The solid state reactions in powder soldering mixtures of Cu-Sn system

Abstract

This paper describes a method of production of massive samples of intermetallic compounds of Cu-Sn system, namely δ-Cu41Sn11 and ε-Cu3Sn, and the subsequent process of production of powders and solder mixtures from these bulk compounds. Various compositions of powders and fluxes and heat treatment cycles were selected for experimental study of solder mixtures sintering. Influence of various fluxes on the δ-Cu41Sn11 and ε-Cu3Sn intermetallic powder sintering and phase transformation inside the solder mixtures are studied and discussed. Pre-alloyed δ-Cu41Sn11 powder inside solder pastes prepared by mixing this powder and commercial RMA flux do not undergo phase transformation after 1 hour annealing at 350oC and 1 hour annealing at 500oC. Location of peaks on the diffractogram indicates only presence of δ-Cu41Sn11 phase. Instead, the same heat treatment of the solder pastes made on the basis of ortophosphoric acid results in the phase transformations inside δ-Cu41Sn11 powder – formation of the continuous set of Cu-Sn solid solutions with maximum 13,5 wt% Sn. X-ray diffraction analysis of two types of annealed solder pastes, which consist of powder mixtures with composition 25 % - Cu3Sn, 25 % - Cu, 50 % - Cu41Sn11 and different fluxes (commercial RMA flux and ortophosphoric acid) indicates the presence of δ-Cu41Sn11 phase and Cu-Sn solid solution with maximum 13,5 wt% Sn.