Tin Whiskers: Attributes And Mitigation

The movement to eliminate lead (Pb), especially active in Japan and the European Union, has resulted in an increasing use of pure tin (Sn) coatings on leads and other external and internal surfaces of capacitors, resistors, and other passive components. This paper discusses the issues of tin whisker growth with respect to passive components. It also presents both a critical analysis of existing published documents on tin whisker nucleation and growth and a summary of very recent experiments that provide further understanding of the potential means of whisker formation mitigation. Many of the proposed mechanisms for mitigation, including control of the immediate underplating material, use of conformal coating, regulating the thickness of the tin coating, use of matte tin electroplating, and annealing or fusing of the tin layer, are inadequate. They likely reduce the incidence of nucleation or growth but do not provide an absolute guarantee of lack of whisker formation. "Lead-Free" Movement Electroplated and dip-coated finishes are applied to electronic components primarily to protect the base metal (conductor) from corrosion and to enhance solderability. The benefits of many tin and tin alloy electroplating processes also include excellent control and uniformity of plating thickness (especially critical in miniaturized, fine-pitched components), good electrical conductivity, and non-toxicity [1]. In addition, tin finishes, especially "bright" finishes, which use organic additives in the plating bath, maintain an aesthetically pleasing shiny surface. For these purposes, a wide assortment of electroplated finishes have been used with much success, including many tin-lead based alloys (ranging from 2% 50% Pb) as well as 100% pure tin [2]. In recent years, legislative pressures (particularly in Japan and Europe) have forced the electronics industry to consider methods of eliminating Pb from their end products and manufacturing processes. These legislative regulations are being developed in response to potential environmental and health hazards that may result from the manufacturing and disposal of consumer products bearing Pb and other hazardous materials [3]. There have also been reports that Japan’s computer makers will start charging fees from consumers for future recycling/disposal of these computers. Although the U.S. is lagging Japan and Europe in introducing similar legislative restrictions, the U.S. electronics industry is reacting to these initiatives, by considering alternative solders and coating systems in order to remain competitive in the world market. With respect to factors such as ease of converting existing tin-lead plating systems, ease of manufacture and compatibility with existing assembly methods, pure tin plating is seen by many in the industry as a potentially simple and cost effective alternative to SnPb-based systems [4]. Many manufacturers have been offering pure tin coated components as a standard commercial (and in some cases high reliability) product for years while others are just now exploring use of Pb-free alternatives, such as pure tin, for the very first time. Despite all of its benefits, there is one major impediment to an across-the-board adoption of pure tin as the solution to pending Pb-free legislative requirements: many pure tin electroplates develop tin whiskers. This possibility, in combination with the lack of accepted methods for testing whisker growth susceptibility, gives rise to major reliability concerns! Concern Over Tin Whiskers Tin whiskers present two reliability issues for equipment manufacturers and users. The first is electrical shorting. Whiskers can grow between adjacent conductors of different potential and cause either a transient short as the whisker is burned open, or a permanent short. Second, whiskers can be broken loose from their substrate and as debris cause mechanical problems with slip rings, optical devices, microelectromechanical devices (MEMS) and similar components.