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SMT-Sept2016

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14 SMT Magazine • September 2016 Diffusion through tin matrix is expected to be impeded by alloying element. One vivid ex- ample is SnPb. This binary system comprises two phases—a Pb-rich phase and Sn-rich phase. The fact that SnPb alloy alleviates tin whisker propensity largely attributes to that Pb slows the diffusion of Sn in the matrix and allows for rapid stress relaxation, which in turn, reduces or prevents continuing whisker growth. Diffusion rates are also grain boundary (g.b.) sensitive; the greater the number of g.b., the faster the diffusion rate is. For a grain boundary rate-determining process, the greater the num- ber of g.b. could lead to a faster rate of whisker growth. Increasing the grain size of the tin plat- ing will reduce the number of grain boundaries, slowing the diffusion. Intermetallic compounds (IMCs) with "suitable" sizes tend to form pref- erentially along grain boundaries. Fewer grain boundaries offer fewer sites for non-uniform IMC growth, resulting in the reduced stress in the system. Additionally, the diffusion rate varies with the crystallographic orientation. The intersti- tial diffusion of Cu/Ni along the c-axis of tin grains is much faster than along a,b-axes. Ac- cordingly, grain orientation can affect diffusion rates in tin matrix. Controlling the grain orien- tation can alter the rate of diffusion. However, tin's anisotropic properties can be reduced by refining its microstructure. Overall, increasing the grain size of the plat- ing or controlling the grain orientation can slow the rate of diffusion. Both lattice diffusion and grain boundary diffusion are participants in tin whisker growth. The relative rate of lattice diffusion and grain boundary diffusion varies with temperature, which plays a role to the actual mechanism of the process, as well as to the growth of whisker. However, as the temperature rises, in the case of tin to above 75°C, the lattice and grain bound- ary diffusion rates start to converge to a simi- lar rate. The relative diffusion rates between lattice and grain boundary in relation to the size, mor- phology, crystal lattice structure and external conditions (e.g., temperature) are more intricate than the first glance—indeed, a complex chal- lenge to "conquer and control!" Reaction and Dynamic of Intermetallic Compounds Intermetallic compounds may exert addi- tional effects in grain structure, as these com- pounds can form in various sizes, geometries and morphologies ranging from small, more- rounded particles to large, long needles. This formation creates either highly localized stress or well-distributed stress or both in the tin lat- tice structure. When IMCs are large, they tend to be dispersed in Sn matrix; if IMCs are small particles, they tend to reside along grain bound- aries. In either situation, IMCs impede tin atom diffusion through Sn matrix or reduce g.b. mo- bility. If IMCs are attracted to the grain bound- aries in the atomic form or as small particles, they may act as impurities that reduce the mo- bility of grain boundaries and thus could pro- mote "abnormal" grain growth. ("protruding" grain growth, i.e., tin whiskers), in lieu of nor- mal grain growth. If IMCs form large particles at the substrate deposit interface or in the bulk of the deposit, their effect is similar to the ef- fect of any embedded particles regardless of its nature. Embedded particles in tin coating are known to promote tin whiskers, as evidenced in published and unpublished results, which indicate that inclusion of inert particles (e.g., Teflon and carbon) increased whiskers. On the other hand, if IMCs impede tin atom diffusion, it could jeopardize the sustainability of tin whisker growth. However, it is important to note that the presence of IMCs is not a necessary condition to the occurrence of tin whisker. This applies THE THEORY BEHIND TIN WHISKER PHENOMENA, PART 4 " Fewer grain boundaries offer fewer sites for non-uniform IMC growth, resulting in the reduced stress in the system. "

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