their electrical and mechanical functions. Resultantly, many interfaces of dissimilar materials exist in the package. Therefore, the reliability related to the interfaces of dissimilar materials is important. This study covered the adhesion strength of bonded dissimilar films and residual stress in multilayered thin films used in electronic packaging. First, the analysis of peel test to estimate the adhesion strength of Cu/Cr/Polyimide system has been conducted. The T peel strengths were measured using a Cu/Cr/polyimide system under varying metal layer thickness and rf plasma pretreatment conditions. Measured peel strength showed reversed camel back shape against the metal layer thickness, which is quite different from the results of the 90° peel test. Elementary analysis suggests that the T peel strength variation is a combined outcome of the plastic bending work of the metal and polymer strips, and when the latter were subtracted from the measured peel strength, interfacial fracture energies, quite independent of the metal layer thickness but increasing with the rf plasma power density were procured. Another interesting outcome is that the peel angle, the angle between the unpeeled ligament and the load axis normal, can be actually predicted and is a useful measure of the root curvature and the interfacial fracture energy. Secondly, the residual stresses in electroless plated Ni(P) films were investigated. Electroless plated Ni(P) films have been widely used as under-bump-metallurgy in microelectronic packaging, however high tensile stresses in the films often cause reliability concerns. Here, Ni(P) films with varying P content were made by changing the pH of the plating solution, and the residual stresses in electroless Ni(P) films in as-prepared samples were all tensile regardless of the P content, but tended to decrease in magnitude with increasing P content. T...