: Dudek, R.; Nylen, M.; Schubert, A.; Michel, B.; Reichl, H.

Abstract
The paper describes theoretical predictions and experimental observations of solder fatigue in different Sn-63Pb-37 solder joints. Experimental characterisation of solder-behaviour is performed by both thermal cycling of Surface Mount (SM) solder joints and mechanical cycling of ring and plug specimen. Detailed studies of the microstructure in solders after temperature cycling as well as after mechanical cycling have shown the same type of microstructural degradation. This degradation can be described by overall coarsening, local coarsening, recrystallisation, crack initiation and propagation. The computational method to assess the cyclic damage of solder is based upon non-linear finite element calculation results. Comparison of calculation and test results have demonstrated better predictive capabilities when the Coffin-Manson criterion takes into account creep strain distribution within the joint and not only its maximum value. It is shown that Plastic Ball Grid Arrays (PBGA) achieve a high solder joint reliability and exhibit no reliability drawbacks when compared to Plastic Quad Flat Packages (PQFP) with a similar pin count. Additionally, solder bump fatigue of underfilled flip chip assemblies is investigated. It is demonstrated that the mechanical stiffness of underfill has a major impact on bump stresses.