Aging studies of Cu-Sn intermetallic compounds in annealed surface mount solder joints

Our previous investigation (1995), revealed the formation kinetics and characteristics of copper-tin (Cu-Sn) intermetallic compounds (IMC) in leadless ceramic chip carrier (LCCC) surface mount solder joints during infrared (IR)-reflow soldering. The present study focuses on the solid state growth of the interfacial Cu-Sn IMC in LCCC surface mount solder joints under prolonged annealing at elevated temperature. A thick Cu-Sn IMC layer at the Sn-Pb solder/Cu interface of a surface mount solder joint (which can be achieved by prolonged aging at high temperature or after long term operation of surface mount technology (SMT) electronic assemblies) makes the interface more sensitive to stress and may eventually lead to fatigue failure of all SMT solder joint. The microstructural morphology of the Cu-Sn IMC layer at the solder/Cu pad interface in all annealed LCCC surface mount solder joints is duplex and consists of /spl eta/-phase Cu/sub 6/Sn/sub 5/ and /spl epsi/-phase Cu/sub 5/Sn IMC. Both Cu-Sn LMC phases thicken as the aging time increases. On the other hand, at the interface close to the component metallization, the growth of both the /spl eta/- and /spl epsi/-phase were shown to be suppressed, with more a pronounced effect on /spl epsi/-phase, by Ni originating from the metallization. The mean total layer thickness was found to increase linearly with the square root of aging time and the growth was faster for higher annealing temperature. The activation energy for the growth of interfacial Cu-Sn IMC layers and the pre-exponential factor, D/sub 0/, for diffusion were found to be 1.09 eV and 1.68/spl times/10/sup -4/ m/sup 2//s, respectively, for the 0805 LCCC surface mount solder joint using eutectic Sn-Pb solder. The pad size and quantity of Sn-Pb solder employed in LCCC surface mount solder joints were shown to have little effect on the solid state growth rate of interfacial Cu-Sn IMC layers.