Tin whisker growth from electroplated finishes: a review

Tin whiskers are filamentary growths that are formed on the surface of electrodeposited tin, which is used extensively in the electronics industry. The presence of whiskers on electroplated finishes has been observed for more than 60 years, but, despite a huge amount of work in this area, a definite mechanism by which whiskers grow remains unidentified. Whiskers pose a significant problem for manufacturers of electrical and electronic equipment, since they are able to grow across and bridge the gap between adjacent electrical components, resulting in short-circuits and other associated failures. For many years, whisker growth was effectively mitigated by the addition of lead to tin electrodeposits. However, recent legislation prohibits the use of lead in new electrical and electronic devices, and thus alternative whisker reduction techniques are being sought. Effective mitigation is critical in ensuring that widespread whisker initiated failures can be avoided. However, since the mechanism, or mechanisms, which cause whisker growth remain unknown, the development of effective mitigation techniques is a significant industrial challenge, but a challenge which must be undertaken. This review examines some of the work undertaken to elucidate the whisker growth phenomenon. A brief history of whisker initiated failures along with key developments in whisker theory is presented. This is followed by a more detailed assessment of the several growth mechanisms hypothesised; dislocation-based, recrystallisation-based and compressive stress-based theories. The structure and properties of tin whiskers, along with factors that affect whisker growth and potential mitigation techniques are discussed.