Living organisms are separated from the outside world by their membranes, which perform important roles for maintaining their lives. Sterols are indispensable com- ponents in eukaryotic organisms for stabilizing membranes composed of phospho- lipid bilayers. Most prokaryotic organisms, on the other hand, do not contain sterols in their membranes, although recent studies indicated that the bacterium Methylo- coccus capsulatus produces sterols 1) and many strains of Mycoplasma, bacteria without cell walls, require sterols as an essential growth factor 2). Some bacterial species synthesize hopanoids which are hypothesized to function like st~rols in eu- karyotic organisms 3). Significance of ergosterol in fungi has been recognized from the fact that the yeast Saccharomyces cerevisiae requires ergosterol for growth, when cultured in a strictly anaerobic condition "'. l:.lectron microscopic studies showed that membrane structures disappeared in the yeast grown anaerobically without a supplement of ergosterol 5.6), indicating the necessity of ergosterol for membrane biogenesis. Further confirmation of the significant role of ergosterol in the membranes was derived from the studies on polyene antibiotics. In the presence of polyenes such as nystatin, filipin and amphotericin B, fungal cells lose selective permeability of the membranes, since polyenes interact with ergosterol, such that the structure of the fungal membranes is disrupted 7).

Discusses the growth mechanisms of tin whiskers and the effective mitigation strategies necessary to reduce whisker growth risks This book covers key tin whisker topics, ranging from fundamental science to practical mitigation strategies. The text begins with a review of the characteristic properties of local microstructures around whisker and hillock grains to identify why these particular grains and locations become predisposed to forming whiskers and hillocks. The book discusses the basic properties of tin-based alloy finishes and the effects of various alloying elements on whisker formation, with a focus on potential mechanisms for whisker suppression or enhancement for each element. Tin whisker risk mitigation strategies for each tier of the supply chain for high reliability electronic systems are also described. * Discusses whisker formation factors including surface grain geometry, crystallographic orientation-dependent surface grain boundary structure, and the localization of elastic strain/strain energy density distribution * Examines how whiskers and hillocks evolve in time through real-time studies of whisker growth with the scanning electron microscope/focused ion beaming milling (SEM/FIB) * Covers characterization methods of tin and tin-based alloy finishes such as transmission electron microscopy (TEM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) * Reviews theories of mechanically-induced tin whiskers with case studies using pure tin and other lead-free finishes shown to evaluate the pressure-induced tin whiskers Mitigating Tin Whisker Risks: Theory and Practice is intended for the broader electronic packaging and manufacturing community including: manufacturing engineers, packaging development engineers, as well as engineers and researchers in high reliability industries.