Fatigue damage in solder joints

Miniaturisation of electronic products is increasingly important for reasons of functionality enhancement and freedom of design. This is realised by integration into silicon and developing miniature packages with high I/O and power dissipation density. Generally this leads to elevated temperatures, not only in the components but also in the solder joints. Elevated temperature cycling leads to fatigue damage propagation eventually resulting in failure of the joint. Numerical tools for the prediction of the fatigue lifetime of the solder joints are needed, not readily available and now under development. These tools will be integrated into simulation software which allows virtual prototyping in the design process of (consumer) electronics. In this paper we look at the behaviour of one single solder bump, which connects an electronic chip (package) to a mother board. Temperature cycling provokes cyclic stresses due to different coefficients of thermal expansion of the combined materials. The cyclic loading results in fatigue damage initiation and propagation in the solder bump. The fatigue damage propagation is simulated with the finite element method by using a smeared crack approach [1]. In every integration point of the mesh the consumed lifetime (cycles) of the material is calculated using the stress state and a damage evolution law. When the damage (consumed life) in a point reaches a certain limit value, a microcrack is introduced. In an iterative procedure more and more new microcracks are initiated while existing cracks are growing in length. The crack growth is associated with a reduction in certain material properties. The simulation procedure will be outlined and effects of different parameter values will be discussed. Simulation results are shown to be mesh independent and compare qualitatively well with experimental results.