An investigation was undertaken of the role of intermetallics in determining the strength of soldered joints, using copper and a lead-tin solder as the materials of interest. Joints formed at various times and temperatures were tested in shear, and measurements of the thickness of the two intervening intermetallic layers formed were taken. The temperature of formation was found to have the greatest effect on the strength, with the strength decreasing as the temperature increased. Failure at the interface occurred in two stages. In all cases, the first stage consisted of fracture within the intermetallic layer, accounting for partial separation of the interface only. The area of the interface showing this kind of behaviour increased with both time and temperature. Final stages of failure involved shear within the solder immediately above the intermetallic layers. The two intermetallic phases found at the interface were determined by X-ray diffraction to be Cu6Sn5 and Cu3Sn. Both intermetallics were found for all joint-forming conditions investigated. The growth of Cu6Sn5 reached a limiting value for times longer than 30min, but increased in thickness with temperature. Cu3Sn also increased in thickness with temperature, but showed linear increases with time; the rate of increase was greater for the higher temperatures.