SMT007 Magazine

SMT-July2014

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18 SMT Magazine • July 2014 To comply with RoHS regulations, Pb-free materials including pure tin (Sn) have been used as surface coating for component leads and metal terminals, and Sn-based alloys as sol- der materials in making solder joints. Because of its economics, availability, manufacturability, compatibility and solderability, pure tin makes a practical replacement for Sn-Pb as a choice of surface coating. Today, most component manu- facturers offer pure tin-coated components. The evaluation of tin whisker propensity and growth rate needs to be put in the context of relative formation rate under a set of conditions. For the electronic and electrical applications, the renewed concern about tin whiskers are largely the result of conversion from tin-lead coating to lead-free (or tin coating) for component leads or PCB surface finish. Thus, the relative perfor - mance in reference to a tin-lead benchmark that has demonstrated satisfactory whisker-resistance is a logical criterion, not the absolute perfor- mance. An SAC (SnAgCu) alloy is lead-free, but a lead-free alloy is not necessarily an SAC. This clarity is particularly important as more viable lead-free alloys become commercially available. And tin whiskering is highly sensitive to an alloy composition including impurities. Phenomena and observations Tin whisker reflects its coined name, which has long been recognized to be associated with electroplated tin coating and most likely occurs with pure tin. Its appearance resembles whiskers. However, they can also form in a wide range of shapes and sizes, such as fibrous filament-like spirals, nodules, columns and mounds (Figure 1). Tin whiskers are often single crystals and electrically conductive. They are normally brit- tle in nature but can be rendered ductile when whiskers are very long and thin. Whisker formation and its resulting shapes and sizes depend on time, temperature, sub- strate, surface condition of the substrate, sur- face morphology, plating chemistry, and plat- ing process. The rate of whisker growth also depends on a list of factors including the above- mentioned. Whiskers sometimes grow up to a few mm long, but usually less than 50 µm, and a few mi- crons in diameter. Whiskers may grow, but they may also be self-annihilating as the electric cur- rent can fuse the whisker if the current is suffi- cient (e.g., typically more than 50 milliamps is often required). The self-annihilation ability var- ies with the whisker's size in length and diameter. This self-annihilating occurrence further contrib- utes to the observed inconsistent or mythical na- ture of the events. Furthermore, the highly dis- parate whisker growth rates have been reported, ranging from 0.03 to 9 mm/year. And whiskers can grow even in a vacuum environment. Among various findings, one experiment indicated that whiskers can be eliminated by controlling the plating process in an equiva- lent way to controlling stresses in materials. The very sharp decrease in internal stress of tin electrodeposits was observed after plating as quickly as within minutes. It is interesting to note that this fast stress release occurs regardless whether initial stress in the deposit is compres- sive or tensile. In the case of compressive or ten- sile stress, the value of the 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 reduces to much lower stress value but remains tensile and high compressive stress remains compressive). It has been observed that the inclusion of organic elements in the tin structure promote tin growth. Organic inclusion or the level of in- clusion is in turn affected by the plating chem- istry [1,2] . And bright tin has exhibited to be most susceptible to whisker formation. Bright tin TIN WHISKERS: CAPSulIzATIoN continues fEATURE figure 1: Tin whiskers appearance, from mound to filament.

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