![]() The size and geometry of the solder joint, as well as the constraint of the adjacent substrate and components in an electronic assembly, are factors that are not assessed in studies of bulk samples, but have been implicated in the resistance of solder joints to tin pest. Despite the increased susceptibility of lead-free solders observed in bulk samples, it has been suggested that tin pest formation in solder joints will be opposed by the physical properties of the solder joint itself. Most of the available research on tin pest in lead-free solders is performed on bulk samples of varying shapes and sizes. In fact, the electronics industry has generally operated under the premise that tin pest will not occur if tin is alloyed. Yet, although tin pest has been reported to occur in the laboratory, there are no examples of tin pest ever occurring in the field. Clearly, this is not acceptable for electronic systems that must be reliable below 13.2 ☌. Nevertheless, some studies of bulk alloy samples suggest that lead-free alloys exhibit greater susceptibility to tin pest than traditional tin‑lead solders. Pure tin is generally not accepted in the electronics industry due to another failure mechanism known as tin whiskers. The transition of the electronics industry toward lead-free solders has once again shifted the discussion to the risks of tin pest, primarily when the solders are alloyed with lead-free materials. ![]() The result has been the transition away from traditional tin‑lead solders to lead-free alternatives containing a substantial percentage of tin. However, since 2006, with the passage of the RoHS directive, the electronics industry has been undergoing a lead-free revolution. ![]() Over the past several decades, tin‑lead solders have been preferred for interconnecting and packaging electronic components. Although traditional tin‑lead solders have been observed to resist tin pest for greater periods of time in studies of bulk samples, studies that have investigated lead-free alloys in electronic packaging demonstrate a resistance of solder joints to formation of tin pest that could be used to present a counter argument to lead-free exemptions. The evidence for tin pest resistance by lead-free alloys in solder joints is strong. In order to make a more complete assessment of the issue of tin pest in lead-free electronics, the entire body of available evidence must be considered. The studies that were not included highlight a disparity between the formation of tin pest in bulk samples of lead-free alloys and in actual leadfree solder joints. Although the exemption was approved, the underlying technical justification for the exemption is incomplete because it used only a portion of the available literature on tin pest in lead-free solders. Exemption 26 in Annex IV of the RoHS was thus approved for the use of lead in various components of electronic packaging intended for applications below −20 ☌. ![]() Publications that demonstrate a greater susceptibility of lead-free solders to tin pest, such as the one by Plumbridge, are cited in the application mentioned above to support the claim that there are no suitable alternatives to tin‑lead solders and that continued use of tin‑lead solders is required to ensure the reliability of certain low-temperature application devices. According to Article 5(1)(a) of the RoHS, exemptions are granted when applications can demonstrate that compliance with RoHS materials bans will result in any one of three conditions, among which is the condition that the reliability of substitutes is not ensured.
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