Electroplated Tin and Tin Whiskers in Lead Free Electronics

Electroplated Tin and Tin Whiskers in Lead Free Electronics by Fred W. Verdi - 11/5/04
Introduction:
The electronics industry is under extreme pressure from the environmental community to remove elemental Lead (Pb) in the form of the commonly used tin-lead solders from electronic components and assemblies. Pure tin is being electroplated onto component terminals as an inexpensive, solderable, drop-in replacement for the tin-lead alloy electroplated component terminal finishes that have been successfully used for some 60 years. This replacement of tin-lead alloy with pure tin makes the formation of whiskers by tin (Sn) and tin alloys in electronic assemblies a major concern for failure (1). When these electrically conductive, single crystal tin whiskers grow significantly, (e.g.>50 microns and sometimes several mm), electrical shorting between fine pitch circuits becomes possible, leading to catastrophic electrical short circuit failures in high reliability systems such as heart pacemakers, spacecraft, or military weapons and radars. Worse yet, the thin filamentary tin whisker can fuse, creating a plasma that can conduct hundreds of amperes, destroying electronic equipment such as power supplies in high-current applications.

Tin whisker growth is not a new problem. The Bell System has documented the growth of tin, cadmium, and zinc whiskers and subsequent short circuit failures, on electroplated hardware in telephone switching equipment as early as 1946 (1). After extensive study, Bell Laboratories recommended alloying the tin with lead, and the result has been essentially whisker free electronic assemblies, using tin-lead alloy plating on component leads and circuit boards and using tin-lead solder attachment in assembly, for the last 60 years.

Because of the recent global environmental emphasis in eliminating lead (Pb) from electronics, a firm metallurgical understanding of tin (Sn) whisker growth and methodologies to predict or mitigate growth are needed. Such mitigation techniques are not conclusively known, making the situation of electrical failure risk due to tin whiskers, particularly in high reliability systems unacceptable. A spontaneously growing, electrically conductive, tin whisker in an expensive and/or life supporting military or high reliability medical assembly can cause catastrophic failure.




Figure 1. Early photograph of spontaneously growing, tenth-inch long single crystal tin whiskers from Bell Laboratories investigation of electrical short circuit failures from tin plated details in telephone switching equipment in the 1940’s.

From the 1940s until the last decade of the twentieth century, tin-lead electroplate and tin-lead solder was routinely used for electronic assembly, with virtually no chance of tin whisker failures. The history since then begins to show the growing menace and attempts at mitigation.

In 1993, M.E. McDowell of the United States Air Force (2) outlined the method used by the USAF in dispositioning Sn plated parts in inventory. No position was taken relative to the prohibition of Sn usage (as previously recommended by Dunn of the European Space Agency (3)). This would prove to be an unfortunate situation, as later events were to show, relative to reliability failures on USAF equipments. Between 1990 and 2004, Brusse from NASA compiled a list of high reliability system failures caused by metal whisker growth. Tin, Zinc, and Cadmium are all prone to whisker growth. This list of failures in the time period from 1990 to 2004 counts scores of well-documented whisker failures in electronic assemblies, from heart pacemakers to NASA and commercial satellites, and many millions of dollars in monetary losses. A typical tin whisker shorting issue is depicted in Figure 2 (After Brusse/NASA ref. 4).




Figure 2 Tin whiskers causing electrical shorts between connector pins after 10 years in service.

This increased incidence of whisker failures has not gone unnoticed by the scientific community.

In the first two years of the 21st century, there were more presentations/papers on Sn whisker matters than in the prior 15 years. The advent of lead free electronics manufacturing (in response to environmental legislation) undoubtedly accounts for much of this renewed interest. Many commercial and military organizations have begun consortia and programs to define mitigation techniques that might be able to stem the tide of whisker failures. As of this writing, no conclusive set of mitigation techniques is widely accepted, and so the only accepted mitigation for tin whiskers is to electroplate component terminals with a solderable metal other than pure tin. Immersion Silver, Electroless Gold, Nickel-Palladium, all can be substituted for tin, but at significantly higher cost than electroplated tin.

Metallurgical Theory:
White Sn (

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