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The phenomenon of tin transformation was first observed and reported over 25 years ago. Although the incidents of transformation had been few and isolated, concern for this problem has grown recently following the expanded use of electronics in m litary weapons systems in arctic regions. In turn, the dependence upon electronics by military agencies has produced the need for greater reliability of such equipment. The complexity of modern electronic equipment has increased the quantity of wiring and components with tin coatings susceptible to transformation. The presence of grey tin powder in electronic equipment may produce a permanent failure from short circuits or improper performance due to intermittency or degradation of high-impedance circuits, with possible adverse effect on equipment reliability. The requirement in ASTM Specification B33-52T, Tinned Soft or Annealed Copper Wire for Electrical Purposes,2 that “Commercially Pure” tin be used for coating copper wire was considered too vague to assure that the tincoating would retain its physical properties at low temperatures after an extended period of time. Although a limit on the quantity and type of impurities allowed in the tin is necessary to achieve a satisfactory tin coating for most applications, recent studies and investigations indicated a need to limit the “purity” of the tin in order to inhibit transformation of the tin to a gray powder at low temperatures. Since the phenomenon of tin transformation is known to take place under the right conditions, a study was initiated in 1956 by Task Group 22 of Subcommittee IV on Conductors of Copper and Copper Alloys of ASTM Committee B 1 on Wires for Electrical Conductors to determine suitable preventive measures to maintain reliability of equipment.
Chief, Electromechanical Devices Area, U. S. Army Signal Research and Development Laboratory, Fort Monmoutli, N. J.