The formation of intermetallic layers between copper wires and lead-tin solders is indicative of the strong metallurgical bond required for a durable solder joint Previous studies have shown that these interracial compounds grow at significant rates (up to 1 µm/day at 100°C), and differences in thermal expansion and hardness between the metals and the intermetallics can lead to the mechanical failure of the solder joint during service. Most previous measurements of the compound formation rate have used elevated temperatures to facilitate the direct observation of the intermetallic thickness by optical microscopy, SEM, or TEM. In this study, Rutherford backscattering spectrometry (RBS) has been used to study the intermetallic growth rate using sputter-deposited thin films of Cu and PbSn. The extraordinary depth sensitivity of RBS (~10 nm) allows these measurements to be made in relatively short times, without the need for heat treatment to accelerate the reaction. Measurements have indicated that at least 23 nm of Cu6Sn5 and 30 nm of Cu3Sn form between Cu and PbSn solder in less than one hour. Faster deposition methods are being investigated to further characterize this rapid reaction. Iron has been proposed as a diffusion barrier material to inhibit intermetallic growth in solder joints. Preliminary measurements of the solder/Fe reaction indicates a much slower rate of growth, with less than 50 nm of total intermetallic forming over 8 hours.