Review of Critical Variables Determining Solder Fatigue Lives

Abstract

AbstractThe fatigue of solder joints is a dominant factor limiting the long term reliability of electrical devices, especially when modern surface mounting is employed to attach devices to wiring boards. This paper will review the critical variables which determine the fatigue of solder and solder interconnections for the joints encountered with surface mounted devices. Consideration will first be given to the variables determining the displacement that a joint experiences due to an imposed thermal cycle. This displacement will then be related to strains in the joint. These strains are then correlated to the fatigue life. This final correlation requires not only a knowledge of the magnitude of the strain which is being applied, but also nature of the joint geometry, type of solder employed, stress state, the definition of failure, the frequency of fatigue cycling, the introduction of hold times, the environment (both thermal and gaseous) and the occurrence of metallurgical changes. The importance of these critical variables will be discussed along with an outline of design approaches to predict the fatigue life. Data for tests on eutectic Pb-Sn solder and for a Pb rich solder will be presented and used to illustrate the influence of critical variables on the fatigue life and to illustrate design approaches.