An intermetallic compound which grows at a rate greater than 1 μm/s has been observed in the liquid/solid reaction at 250°C between molten eutectic SnPb solder and solid Pd. The intermetallic PdSn3 does not form as a diffusion barrier between the reactants, rather it grows as lamellae into the molten solder. The growth direction is normal to the liquid/solid interface, where the molten solder between the lamellae serves as fast diffusion channels during the reaction. For comparison, molten Sn (Pb-free) reacts with Pd at a rate slower by one order of magnitude. The intermetallics, in this case, grow as a diffusion barrier between the Sn and Pd. The role of Pb in the extremely fast eutectic soldering reaction is discussed, in view of the trend of searching for a Pb-free solder in electronic packaging.