Kinetic analysis of the soldering reaction between eutectic SnPb alloy and Cu accompanied by ripening

The wetting reaction of molten eutectic SnPb on Cu leads to Cu-Sn intermetallic compound formation, but not compounds of Cu-Pb since they do not exist. Cu-Sn compounds do not form layered structures between the solder and Cu, rather the ${\mathrm{Cu}}_6$ ${\mathrm{Sn}}_5$ phase has grown as scalloplike grains into the molten solder. The scalloplike grains grow larger but fewer with time, indicating that a ripening reaction has occurred among them. However, the ripening is not a constant volume process since it is accompanied by the soldering reaction. Between the scallop grains, there are molten solder channels extending nearly all the way to the Cu interface. In aging, these channels serve as fast diffusion and dissolution paths of Cu in the solder to feed the reaction. A kinetic analysis of the soldering reaction accompanied by the ripening reaction is given. In the analysis, the growth of the scalloplike grains is supplied by two fluxes: the flux of interfacial reaction and the flux of ripening. The latter was formulated by the Gibbs-Thomson equation. The former was obtained by measuring the rate of consumption of Cu in the reaction. The measurement was carried out by determining the change of the total volume of scalloplike grains (and in turn, the Cu content in the grains) as a function of time and temperature. A reasonable agreement has been obtained between the calculated scallop-grain growth based on kinetic analysis and the experimentally measured values. A discussion of the Cu consumption rate is given, since it is important for applications in electronic packaging technology. © 1996 The American Physical Society.