| The spontaneous growth of tin whiskers from the tin surface of copper-tin bimetal layers is thought to be driven by the stress resulting from the formation of interfacial reaction products between copper and tin. Systematic experiments were conducted on clean, vapor-deposited bimetallic copper-tin layers as well as electrodeposited films to (i) characterize the intermetallic growth kinetics, (ii) measure the stress evolution with time, and (iii) identify possible mechanisms to transfer the stress to the tin surface. A suite of experimental techniques was used in the investigation including an in situ optical technique to measure stress evolution in the bimetallic structure, X-ray diffraction (XRD), mass change measurements, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to monitor intermetallic growth kinetics, and TEM of the bilayer cross-section to obtain important insights into microstructural evolution and operating deformation mechanisms in the tin layer.; Stress measurements using the in situ optical technique confirmed the tin to be in a state of compression, the compressive stress evolving with time and reaching a constant level. The saturation of stress in the tin film is interpreted as corresponding to the onset of plastic deformation and dynamic recovery, a response expected of a pure metal loaded at 0.6T m at quasi-static strain rates. The intermetallic, Cu 6Sn5, forms at the Cu-Sn interface and grows into the tin, exhibiting preferential growth at the tin triple junctions and grain boundaries, and at the copper triple junctions and grain boundaries that intersect the tin layer as well. The growth is initially interface-controlled and, upon reaching a critical particle size, becomes diffusion-controlled.; Cross-section TEM confirmed dislocation emission from the intermetallic/tin interface and the rearrangement of these dislocations to generate subgrain boundaries by glide and climb, the latter being accomplished by point defect generation to accommodate misfit stresses and their subsequent migration to dislocations. Dislocation pile-ups at the tin oxide, subgrain rotation, as well as subgrain boundary intersection with the tin surface offer means to transmit stress to the tin surface and provide a plausible mechanism for cracking the oxide layer, a feature that is believed necessary for whisker nucleation and growth. |
