SMT007 Magazine

SMT-Feb2015

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52 SMT Magazine • February 2015 electronics, advantages of tin include low cost and excellent engineering properties, includ- ing high corrosion resistance and solderablity. With the elimination of lead, tin has become a common surface finish for plated package ter- minations used on electronic devices. Research dating back to the 1950s has demonstrated that the addition of lead to tin significantly mitigates the occurrence of tin whiskers. While quality control tests for tin whiskers have been established, no tests are available to assess the long-term elimination of the tin whisker failure risk [3] . Tin whiskers are electrically conductive and can carry, depending on their cross-section- al area, sustained electrical current up to 10 mA without fusing. At higher current levels, the tin will melt and break the electrical circuit. In the presence of a high in-rush current (>1000A/s) and with voltages at or above 12 V, tin whiskers can strike a metal vapor arc operating at tem- peratures in excess of 1000°C. Once initiated, a metal vapor arc can be sustained with continued voltage, current, and material [4] . For electronics, the formation of an unintended electrical short can cause a product to malfunction. For control systems, such as an automotive engine control or automotive stability control system, a tin whisker-induced malfunction can create unsafe operating conditions. While extensive research has been con- ducted into whisker formation, methods to ef- fectively assess the propensity of tin-finished surfaces to form whiskers in long-term service applications remain elusive. Research indicates that tin whiskers form in response to stress im- balance in the tin film. Vianco et al., have re- cently reported that tin whiskers form through dynamic recrystallization when the strain with- in the tin film is greater than a critical value [5] . Even as our understanding of the processes pro- moting whisker formation increases, the chal- lenge of finding effective test methods to pre- dict long-term tin whisker growth remains. This challenge stems from a variety of factors, both external and internal, which can change the stress state in the tin film, giving rise to whisker growth. For instance, contact forces in connec- tor applications have been shown to give rise to rapid whisker growth. In field applications, en- vironmental factors such as corrosion and sur- face oxidation can promote whisker formation. Additionally, temperature cycling of tin-plated systems can give rise to stress states within the tin film due to the mismatch of temperature expansions rates between the tin and substrate to which the tin is applied. Internally, the for- mation of intermetallic compounds between the tin and the substrate material to which it is plated can also over time create a stress state favorable for whisker formation. Further, grain structure and plating thickness have been dem- onstrated to play a role in whisker growth. For part manufacturers, the use of matte tin plating, high-temperature annealing, and the application of a nickel underlayer have become common approaches for mitigating tin whisker formation. With regard to matte tin plating, this specification typically denotes a dull surface ap- pearance that arises from larger (approximately 1 to 5 micrometer) surface grains. As whiskers form from new grains at or near the free surface Figure 1: Tin whisker bridging adjacent terminals on feed-through tin-plated connector of an accel- erator pedal position sensor that led to intermit- tent resistance between the two terminals [2] . Feature TIN WHISKerS reMaIN a CONCerN continues

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