With the recent discovery that the addition of copper to thin aluminum films increases the lifetime to electrical failure of the films, a large effort has been made to understand the role of the copper in the process. The present paper describes experiments on various types of thin polycrystalline films designed to understand the solid solution and precipitation effects in the films. It is shown that precipitation is different from the bulk and in turn different from thin‐film work on unsupported single‐crystal films. Possible mechanisms relating copper additions to electromigration lifetime are considered and the effect of total copper distribution in the films is found to be important. Differences in residual gas pressure in the vacuum system can alter precipitation processes.