Solder Joint Loading Conditions under Torsion Test

More than 10 years ago a so-called mechanical deflection system (MDS) for highly accelerated tests has been developed in order to evaluate the solder joints reliability. The objective was to replace long accelerated thermal cycling (ATC) tests by shorter mechanical ones. This promising concept has not been recognized as an industrial standard test method because of the misunderstanding of solder joint loading conditions. In the field, fatigue failures of solder joints are principally induced by shear stress as a consequence of relative displacement between package and board solder pads. Indeed, any accelerated test should take it into account. Based on this fact, this work is focused on the understanding of stress distribution and on the definition of torsion test conditions to ensure the fact that the solder joints are stressed in shear mode. Three main actions, finite element analysis, dedicated test program and strain gage measurements, have been conducted in parallel to guaranty consistency of data results and model calibration. Firstly, on the board we observed shear strains as the main behaviour and important edge effects for low and high torsion angles values. An area of homogeneous strain was identified: finite element analysis (FEA) and strain measurements were in good agreement. Secondly, on the boards equipped with BGA (ball grid array) package, we analyzed the stress distribution in solder joints. Experimental torsion tests and cross section analyses were made to identify the failure mode. Components placed in the homogeneous strain area showed equivalent failure mode and the observed cracks confirmed the shear stress as fatigue mechanism