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64 SMT Magazine • February 2015 high-reliability industry that at least 3% lead is needed for best practice. What are Tin Whiskers? Tin whisker growth was documented as early as 1946. Tin whiskers are single crystal growths which are known to achieve lengths of up to and over one inch. Tin whiskers grow unpredictably on pure tin surfaces, and whis- kers can cause catastrophic short circuits as they are electrically conductive. The current theory is that compressive stresses in the tin layer are relieved with whisker growth. Compressive stresses can come from the plating conditions, from the substrates, or from external scratches or bends on the pure-tin surface. There are a number of other circumstances that promote the occurrence of tin whisker growth through intermetallic formulation, including mechani- cal, thermal, and electrical stresses from the sur- rounding environment of the pure tin surface. Tin whiskers have even been known to grow from the surfaces of hardware sitting on a shelf only exposed to typical ambient storage condi- tions. AEM has been able to grow whiskers with either precious metal ceramic chip capacitors (silver electrodes, silver termination and nickel barrier below the tin) or base metal (nickel elec- trodes, copper termination and nickel barrier below the tin) versions of the same OEM device. Both versions of this pure-tin ceramic chip ca- pacitor grew measurable whiskers after 88 tem- perature cycles between -50°C and +85°C. Issues with Lead-Free electronics Components need to withstand higher cir- cuit board assembly temperatures related to lead-free implementation. Lead-free solders typ- ically require re-flow temperature 40°C above that of tin-lead alloys. Instead of 220°C for tin-lead, lead-free solders are typically recom- mended to re-flow at 260°C. Components and circuit boards built to withstand typical tin-lead soldering temperatures have to be re-qualified to withstand the 40°C higher process tempera- ture. Equipment conversion and other associ- ated process and material changes have resulted in billions and billions of dollars of global man- ufacturing costs related to meeting the various environmental directives. The most critical lead-free issue has to be the propensity of pure-tin plated components to de- velop whiskers. There is as yet no pure-tin plat- ing that can claim to be whisker-free. Whiskers appear at unpredictable times and neither the onset conditions nor the growth habits are pre- cisely understood. The fact is that pure tin, in its popularly used configurations, whether plated or dipped,all grow tin whiskers under various conditions. There are many documented fail- ures where whiskers are believed to be the root cause; the NASA website captures a good num- ber of examples, as does the CALCE website. Prevention of Tin Whiskers Tin whiskers risks are foremost in the minds of design and component engineers in indus- Feature Figure 2: example of typical component end metallization with 100% tin plate prior to temperature cycles. Figure 3: precious metal base construction Mlcc (Ag electrodes, Ag termination, ni & Sn). THe uNPredICTabILITy OF TIN WHISKerS eNdureS continues

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