After years of study, tin whiskers continue to fascinate, perplex and astound.

As the compliance date for eliminating lead approached, component manufacturers began to implement Pb-free surface finishes for device leads. Numerous manufacturers selected pure tin.^1,2,3,4 This change sparked concern since pure-Sn plating has a well-known reliability problem: the potential to spontaneously grow tin whiskers.

Tin whiskers are electrically conductive crystalline structures that grow from pure tin-plated surfaces. Commonly growing as hair-like filaments, they may also take other forms such as “odd-shaped eruptions” and “nodules.” The main reliability concern associated with tin whiskers is their potential to cause transient or catastrophic electrical short circuits. Tin whiskers may also break free from their Sn-plate growth surface and find their way into, and interfere with, the operation of mechanical assemblies.^4,5

To better understand tin whisker growth, evaluate associated reliability risks, and develop mitigation techniques, Raytheon, along with industry, government, and academia colleagues, performed several experiments. This article shares observations and scanning electron microscope (SEM) images of tin whiskers captured during some of these investigations and experiments.

Tin whiskers remain a reliability concern, especially for space and military applications. This issue is especially a concern for equipment with long life requirements, since tin whisker initiation can occur soon after plating or can lie dormant for years before initiating.^2,4,5

There are numerous documented cases of tin whiskers causing equipment failure. The NASA Goddard Tin Whisker website lists many examples of satellites, military, medical, and industrial/power equipment that failed due to tin whiskers.⁵ At Raytheon, prior to implementing strict controls for pure Sn-plate usage, tin whiskers caused the failure of a rocket motor initiator by shorting across the device’s 0.010˝ spark gap.

Tin whiskers are single crystal structures whose growth mechanism(s) are not completely understood. They are reported to grow as long as 10 mm, but commonly grow to lengths less than 1 mm.⁵ Raytheon has documented multiple examples of whiskers growing greater than 1 mm in length, such as the 2.7 mm long whisker found growing on a connector shell shown in Figure 3.
 
Along their length, filament whiskers vary from being straight, kinked, curved or spiraled (Figures 1 to 7). Figure 4 also shows whiskers in the same general growth area can have widely varying filament thicknesses. Figure 5 provides an example of a kinked filament tin whisker having multiple bends along its length.

For one tin whisker study, Raytheon participated with an industry working group to evaluate the ability of different types of conformal coatings to mitigate tin whisker growth.6 For this investigation, bright Sn-plated brass substrate coupons (1 x 3˝ Sn-plated brass substrate strips) were selected due to their propensity to grow whiskers. As shown in Figures 1 and 8, this methodology was very successful in producing significant quantities of tin whiskers for experimentation and study. It was not uncommon for thousands of tin whiskers to grow on each Sn-plated coupon.

Figure 8 shows the results of the experiment in which silicone conformal coat was used to partially cover a Sn-plated brass coupon. The image shows that silicone conformal coating significantly reduced tin whisker growth in coated areas. This experiment, however, also concluded conformal coating did not prevent all whisker growth. Figure 9 shows a tin whisker penetrating through the conformal coat. Other whiskers were additionally observed to have punctured through the silicone conformal coat, especially in areas of thin coating. This experiment helped establish that conformal coating can be used to help mitigate tin whisker risk, but cannot be completely relied on to eliminate all risk.

Besides filament whiskers, tin whiskers can take on many unusual shapes. We label these as odd-shaped eruptions. Figures 10 to 12 provide SEM images of this category of tin whiskers.

During our experimentation, several tin whiskers were observed with unusual features that are difficult to describe and are best documented by providing images. The tin whisker featured in Figure 13 has a thick, straight base with multiple contorted appendages emanating from this base. Additionally, as highlighted in Figure 14, a portion of this unusual whisker splits off around a kink and then rejoins with the main whisker body. This splitting characteristic is rare, but not unique, as it was observed on other tin whiskers. Figure 14 also provides a good perspective on the relatively large size of tin whiskers as compared to the small size of the plating grains.

The unusual tin whisker featured in Figure 15 (growing among several straight filament whiskers) is equally difficult to describe. This whisker has several contorted appendages and a base comprised of two separate sections that merge together at the tip. Also making this tin whisker highly interesting is that it appears to have a straight filament whisker growing from its surface.

Figure 16 provides a SEM image from the perspective of looking down from the top of a tin whisker along its length to the Sn-plated surface. In this image, a hole in the Sn-plating surface is seen where the tin whisker emerges from the tin plate. During the months and years of our experimentation, as tin whisker growth progressed, we observed an increasing number of Sn-plating voids. These plating voids are areas of tin depletion that occur as the tin whiskers grow. In many instances, the voids were not located close to tin whiskers, implying some tin whisker growth occurs due to long range transport of tin. Figure 16 also provides excellent detail of striations that frequently run the length of filament tin whiskers.

Figure 17 is a high magnification SEM image of the tip of a tin whisker. This image, which is not typical of all tin whisker tips, provides significant detail of this tin whisker’s surface. In this image, the whisker tip appears to be comprised of multiple small strands. In contrast, Figure 18 shows a Focused Ion Beam (FIB) cross-section of a filament tin whisker with solid construction throughout.

From published literature, it is universally agreed tin whiskers grow from the addition of tin to their base and not from addition of tin to their tips.⁷ NASA has used time-lapse photography to document this growth.⁵ Interestingly, however, branching tin whiskers have been observed. Figures 19 and 20 show different magnification images of a tin whisker that has significant branching. It is not known how tin whiskers are able to branch with growth occurring from their base.

Another observation from experimentation was that there were noticeable differences in tin whiskers based on the substrate metal used. Figure 21 shows tin whiskers grown from a Sn-plated coupon with an Alloy 42 substrate. Typically, these whiskers were much shorter in length than those grown with brass substrates and grew from mound-shaped structures or “nodules.”

One tin whisker growth theory is that whiskers sprout through weak areas in the surface oxide layer. In Figure 22, tin whiskers are observed emerging from a cracked surface region. The whisker on the right side of the image has lifted off and still has a portion of the surface layer attached to its tip. In Figure 23, numerous tin whiskers sprout along surface scratches. Theories as to why tin whiskers are prone to grow along surface scratches include weaknesses in the surface oxide and the presence of increased stresses along the scratch.

FIB cross-sectioning was also used to investigate tin whiskers. We sectioned several tin whiskers and then cut rectangular trenches in the tin plating beneath them. We were surprised to find that after FIB trenches were cut, new tin whisker growths emerged horizontally (as shown in Figures 24 and 25) out of the FIB trench walls near the location where the original tin whisker had sprouted vertically from the surface.

The study of tin whiskers has been fascinating. During the months and years of investigating and experimenting with tin whiskers, we were continually surprised, perplexed and even sometimes astounded by their complexity, appearance and behavior. Each time a SEM examination was performed, we thought we had seen all that there was to observe about tin whiskers, only to find during our subsequent examinations, there was more to be discovered. 

Acknowledgments

The authors would like to recognize those who contributed to this research effort. Bill Rollins has been a driving force at Raytheon and in industry in the area of tin whisker research and whisker mitigation. Tom Woodrow (Boeing) provided the first Sn-plated brass and Alloy 42 test coupons in which initial tin whisker growth studies were conducted. Joe Colangelo (Raytheon) was the lead investigator on the rocket motor initiator failure analysis efforts. John Wolfgong, Ph.D., (Raytheon) was lead investigator and co-author of “Surface Oxidation as a Tin Whisker Growth Mechanism.” Bill Shieldes (Raytheon) provided support and funding to conduct tin whisker studies. Phuc Dinh Ngo (Microtech Analytical Labs, Inc.) provided FIB cross-sectioning services.

References
1. M. Warwick, “Implementing Lead Free Soldering – European Consortium Research,” Journal of SMT, vol. 12, no. 4, October 1999.
2. CALCE-EPSC Lead Free Forum, Global Transition to Pb-free/Green Electronics, 2004.
3. C. Xu, Y. Zhang, C. Fan and J. Abys, “Understanding Whisker Phenomenon: The Driving Force for Whisker Formation,”
CircuiTree, April 2002.
4. M. Mosterman, “Mitigation Strategies for Tin Whiskers,” CALCE-EPSC, August 2002.
5. NASA Goddard Tin Whisker Website, nepp.nasa.gov/whisker/.
6. T. Woodrow, B. Rollins, P. Nalley, B. Ogden, Tin Whisker Mitigation Study: Phase 1: Evaluation of Environments for Growing Tin Whiskers, draft report, released August 1, 2003, calce.umd.edu/lead-free/tin-whiskers/restricted/Phase1Draft2.pdf.
7. K.Subramanian, “Lead-free Electronic Solders: A Special Issue of the Journal of Materials,” page 359.

Robert R. Ogden is senior reliability engineer at Raytheon Space and Airborne Systems; r-ogden1@raytheon.com. Robert F. Champaign is principal failure analysis engineer at Raytheon Failure Analysis Lab, NCS Division.