Growth kinetics of interfacial Cu-Sn intermetallic (IMC) layer and its effect on the shear strength on practical LCCC surface mount solder joints were studied for isothermal;aging at 70, 120, 155 and 170 degrees C. Only normal Cu6Sn5 (eta-phase) intermetallic was found in the interfacial IMC layer of as-soldered solder joint, whereas the duplex structure of both ii-phase and E-phase Cu,Sn existed in all annealed joints. The growth kinetics of the overall interfacial IMC layer can simply be described by classical kinetic theory for solid-state diffusional growth with an activation energy of 1.09 eV and interdiffusion coefficient of 1.61 x 10(-4) m(2) s(-1). The relatively higher activation energy, as compared with that found for bi-metallic couple of eutectic Sn-Pb solder on copper, is attributed to the dissolution of Ni from the component metallization into the bulk Sn-Pb solder. In addition, the shear fractures in all the solder joints investigated are shown to be ductile in nature and confined in the bulk solder rather than through the interfacial IMC layer. A linear reduction in shear joint strength was observed with an increase in intermetallic layer thickness up to similar to 5.6 mu m. Such a reduction in joint strength is due to a continuous removal of Sn from the bulk solder for the growth of interfacial IMC layer and flattening of the solder/IMC layer during isothermal aging of the solder joint. (C) 1998 Elsevier Science S.A. All rights reserved.
