Strain relaxation has been measured in Pb-alloy films on Si substrates during cooling to 4.2K. The amount of strain relaxed was found to be strongly dependent on the film thickness h increasing from about 0.2 to 0.6 % as h was increased from 0.2 to 1 µm. The failure of Josephson tunnel junctions made with similar Pb-alloy films was shown to occur by the formation of a microbridge of about 1000 Å size through the tunnel barrier. It is proposed that the microbridge forms due to strain relaxation in the junction electrodes