Characterization and Modeling of Intermetallic Phases Formed During Soldering of Sn-Pb-Ag Alloy on Copper Substrates

In solder-copper diffusion couples an intermetallic compound layer forms when tin in the solder alloy reacts with the copper substrate. During reflow soldering, the intermetallic compound may also dissolve from the layer into the molten solder. Experimental studies using 62% tin-36% lead-2% silver solder on copper substrates were done in order to evaluate the growth and dissolution behavior of these intermetallic layers under reflow conditions and during solid state aging. For isothermal reflow experiments, intermetallic layer growth followed a power law dependence on time, (time)$\rm\sp{n},$ with a growth exponent of n = 0.31 to 0.37. The influence of dissolution was demonstrated by comparing the above results to experiments which used Cu saturated solder. Growth rate increased and the growth exponent changed to n = 0.25 when Cu saturated solder was used. Numerical results and metallographic observations were in agreement with a proposed growth model which is based on two assumptions: (1) the rate limiting mechanism is Sn diffusion through the layer via grain boundaries and (2) grain boundary density decreases over time due to coarsening. This model predicts a growth exponent of formula Analyzing growth data according to this model indicated an activation energy for grain boundary diffusion of approximately 25 kJoule/mole. Copper dissolution during reflow was measured. Data follow reasonably well the Nernst-Brunner model for dissolution, however, results were somewhat inconsistent. Numerical and morphological evidence suggest that the surface reaction was the rate limiting mechanism for dissolution. An estimate for the activation energy for dissolution is 8 to 19 kJoule/mole. Solid state aging experiments exhibited a continuous increase in growth exponent with increasing initial layer thickness from n = 0.33 for thin layers (initial thickness of 1 micron) to n = 0.5 for thicker layers (10 microns). The change in growth kinetics was related to the development of a Pb rich layer adjacent to the intermetallic. Initially, diffusion through the intermetallic layer may be rate limiting. After a continuous Pb layer develops, diffusion of Sn through the Pb-phase may be the rate limiting mechanism for intermetallic growth