A Joint-Scale Test Specimen for Tensile Properties of Solder Alloys

As modern electronic devices continue to get smaller and smaller, the size of the solder interconnects is also getting smaller. As well documented in the literature, mechanical properties of Pb-free Sn-rich solder alloys such as SnAgCu (SAC) display strong length-scale effects due to (i) their coarse-grained structure and the strong anisotropy of Sn grains; and (ii) the complex dendritic structure of the β-Sn and the interspersed eutectic Ag-Sn eutectic material. Consequently, the mechanical behavior of SAC solder alloys at the joint-scale differs significantly from that measured from bulk test specimens. The consequence of the length-scale effects described above is that accurate evaluation of material properties used for reliability estimation of such miniature devices should also be done using joint-scale specimens. Since functional solder joints experience a multiaxial state of stress, the test program should address both tensile and shear properties. This paper addresses the measurement of tensile behavior and the measurement of shear behavior is addressed elsewhere in the literature. The overall design of the tensile specimen geometry is a modified joint-scale version of the typical dog-bone shaped tensile test specimens used for testing tensile properties of homogenous isotropic bulk materials. The effect of stress triaxiality (caused by interactions between the solder and the substrate in joint-scale specimens) on the monotonic tensile behavior of the grain-scale SAC305 test specimens are investigated and compared to the behavior of conventional dog-bone specimens reported in the literature. Based on these observations, a hybrid inverse methodology, based on combined testing and FEA, is proposed for extracting accurate uniaxial tensile properties of SAC305 using joint scale tensile modified dog-bone test specimen. The proposed test method is believed to be effective for replicating the grain-scale behavior and the grain-boundary interactions, which exist in functional solder joints. Some of these effects would be missed in bulk homogeneous specimens.