Thermomechanical fatigue of solder joints in electronic component assemblies for space applications

Failure assessment of electronic component assemblies is crucial for the space industry in order to design reliable modules and to further enhance product lifetimes. Printed board assemblies, i.e. components soldered on printed circuit boards, are exposed to thermal cycles that are responsible for fatigue cracking in solder joints due to the mismatch in thermal expansion coefficients of the constituting elements. The problem is highly complex both to characterize exhibiting wide dispersion in the data due to inherent variability in assembly conditions and materials and also to model due to the thermal viscoplastic nature of the solder joint material and the mixed mode failure process. The objective of this thesis is to determine the impact of material and geometry parameters on the resistance to fatigue cracking of ceramic rectangular end-capped chip component solder joints to both increase the predictive capabilities of integrity assessment schemes and to guide towards more durable designs. A machine learning approach is used to rationalize the set of experimental results which includes some novel 3D tomography characterizations of the damage process occurring during cyclic thermal loading. Crack propagation simulations are performed using a finite element model with a traction-separation law to represent the failure process. The predictions are successfully assessed towards machine learning-processed experimental data, giving trust in the validity of the model. In particular, the high sensitivity of the component characteristic dimensions on thermal ageing reliability is properly captured. Simplified failure indicators are extracted from static simulations, allowing fast predictions but being unable to capture crack growth behaviour. Phenomenological failure power law models are identified based on experimental data and the previously developed cohesive zone model describing the fatigue cracking process. The analysis proves that the fillet and its shape play a primary role in setting the fatigue lifetime of the solder layer.