by Tim Ellis
With Navy sustainment looking at 30 year lifecycles, the formation of tin (Sn) whiskers is a cause for concern. The micrograph of Sn whiskers shown in figure 1 was produced by the EMPF in only three days. Whiskers of this type are known to cause electrical shorting in electronic components. EMPF investigations found that simple changes in the plating bath temperature and storage conditions could change the amount and length of whiskers produced. With that in mind, you may wonder how close suppliers are watching Sn plating on COTS parts from overseas, purchased sub-assemblies or reworked formerly leaded parts? The present mitigations strategies for Sn whiskers are: control of the finish to matte, reduction in organic contamination, minimization of mechanical compressive stress and continued use of lead in conflict with future industry practice. This is the first of a three part series on Sn whiskers that will address the implications of Sn whiskers on Navy electronics.

Since 1988 several weapons systems have failed due to Sn Whiskers, examples are the Phoenix Missile, F - 15 Radar, Patriot Missile and Avionics relays (1, 2, 3, 4, 5, 6). If the military is depending upon these systems to perform without failure when deployed, then it should be concerned with the Sn whisker issue facing electronics manufacturers. Sn whiskers have been an issue off and on over the last 50 years. However, with the advent of Pb - free solders mandated by both Europe and Japan to address environmental issues whiskers seem to have returned. Figure 2, shows typical Sn whiskers growing on a plated surface (7). Since these whiskers can develop aspect ratios >1000, i.e. Length/Diameter, their presence which can lead to shorting threatens the viability of an electronic device using high Sn materials. It has been reported that Sn whisker formation has lead to failure in heart pacemakers do to shorting to the package, grounding/shorting in avionics radar and relays, and the loss of satellites from process failure, terminal shorting and relay breakdown. Unfortunately, little is known about how whiskers form and why do they appear so metallurgically stable once developed.

Metallurgically the nucleation and growth Sn whisker are a balance of mechanical stresses, alloy chemistry, crystallography and morphology. All of these inputs can be affected and affected by the plating and assembly processes used to assemble the electronic component. The present phenomenological model is based upon compressive stresses leading to the nucleation and growth of Sn whiskers (8, 9, 10). Although in these same studies other elements, e.g. Cu, Zn, are found to effect whisker growth in addition to the nature of the surface finish, i.e. bright, matte or satin. Stress development can also be enhanced due to thermal treatment which aids in the formation of intermetallic compounds; a specific example is Cu6Sn5 which is often found associated with whisker formation.

At this time an analytical model to predict Sn whisker formation does not exist. Present models for dendrite and cell formation in solidification nor vapor phase whisker formation describe Sn whiskers adequately for predictive use (11). The issue is that in the Sn whisker case the growth happens within a uniform temperature field. Unlike phase change growth which balance thermal gradients against material transport mechanisms the isothermal transport to produce Sn whiskers is more akin to the heat treating of Steel or the aging of Aluminum (Al). In isothermal nucleation and growth is produced by the elimination of a metastable phase formed by rapid cooling from an elevated temperature. For example Al aircraft alloys are solutionized by heating above 400C to dissolve and evenly distribute the alloying elements. The material is the rapidly quenched, e.g. in water, to lock the dissolved atoms in solution. Holding the Al alloy at or slightly above room temperature allows the formation of a fine distribution of particles of alloying elements which provide strengthening. In the Sn whisker case it has been show that the whiskers have a body centered tetragonal crystal structure, white - Sn, with a density of 7.23 g/cm3. However, Sn also has several other crystal structures with much lower densities, - Diamond Cubic at 5.23 g/cm3, SnII and - Sn at g/cm3 (12). Therefore a proposed mechanism is that the difference in density, or atomic packing, drive the nucleation and growth of a Sn whisker from a low density phase to a high density one.

To develop an understanding to predict and control Sn whisker formation is an issue semiconductor and electronic assembly engineers to come to grips with. Process control in the factory and product reliability in the field require it. This is especially true in high reliability areas with long life cycle expectancies. It is unfortunate that the commercial market at large with the possible exception of telecommunications with probably not be of assistance as Sn whisker often manifests themselves well outside of the usual reliability window. The EMPF is presently developing manufacturing and reliability processes with Pb - free alloys and supporting that work by developing a thorough understanding of the underling metallurgical science involved. The final goal is to predict, process and protect against failure in high reliability applications.

References:

1.Military Airplane: G. Davy, "Relay Failure Caused by Tin Whiskers", Northrop Grumman Electronic Systems Technical Article, October 2002

2.Patriot Missile: Anoplate WWW Site: Suspected tin whisker related problems (Fall 2000)

3.Phoenix Air to Air Missile: L. Corbid, "Constraints on the Use of Tin Plate in Miniature Electronic Circuits", Proceedings 3rd International SAMPE Electronics Conference, pp. 773-779, June 20-22, 1989.

4.F-15 Radar: B. Nordwall, "Air Force Links Radar Problems to Growth of Tin Whiskers", Aviation Week and Space Technology, June, 20, 1986, pp. 65-70

5.U.S. Missile Program: J. Richardson, and B. Lasley, "Tin Whisker Initiated Vacuum Metal Arcing in Spacecraft Electronics," Proceedings 1992 Government Microcircuit Applications Conference, Vol. XVIII, pp. 119 - 122, November 10 - 12, 1992.

6.U.S. Missile Program: K Heutel and R. Vetter, "Problem Notification: Tin Whisker growth in electronic assemblies", Feb. 19, 1988, memorandum

7."Tin Whisker Observations on Pure Tin-Plated Ceramic Chip Capacitors", J. Brusse, Proceedings of the American Electroplaters and Surface Finishers (AESF) SUR/FIN Conference, June 24-28, 2002, pp. 45-61

8."Understanding Whisker Phenomenon: The Driving Force for Whisker Formation", C. Xu, Y. Zhang, C. Fan, J. Abys, Circuit Tree Magazine, April 2002, pp. 94-105

9."A Laboratory Study of Tin Whisker Growth", B.D. Dunn, European Space Agency (ESA) STR-223, pp. 1 - 50, September 1987

10."The Formation of Whiskers on Electroplated Sn Containing Cu," K.-W. Moon, M.E. Williams, C.E. Johnson, G.R. Stafford, C.A. Handwerker, and W.J. Boettinger, Proceedings of the Fourth Pacific Rim International Conference on Advanced Materials and Processing, The Japanese Institute of Metals, Sendai, Japan, 2001, S. Hanada, Z. Zhong, S. W. Nam and R. N. Wright, eds., pp. 1115-1118

11."Whiskers and Dendrites", R. Trivedi and A. Karma, Encyclopedia of Applied Physics, Vol. 23 pp. 441 - 459

12. Metastable Phase Equilibria in Lead - Tin Alloy Systems: Part II Thermodynamic Modeling, H. Fecht, M. Zhang, Y Chang and J. Perepesko, Met. Trans. A Vol. 20A May 1989 pp. 795 - 803

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