The Microstructure and State of Stress of Sn Thin Films after Post-Plating Annealing: An Explanation for the Suppression of Whisker Formation?

The post-plating annealing (´post-bake´ treatment, annealing at 150degC for 1 h) of pure matte Sn thin films on Cu-based substrates is known to be an effective whisker mitigation treatment. The microstructure and the stress state of Sn thin films on Cu-substrates produced with and without a ´post-bake´ treatment have been investigated using scanning electron and focused ion beam microscopy (SEM & FIB), X-ray photoelectron spectroscopy (XPS) and ageing time and depth-dependent X-ray diffraction (XRD) stress analyses. A comparison of the measured stress-depth gradients in the near-surface regions of the Sn thin films produced with and without ´post-bake´ treatment indicates differences that might provide a plausible explanation for the observed long-term resistance against whisker growth of Sn thin films subjected to a ´post-bake´ treatment. In specimens subjected to a ´post-bake´ treatment, an almost depth-independent tensile stress state prevails in the surface region even after prolonged ageing.