Measuring the Stress Dependence of Nucleation and Growth Processes in Sn Whisker Formation

Sn whiskers/hillocks are believed to form due to stress in the layers, but the dependence on the stress has been difficult to quantify. We therefore used the thermal expansion mismatch between Sn thin films and Si substrates to induce controlled stress by heating. This enables us to measure the average stress in the layer (using wafer curvature) at the same time as we monitor the nucleation rate (using optical microscopy). Scanning electron microscopy of the surface after intervals of heating is also used to quantify the whisker volume as a function of stress and time. The results allow us to determine the dependence of the whisker nucleation rate and the growth rate on the applied stress. They also show that whisker formation is not the dominant mode of plastic strain relaxation in the Sn layer.