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A comparative study of solder fatigue evaluated by microscopic in-situ analysis, on-line resistance measurement and FE calculations

: Dudek, R.; Faust, W.; Vogel, J.; Michel, B.


Ernst, L.J. ; IEEE Components, Packaging, and Manufacturing Technology Society:
Thermal, mechanical and multi-physics simulation and experiments in micro-electronics and micro-systems, EuroSimE 2005 : Proceedings of the 6th International Conference on Thermal, Mechanical and Multi-Physics Simulation and Experiments in Micro-Electronics and Micro-Systems, April 18, 19, 20, 2005, Radisson Hotel, Berlin, Germany
Piscataway, NJ: IEEE Order Department, 2005
ISBN: 0-7803-9062-8
ISBN: 0-7803-9063-6
International Conference on Thermal, Mechanical and Multi-Physics Simulation and Experiments in Micro-Electronics and Micro-Systems (EuroSimE) <6, 2005, Berlin>
Conference Paper
Fraunhofer IZM ()

With the introduction of lead-free solder materials various efforts have been made to characterize their creep behaviour. However, much less is known about their fatigue behaviour, i.e. the local deformations within the joint and the progress of microstructural degradation and cracking. During the course of the European project MEVIPRO a combined numerical-testing methodology has been developed for the evaluation of the fatigue failure mode of tiny material volumes loaded in shear, because no standard testing methodology is applicable. To this end small lap-shear specimens are mounted in a loading frame with slightly different thermal expansion, causing shear loading of the joint material when subjected to thermal loads. In-situ deformation analysis of the joint surface is an integral part of the procedure. This test, denoted as ?Thermal Lap Shear Test?, represents the most important kind of mechanical loadings observed in electronic applications, the thermal mismatch induced load, very well. The microscopic in-situ analysis of the deforming joint surface allows the observation of the local joint deformations at different temperatures, which can be also played as a video sequence to visualise the local movements of the microstructure. The deformations at different temperature and deformation stages can also be analysed by the MicroDAC (grey value image correlation) technique. This provides a good means to adjust the overall joint deformations to those calculated by FE analysis and to compare the local results obtained by the test and the numerical simulation on the displacement field level. The described methodology shows universal application potentials regarding studies on the fatigue induced degradation of solders with spurious alloying elements, electronic adhesives, encapsulants, etc., whereby also the fatigue controlled crack propagation can be studied. The technique was applied to analyze the deformation and failure behaviour of SnPb-, SnAg-, and SnAgCu specimens at different stages of thermo cycle tests. The lap shear specimens were made of ceramic substrates containing solder joints in the thickness range 40-100 microns. The fatigue progress is studied in detail for Sn95.5Ag3.8Cu0.7 solder. Microscopic observations revealed that sliding of the boundaries of the Sn-rich phases is the dominant deformation mechanism. Sliding between Sn-rich phases and larger intermetallic inclusions is also observed. When compared to FE analysis results it is noted that the deformation localizes along the path of local maximum equivalent creep strain, which crosses the joint diagonally, see Fig. 1 and 2. On-line monitoring of the electrical joint resistance can indicate joint cracking by a slight increase of the maximum stationary joint resistance as well as by transient resistance peeks which occur particularly during the beginning of the temperature ramps. However, the stationary resistances are not sensitive to cracking, even for nearly or totally fractured joints. Joint failure prediction by application of Manson-Coffin type criteria using maximum or mean values averaged along the damage path gives reasonable results. The use of the cyclic equivalent creep strain or dissipated creep strain energy density revealed almost the same critical cycle numbers.