SMT007 Magazine

SMT-May2015

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10 SMT Magazine • May 2015 Before we delve into the practical questions stated above, let's look at well-established find- ings and observations in the literature, published or unpublished. Studies indicate that whis- kers can be mitigated by controlling the plat- ing process in an equivalent way to controlling stresses in materials. The very sharp decrease in internal stress of tin electrodeposits was observed within minutes after plating. This fast stress release can occur regardless of whether initial stress in the deposit is compressive or tensile. In either case, the value of stress drops to very low numbers, but it remains being of the same type as the initial stress form (i.e., high initial tensile stress re- duces to much lower stress value, but remains tensile; high compressive stress re- mains compressive). It has also been observed that the inclusion of or- ganic elements in tin struc- ture promotes tin whisker growth. Organic inclusion or the level of inclusion is, in turn, affected by the plat- ing chemistry and its con- trol. And the bright tin has exhibited to be most suscep- tible to whisker formation. It is evidential that recrys- tallization and grain growth prior to whisker formation have occurred in bright tin deposit, showing large irregular shape grains as the precursor for whiskers. When comparing between Cu substrate and Ni substrate, Ni substrate tends to retard the whisker formation. This is evidenced by the successful use of Ni barrier layer to miti- gate tin whisker in many incidents, albeit not exhaustively. This phenomenon, related to in- ter-diffusion rate and intermetallic formation, correlates well with the relative diffusion rates between Cu and Sn vs. Ni and Sn. Another ob- servation showed that the external forces exert- ed to tin plating, such as bending, stretching, torque, scratches, nicks, and exacerbated whis- ker growth in the physically stressed region. Still another experiment exhibited that whisker formation involved a shelf time. How- ever, the shelf time varies without a straightfor- ward correlation with temperature, humidity and other environmental condi- tions. Studies showed that a moderately warm tempera- ture served as the 'green- house' that nurtured whis- kers, yet a high tempera- ture (e.g., above 150 o C) in- hibited whisker formation. Furthermore, the highly disparate whisker growth rates have been reported, ranging from 0.03–9 mm/ year. It is noted that whis- kers can grow even in a vacuum environment. Against this backdrop, what is the energy driver (or stress/strain gradient) to grow this whisker-like thing protruding from the surface of a coating or met- al substrate? It is interesting to note the fundamental sciences that separate the mecha- nism between the two most common tin-based materials: solder joint fa- tigue failure vs. tin whisker. The physical and mechani- cal behaviors of solder alloys and solder joints have been un- derstood through microstructure examinations in conjunction with mechanical and physi- cal tests. The evolution of microstructure over time and microstructural changes in response to temperature and other external parameters, both environment and in-circuit conditions (if in electronics), provide further understand- ing of solder's degradation and failure modes. In general, solder joint behavior can be ex- plained through micron-scale mechanisms. However, considering the various phenomena THE THEORy BEHIND TIN WHISkER PHENOmENa, PaRT 1 continues sMt prospeCts & perspeCtives still another experiment exhibited that whisker formation involved a shelf time. However, the shelf time varies without a straightforward correlation with temperature, humidity and other environmental conditions. studies showed that a moderately warm temperature served as the 'greenhouse' that nurtured whiskers, yet a high temperature (e.g., above 150 o C) inhibited whisker formation. " "

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