Material Factors Influencing Metallic Whisker Growth
Type of DegreeDissertation
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Whiskering refers to the formation of slender, long, metallic filaments, much thinner than a human hair, that grow on a metallic thin film surface. They are readily observed for pure and alloyed zinc (Zn), silver (Ag), cadmium (Cd), indium (In), and tin (Sn) surfaces. The longest reported whisker length is 4.5 mm long but most high-aspect ratio whiskers range from 1-500 µm. The focus of this research is upon Sn whiskers. Sn whiskers pose serious reliability problems for the electronics industry and are known to be the source of failure in a wide range of electronic devices, such as nuclear power facilities, heart pacemakers, commercial satellites, aviation radar, telecommunication equipment, and desktop computers. The problem with whiskering has been recently exacerbated by the worldwide shift to lead (Pb) free electronics and the continuing reduction in electrical contact pitches. A thorough understanding of the growth mechanism of Sn whiskers is urgently needed. Currently, there is no universally accepted model that explains the broad range of observations on whiskering. The goals of this research are: 1) to develop a more detailed understanding of the physical mechanisms leading to the initiation and growth of Sn whiskers and 2) to outline reasonable mitigation strategies that could be followed to reduce or eliminate the problem of Sn whiskers. The major contributions of this work are: • A reliable method for growing Sn whiskers with predictable incubation times has been developed and tested. • A surface oxide is not necessary for whisker growth. • Intermetallic compounds (IMC) are not necessary for whisker growth. • Smoother, not rougher, substrate surfaces promote whisker growth. • Whiskers grow under both compressive and tensile thin film stress states. • Whisker growth increases with externally applied compression and tension forces. • Sn whiskers are composed of pure Sn except for the expected thin, native Sn oxide on their surface. • For Sn on brass, the atom feedstock for whiskers lies within the film exclusively; the brass substrate does not contribute to whisker production. • The volume of film consumed by a metallic whisker is a simple volumetric calculation. • There are likely to be multiple mechanisms of whisker growth depending on the substrate – thin film system. • In general, the thickness of a metallic film does not have an effect on whisker growth qualities.