We have used scanning electron microscopy to study the interfacial morphology of the Cu-Sn compounds formed between a eutectic SnPb alloy and Cu at 200°C. A selective etching reveals the three-dimensional morphology of the Cu-Sn compounds. On the solder side, the compounds grow rapidly as big scallops and the interface becomes extremely rough as compared to the Cu side of the interface. In order to understand this rapid and extremely irregular growth of the Cu-Sn compounds, we propose that it is caused by the dissolution of Cu into the liquid solder and the coarsening of the scallop-type compounds by Ostwald ripening. The growth of the Cu-Sn compounds has a serious impact on solder joint rework in electronic packaging.