Time-Dependent Ductility of Electro-Deposited Copper

Abstract

Mechanical ductility property of copper deposits is one of the most important factors necessary to provide a reliable component for the flexible printed circuit boards. Copper deposits are usually fabricated by either an electroless technique or an electrode position process. Among many parameters controlling the deposits quality, it is believed that the current density, bath temperature, the film thickness, annealing temperature and time, and the presence of impurities exhibit particularly strong influences on the mechanical ductility property. In our preliminary studies, it was found that the ductility obtained by the mechanical bulge testing showed a three-stage characteristics in terms of a room temperature annealing; namely (i) initial low ductility regime (from the right-after-copper-deposition until 12 hours aged at room temperature), (ii) transitional rapid increasing ductility regime (from 12 hours to 24 hours), and (iii) the high ductility regime (after 24 hours). In this study, X-ray diffraction analyses of the diffracted line width, the microstrain from which the dislocation density was estimated, residual macrostress, and the scanning electron microscopic observations were conducted and results were correlated with the characteristic three-stage ductility change behavior described above. Results of this study indicate that grain size and residual stress changes are strongly associated with improvement in copper ductility during the room temperature annealing.