Mechanical properties of lead-free solder joints - The size efffect

It has been known for some time that various mechanical properties of solder joints depend critically on their size, especially on the gap to thickness ratio. This can have serious effects on brittleness and ductility of these joints and, as a consequence, on their long time stability. Thus it was the aim of this study to investigate the influence of decreasing gap size on tensile, shear, and stress relaxation behavior of solder joints, and to find possible variations with changes in the solder and/or substrate material. The model solder joints were prepared from Sn-3.5Ag and Sn-10.0In-3.2Ag solders using both Cu and Ni substrates. They were of rectangular form with gap sizes varying from 850 down to 25 Am, and they were obtained by a reflow soldering procedure in order to achieve near-industrial process conditions. The microstructure and the complex fracture and crack propagation modes were characterized by scanning electron microscopy.