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Mechanical behaviour and reliability of Sn3.8AgO.7Cu solder for a surface mount assembly

By Pradeep Hegde


A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.The demands for compact, light weight and Iow cost electronic products have resulted\ud in the miniaturisation of solder interconnects to a sub-millimetre scale. With such a\ud reduction in size, the solder joints cannot be assumed to behave in the same way as\ud bulk solder in terms of reliability due to the fact that their material behaviours are\ud influenced by the joint size and microstructure. The complexity of their reliability\ud assessment is furthermore compounded by the demand for the replacement of\ud traditional SnPb solder alloys with lead-free alloys, due to the presence of the toxic\ud and health hazardous element (Pb) in the former alloy. However, these new lead-free\ud alloys have much less history of industrial applications, and their material and\ud reliability data is not as well developed as traditional lead-based alloys. In addition,\ud most previous reliability assessments using finite element analysis have assumed a\ud uniform distribution of temperature within the electronic assembly, which conflicts the\ud actual temperature conditions during circuit operation. Therefore, this research was\ud undertaken to analyse the effect of solder joint size on solder material properties from\ud which material models were developed, and to determine the effect of an actual (nonuniform)\ud temperature distribution in an electronic assembly on the reliability of its\ud solder joints. Following a review of lead-free solders and potential lead-free alloys,\ud lead-free solder microstructures, and the reliability issues and factors affecting the\ud reliability of solder joints, the practical aspects of this research were carried out in two\ud main parts.\ud The first part consisted of substantial work on the experimental determination of the\ud temperature distribution in a typical surface mount chip resistor assembly for power\ud cycling conditions, and the stress-strain and creep behaviour for both Sn3.8AgO.7Cu\ud solder joints and reflowed bulk solder. This also included building material models\ud based on the experimental data for the solder joints tested and comparison with that for\ud bulk solder. Based on the comparison of the material properties, two extreme material\ud models were selected for the reliability study. Size and microstructure effects on the\ud solder material properties were also discussed in this part.\ud The second part comprised of extensive finite element analysis of a surface mount\ud chip resistor assembly and reliability assessment of its solder joints. The simulation\ud began with elasto-plastic analysis for 2D and 3D chip resistor assemblies to decide\ud upon the kind of formulation to be used when the full complexity of both plasticity\ud and creep is considered. The simulation was carried out considering the determined\ud non-uniform temperature distribution and idealized or traditional uniform temperature\ud condition. The solder joint's material properties were modelled using the two material\ud models determined from the experimental results. The effect of temperature\ud distribution during thermal cycling and of the selected material models on the solder\ud joint reliability was demonstrated using finite element analysis and subsequent fatigue\ud life estimation.\ud In summary, this research has concluded that the material behaviour of the solder joint\ud is different from that of bulk solder due to the effect of its size and microstructure. The\ud anisotropic behaviour of the solder joint cannot be ignored in reliability studies, since\ud it has a significant effect on the solder joint's fatigue life. The research also showed\ud the significant effect of an actual (non-uniform) temperature distribution in the\ud electronic assembly on the solder joint fatigue life

Publisher: © Pradeep Hegde
Year: 2008
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