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The overall demand to reduce solar energy costs gives a continuous drive to reduce the thickness of silicon wafers. Handling and bowing problems associated with thinner wafers become more and more important, as these can lead to cells cracking and high yield losses. In this paper a discussion of the microstructure and mechanical properties of the aluminium on the rear side of a solar cell is presented. It will be shown that the aluminium back contact has a complex composite-like microstructure, consisting of five main components: 1) the back surface field layer 2) a eutectic layer 3) spherical (3 - 5 Â¿m) hypereutectic Al-Si particles, surrounded by a thin aluminum oxide layer (200 nm); 4) a bismuth-silicate glass matrix; 5) pores (15 vol.%). The Young's modulus of the Al-Si particles is estimated by nanoindentation. These results are used as input parameters for an improved thermomechanical multiscale model of a silicon solar cell.