Tin whiskers in electronic packaging : mechanism and modeling.

Thesis (Ph. D.)--University of Rochester. Dept. of Mechanical Engineering, Materials Science Program, 2011.Tin whisker is an important issue in the electronic industry. The needle-like single crystal growing from thin tin film may short the circuit causing the electronic device to fail. This PhD research utilized a self-designed fixture to apply calculable and controllable compressive stress across the tin film which can drive tin whisker growth. A high temperature annealing in a vacuum atmosphere accelerated whisker growth, greatly shortening each experimental period to one week. A series of quantitative methodology combining MATLAB programming and Surface Mapping facility was developed to analyze experimental images and data which would describe the whisker growth in an accurate way. Multiple tools such as SEM, OM, TEM, EDS, FIB were used to analyze tin whiskers and tin films. The main purpose of this research is to identify a mechanism to explain the whisker initiation and development. Therefore, various environmental factors such as stress, thickness of tin film, time and underlayers were examined in order to find out how the whiskers grew. The relationship of whisker volume with stress was determined, which yielded to an inverse impression creep model. The thickness of the tin film affected whisker growth in a way that the thicker tin film might restrain whisker growth. The whisker growth appeared a rising trend with the increasing of temperature. The development of tin whiskers with time could be separated as four stages: nucleation, growth, slow growth and saturation. Huge tin hillocks grew from the film coated with a nickel/chromium underlayer. Thorough analysis identified that the tin atom transportation path was the interface between the tin film and the substrate. Quantitative analysis determined a big mass migration of tin atoms which could be explained by a form of fluid flow. A mysterious phenomenon--hollow whiskers accompanying tin depleted areas were found on the tin film which had a chromium underlayer. The hollow whiskers and depleted areas were further examined using a focused ion beam. Most of these whiskers/hillocks were identified as single crystals, though some of the hollow whiskers were amorphous due to a mixture of chromium inside the tin whisker. The results confirmed the mechanism of tin atoms interfacial fluid flow. Needle whiskers appeared on the tin film without any underlayer. The study on different underlayers further proved that the interface between tin and underlayers played a key role in whisker growth