Experimental investigation and numerical modelling of microstructure evolution as damage initiation indicator in SnAgCu solder joints under thermomechanical loading
Date
2024
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Stuttgart : Materialprüfungsanstalt (MPA), Universität Stuttgart
Abstract
Automotive electronics are during their operational loads imposed to harsh environmental conditions, which put a challenge for reliability considerations of electronic control units. The technology trends of autonomous, connected driving and powertrains electrification results in an increased number of electronic components, highly impacting the product designs and reliability requirements. Lifetime prediction methods allowing to tailor reliable and robust designs are therefore of great importance. The reliability prediction of solder joint connections subjected to cyclic thermally-induced mechanical load is a popular research topic in the electronic packaging community, with numerous deformation and lifetime models proposed in the last 20 years. Such a method is often supported by numerical prediction of deformation, and Coffin-Manson-like relation is used for prognosis of a lifetime based on creep or viscoplastic strain extracted from simulated stabilised temperature cycles. The material models are typically calibrated based on isothermal tests and used for the prediction of complex deformation behaviour under variable temperatures. Different fraction of the research community focuses on understanding the physics behind solder joint failure, where numerous studies reveal that under thermo-mechanical load, cracks are typically observed in finely grained regions, even though at as reflowed solder state only a few coarse grains are present. This indicates that during cyclic loading history, a substantial evolution of solder microstructure is a precursor to damage initiation and propagation.
The limitations of the above-mentioned aspects of the lifetime prediction of solder joint connections are the focus of this work. The microstructure evolution is recognised as an essential part of solder deformation history, which determines the location and time point of damage initiation and propagation. For the description of grain size evolution in the viscoplasticity framework, a numerical modelling approach is suggested. The parameter identification of the recrystallisation model is based on multiple low-cycle fatigue tests, allowing intermediate steps-based quantification of grain size during loading history. This study reveals grain size saturation values, which are compared with quantified grain sizes from micrographs of BGA solder connections subjected to different load modes and temperature conditions. The indication of a saturated grain size allows to define its absolute value as an indicator of damage onset.