Aerospace Scientists Forge New Path For Tin Whisker Research

Mark Peterson, left, and Scott Sitzman view an image of tin whisker diffraction patterns transmitted to them from a scanning electron microscope. (Photo: Eric Hamburg)

Aerospace scientists have been studying the issue of tin whiskers for decades. The current group of Aerospace scientists researching tin whiskers was the first to present an entirely new way to analyze and characterize them.

Tin whiskers are hair-like crystalline structures that sometimes grow from tin-finished surfaces. They can grow to be several millimeters in length, long enough to short-circuit electronic systems by bridging the circuit elements, and thus causing system failures. They are a phenomenon that was first noted in the 1940s and ’50s, and the precise mechanism for their formation is still unknown, although there are several working theories on the subject.

“The study of tin whiskers is especially vital when concerning space instruments because of the potential to cause system failures while in space, making timely repair impossible,” said Mark Peterson, one of five Aerospace scientists responsible for this new line of research.

Peterson, along with Scott Sitzman, Brendan Foran, Maribeth Mason, and Miles Brodie, all from the Microelectronics Technology Department, Electronics and Photonics Lab, started with one of the origin theories — that tin whiskers sprout in response to compressive stress in the tin plating.

“Even though whiskers have been around since the ’40s, some of the fundamental formation and growth mechanisms are still not understood,” Peterson said. “Once we understand those mechanisms, then we can find ways to prevent whisker formation.”

“Most metals are made of aggregates of individual crystals called grains. We know that the tin grains in the plating are under stress,” said Sitzman.  A whisker is a single grain growing out of the plating, and its growth acts as a stress relief mechanism.  “We want to analyze the root of the whisker to better understand how and where it grows. We especially want to study its deformation characteristics for comparison to neighboring grains in the plating.”

“The question was, ‘So how do we access the whisker root and study the grains around it without introducing damage and artifacts?’ There are a lot of challenges to doing this. A lot of the work was overcoming those challenges,” he added.

They employed the use of Transmission Kikuchi Diffraction (TKD), which is an electron backscatter diffraction-derived technique that uses a scanning electron microscope.

“It is a technique that can analyze crystalline materials in many different ways,” Sitzman explained. “We used this technique to look at these crystals in detail at high resolution, how they were oriented, how they deformed as a response to the stress, and mapped that out on a micro-nano scale. The technique is new enough that it has not been applied in this way to the tin whisker issue yet.”

The technique works by pointing an electron beam at a crystal to create diffraction patterns. In this case, a small slice of the sample containing a whisker root and some surrounding grains was first thinned down to 100 nanometers or less, allowing much of the beam to pass through it. The resulting patterns were then analyzed.

“Using this technique, we can study crystallographic structure and orientation,” Sitzman said. “The size, shape, and orientation of grains all have an influence on the properties of materials, such as strength, resistance to cracking, and electrical properties. We are particularly interested in how the grains are deformed internally.  Aerospace has world-class facilities and staff, and using some of the other available techniques here, we can look at the material in different ways and corroborate this work.”

“The predictive power that would come with understanding the fundamental mechanisms of whisker growth would be invaluable,” Peterson added.

Peterson said scientists at Sandia National Laboratories, the originator of the whisker formation theory the Aerospace team was working within, are interested in a future collaborative research effort along this line of exploration.

“We are not working in a vacuum, but building on previous work,” Sitzman said. “It is a comprehensive effort.”

—Heather Golden