Importance of Creep Fatigue Interaction in Reliability of Solder Joints

The reliability and the accompanying fatigue law for solder joints are a challenge for the aerospace and defense industries. Severe service loadings and long lifetime requirements generate plastic and viscous deformations even at room temperature, which have to be taken into account in the fatigue design. This paper presents the results of a Design of Experiment performed with an innovative test bench developed for the fatigue analysis of assembled package. The home-maid test bench permits to impose cyclic shear loadings with or without dwell time at different temperatures on the package. Package failure is defined by a 120% increase of daisy-chain electrical resistance for 5 measurements. The influence of several parameters on the number of cycles to failure is presented. A reduction of the number of cycles to failure is demonstrated when (i) increasing applied force (increasing plastic strain), (ii) increasing dwell time (increasing creep strain), (iii) increasing temperature, (iv) and increasing both temperature and dwell time. Moreover, mechanical parameters, such as the plastic displacement and the force relaxation, are monitored during the fatigue test in order to evaluate the damage in the solder joints. The analysis of the experimental campaign gives a precise landscape of the fatigue influence monitored mechanical parameters and provides the quantitative information to define a fatigue law which takes into account creep-fatigue interaction.