How tin whiskers grow

Late Stone Age metal smiths added a little tin to copper to usher in the eponymous Bronze Age; over the ensuing five millennia, many new combinations and applications of the two metals have appeared. Today, for example, a thin tin coating on a copper substrate often works like solder to interconnect electronic components of various kinds, such as are found in medical devices and satellite equipment. Unfortunately, micron-sized tin whiskers (see figure) sometimes arise spontaneously and can short out the equipment, with great technological and economic repercussions. After decades of widespread effort, the actual mechanism underlying such whisker growth has only now been elucidated. Led by Eric Mittemeijer, a group from the Max Planck Institute for Metals Research in Stuttgart, Germany, working with the Robert Bosch company and Argonne National Laboratory in Illinois, examined growing whiskers and their crystallographic environment. Using Laue diffraction measurements made at Argonne’s Advanced Photon Source, the researchers noted that at the Cu–Sn interface, Cu ₆Sn₅ develops along the tin grain boundaries and is most pronounced directly beneath a whisker’s root. That observation, coupled with residual strain measurements, led the team to propose that deep penetration of Cu ₆Sn₅ into the 3-µm-thick tin layer induces in-plane compressive strains near the Cu-Sn interface and in-plane tensile strains nearer the surface. Out-of-plane and in-plane strain gradients—not the strains themselves—then provide the driving force that leads to whisker growth by transporting Sn atoms to the whisker nucleation site as a strain-relief mechanism. (M. Sobiech et al., Appl. Phys. Lett. 94 , 221901, 2009 http://dx.doi.org/10.1063/1.3147864 .)