A History of Tin Whisker Theory: 1946 to 2004

A History of Tin Whisker Theory: 1946 to 2004 George T. Galyon IBM eSG Group Poughkeepsie, New York [email protected] ABSTRACT over to pure tin electroplating, but quickly found that pure tin Tin whiskers are filamentary growths that spontaneously grow had whisker problems very similar to those experienced with from electroplated tin surfaces (Figure 1). Whisker growth cadmium platings. These findings were reported in a 1951 starts after an incubation period that varies from seconds to paper by K.G. Compton, et al [4 ]. Bell Laboratories years. Typical whisker diameters range between 1-5 microns immediately initiated a long-term tin whisker study, as did and whisker lengths range upwards of 5000 microns. Tin other large research organizations such as ITRI (the whisker growth rates under ambient conditions range around International Tin Research Institute), ESA (the European 0.1 Å/second. Whiskers are highly conductive and readily Space Agency), Northern Electric, the L.M. Ericsson short out electrical components by bridging gaps between Telephone Company (Sweden), and others. closely spaced electrical conductors. Tin and tin alloys have been widely used in the electronic industry for over 50 years THE 1950s and whisker problems have always been a concern and a The 1950s saw the establishment of whisker mitigation problem. There has been a long history of whisker related practices still in use today. Whisker fundamentals were electronic failures due to pure tin without any lead content. In addressed and most of the basic concepts currently used to the past, lead (Pb) additions to tin (Sn) were widely used to describe whisker formation and growth were proposed in one mitigate whiskering. However, on July 1, 2006 the European form or another during the 1950s. As the decade progressed Union will enforce the Reduction of Hazardous Substances new experimental tools such as the SEM (scanning electron (RoHS) directive, which requires lead (Pb) removal from microscope) and TEM (transmission electron microscope) electronic assemblies, thereby exposing a new generation of were used to gain new insights into the physics of whisker electronic equipment to massive field service failure rates formation. Some highlights from that 1950s research will be because of unmitigated whisker growth. Concern over briefly reviewed in the following sections. whisker-related reliability problems led to NEMI (National Electronics Manufacturing Initiative) forming a number of The 1950s: Compton-Mendizza-Arnold whisker-related projects to review the problem and Bell Laboratories became very active in tin whisker research recommend appropriate action. One of the projects is the and their first external publication was in 1951 by K.G. NEMI Tin Whisker Modeling Project, currently chaired by Compton, A. Mendizza, and S.M. Arnold [4]. This article this author. As part of the overall study of whisker established that whisker formation occurred spontaneously on fundamentals, a comprehensive tin whisker literature search electroplated cadmium (Cd), zinc (Zn), and tin (Sn) and also was conducted by this author, the results of which have been reported that whisker growths were found on an aluminum (Al) published by NEMI [1 ] and made available at casting alloy and on electroplated silver (Ag) exposed to an http://www.nemi.org/projects/ese/tin_whisker_activities.html. atmosphere of hydrogen sulfide (H2S). The authors inferred, This paper will present a selected set of papers from this but did not conclude, that the whiskers were “not compounds reference along with a summary of some key points from the but are metallic filaments in the form of single crystals.” author’s “Integrated Theory of Whisker Formation and Growth” [2]. The 1950s: Koonce-Arnold In 1954 Koonce and Arnold of Bell Laboratories published a Key words: tin, whisker, lead, RoHS, NEMI, whisker very short letter to the editor [5], which stated that whisker mitigation, whisker fundamentals, integrated theory of growth occurred by the continual addition of material to the whisker formation and growth. base of a whisker rather than by addition of material to the tip of the whisker. Koonce and Arnold came to this conclusion THE 1940S by utilizing electron micrographs of a growing whisker over During WWII the electroplated material of choice for several weeks. They observed that the morphology of the electrical components was electroplated cadmium (Cd). whisker tip was maintained over time as the whisker grew in Repeated failures of electrical hardware led to a finding that overall length, meaning that the whisker was being pushed up many failures were due to shorting from cadmium whiskers. from material added to its base. This fundamental observation These findings were summarized in 1946 by H.L. Cobb [3]. has never been contradicted and is accepted today without Starting in 1948 similar failures were experienced by Bell reservation. Telephone on channel filters used for multi-channel transmission lines. Bell Labs immediately initiated a change- The 1950s: dislocation theorists necessary for the whisker atoms to move into the whisker Three dislocation-based theories were proposed during the grain at the sub-surface grain boundaries. 1950s to account for the Koonce-Arnold observation. The basic idea was to construct a dislocation mechanism that Ellis and his co-authors did more than just cast doubt on the moved toward the film surface and deposited a layer of atoms relevance of dislocation growth mechanisms for whiskers. at that surface. The continual operation of the dislocation They proposed, for the first time, that recrystallization played mechanism would grow the whisker. The first such a key role in the formation and growth of whiskers. The exact dislocation mechanism was published in 1953 by J.D. Eshelby quotation is: [6], who proposed a mechanism whereby a Frank-Read source emitted dislocation loops that first expanded in their source “Whisker growth is but a special case of recrystallization and plane to some boundary and then subsequently glided to the growth involving mass transport…should promote many surface, depositing an extra half-plane of atoms at that surface. experiments to examine its validity”….W.C. Ellis, et al., 1958. Figure 2 is a representation of the Eshelby mechanism. Also in 1953 F.C. Frank proposed a different mechanism [7 ] Ellis and his co-authors also established that whiskers did not involving a rotating edge dislocation pinned to a screw grow from an as-plated crystalline structure; that dislocation located perpendicularly to the surface. Each recrystallization was necessary to transform the as-plated revolution of the rotating edge dislocation deposited an structure into a structure amenable to the formation and additional half-plane of atoms on the surface. Figure 3 is a growth of a whisker. Ellis’ proposals were based on indirect representation of Frank’s mechanism. In 1957, S. Amelinckx, evidence and were not stated as established fact. The et al. [8], proposed a whisker dislocation theory that described concluding statements urged that future experimentalists a helical dislocation moving toward the surface by a climb consider the proposals and attempt to prove/disprove their mechanism and depositing a half-plane of atoms for each validity. complete loop of the spiral reaching the surface. Figure 4 shows a schematic of the Amelinckx helical dislocation. The 1950s: Fisher-Darken-Carroll Early speculations on driving forces for whisker growth All subsequent dislocation proposals are essentially variants of culminated in a landmark 1954 paper from R.M. Fisher, L.S. the above 1950 dislocation theories. It is worth noting that Darken, and K.G. Carroll of U.S. Steel [11]. They applied there has not yet been any experimental evidence supporting clamping forces to tin films electrodeposited onto steel the existence of any of the above described dislocation substrates and observed the resultant whisker growth rates. mechanisms and it is probably fair to say that the majority Figure 5 shows the clamp design. The results showed that opinion of current theorists is that dislocation mechanisms are applied compressive forces up to 7500 psi accelerated whisker not involved in whisker growth. This disavowal of growth rates from less than 1 Å/sec to as high as 10,000 Å/sec. dislocations was first stated by G.S. Baker in 1957 [9] and The seminal importance of the Fisher, et al., paper is that it subsequently by W.C. Ellis, et al., of Bell Labs in 1958 [10]. established the premise that whisker growth is driven by Baker’s conclusions were based on the observation that compressive stresses within the base material. It is worth whiskers did not grow with a root that was coherent with the noting that this effect has been twice corroborated in the base material, and that kinked whisker growth was published literature. The first corroboration was in 1962 by incompatible with dislocation mechanisms for whisker growth. V.K. Glazunova [12] and the second was by C.H. Pitt and R.G. The Ellis monograph will be discussed in more detail in the Henning in 1964 [13]. Glazunova studied tin-coated brass and following section. steel, whereas Pitt and Henning evaluated tin with both steel and copper substrates. The 1950s: W.C. Ellis The Ellis, et al., 1958 monograph dealt at some length with the This author would comment that we now can be fairly certain theoretical aspects of whisker growth and established concepts that the Fisher-Darken-Carroll experimentation also which form much of the basis for the present day debates on accelerated a recrystallization event of the kind that was whisker fundamentals. Ellis compiled all the then available subsequently proposed by W.C. Ellis in 1958. The clamping data on whisker growth orientations and showed that not all pressure apparently extruded some tin material from the clamp whisker growth directions were coincident with dislocation edges and this extruded material had a number of glide-planes.

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