Interfacial morphology and concentration profile in the unleaded solder/Cu joint assembly

A joint assembly of lead-free solder/intermetallic layers/copper was prepared by hot-dipped solder coated on a copper substrate and then by thermal ageing at 100, 725, 150, and 170 degrees C for 50, 100, 200, and 600 h, respectively. Results of interfacial morphologies and concentration profiles on the solder/copper joint were presented. Optical and scanning electron microscope (OM and SEM) were used to measure the thickness of intermetallic layers and then to illucidate the development of microstructure at the joint assembly. The phases of intermetallic compound were identified to be Cu3Sn and Cu6Sn5 by both X-ray mapping in electron probe micro-analysis (EPMA), and X-ray diffraction. The intermetallic layers, subtracted from the initial thickness formed by hot dipping, showed a linear dependence on the square root of ageing time at various ageing temperatures. The diffusion coefficients of intermetallic compounds are estimated by an Arrhenius equation, and the preexponential terms of Cu3Sn layer and Cu6Sn5 layer are 7.10 x 10(-7) cm(2) s(-1) and 6.1 x 10(-3) cm(2) s(-1), respectively. The associated activation energies of Cu3Sn layer and Cu6Sn5 layer are 57.03 kJ mol(-1) and 83.76 kJ mol(-1), respectively. A model of a diffusion-controlled mechanism is used to fit the concentration profiles of the joint assembly, and exhibits a fairly good quantitative agreement with the measured data. The initial thickness formed as soldering proceeds is also taken into account to evaluate the apparent thickness by introducing a term of corrected ageing time. (C) 1999 Kluwer Academic Publishers.