Abstract
During reflow soldering, an intermetallic compound layer forms when tin in a molten solder alloy reacts with a copper substrate. In addition, the intermetallic compound dissolves from the layer into the moltne solder until the solder approaches Cu saturation. Experimental studies using 62 tin-36 lead-2 silver solder on copper substrates indicate that both layer growth and dissolution occur for the initial 2 to 4 minutes of reflow. Intermetallic growth rate increases when the solder is saturated with Cu. Under saturated conditions, layer growth follows a (time) exp 0.25 dependence with an activation energy of 9 KJoule per mole. During reflow accompanied by dissolution (non-saturated solder), layer growth follows a (time) exp 0.37 dependence with an apparent activation energy of 8 KJoule per mole. Results from dissolution rate experiments fit the Nernst-Brunner model for dissolution. Activation energy for dissolution is approx25 KJoule per mole. A complete analysis of intermetallic layer formation must treat layer growth and dissolution as interrelated phenomena which may occur simultaneously. For predicting net layer growth in real systems the growth exponent and dissolution rate will be a function of: (1) growth kinetics, (2) dissolution kinetics and (3) solder volume and substrate area. The Cu sub 6 Sn sub 5 layer exhibits a scalloped morphology. This morphology is observed whether the interface is growing, dissolving or relatively stable. Therefore, it is likely the result of a grain boundary wetting phenomenon. As layer growth progresses there is a concurrent coarsening of the scallops. Under combined growth/dissolution conditions this can be explained in terms of a competitive growth mechanism at the advancing intermetallic-Cu interface.