Why they grow? Getting to the roots of lethal metal whiskers

Sep 29, 2009
(a) This image from an ion beam microscope shows the growth morphology of a tin whisker. (b) A model of the forces that cause spontaneous whisker growth on tin-plated copper. The tin coating is a few micrometres thick. Image: Max Planck Institute for Metals Research, Stuttgart

(PhysOrg.com) -- A short circuit can be quite hairy: satellites have failed, a NASA computer centre was repeatedly paralysed and the US public heath authority recalled thousands of pacemakers - all because tin whiskers caused a short circuit in the electronic components of these devices.

A team of scientists from the Max Planck Institute for Metals Research has been working with Robert Bosch GmbH to measure the forces that trigger this metallic hair growth. Tin whiskers can be up to a few millimetres long and just a few micrometres in diameter. They sprout from the tin used to solder and plate made of copper. Understanding in detail what makes the whiskers grow is the first step on the way to preventing their growth. (Applied Physics Letters, June 2009)

The systems in a NASA computer centre failed after new data storage had been installed; at least 18 short circuits occurred in the high-performance computers before technicians found the reason: the replacement of the storage devices dislodged metal whiskers from the base construction and their subsequent distribution via air circulation caused system failure by bridging the electrical circuits of the supercomputers.

Researchers working with Eric J. Mittemeijer at the Max Planck Institute for Metals Research together with colleagues from the Robert Bosch GmbH, Argonne National Laboratory in Illinois and Oak Ridge National Laboratory in Tennessee have revealed the forces that cause these whiskers to sprout from tin-plated copper. According to their findings, the pressure of the tin atoms at the base of the film needs to be higher than at the surface. At the same time, there must be a pressure difference on the film’s surface plane: The pressure at the root of the tin whisker must be lower than it is further away. "You can compare it to a toothpaste tube," says Matthias Sobiech, who carried out the experiments. "When you press the sides, toothpaste comes out of the top." The pressure, which physicists also call stress, is created because an intermetallic tin-copper compound forms at the tin-copper interface that grows further into the tin film.

X-ray investigations provided a detailed picture of the distribution of the stress in the tin film. The researchers determined the stress differences between the base and the surface of the tin film in their laboratory in Stuttgart by measuring step by step the vertical mechanical stresses. In order to measure the distribution of stress in the surface plane around a growing whisker, the researchers had to use a method with very high spatial resolution in the sub-micron range. These micro stress measurements were taken using the synchrotron at the Advanced Photon Source at Argonne National Laboratory by means of the Micro Laue Diffraction method: A very fine beam of X-rays, around 300 nanometres in diameter, scanned the surroundings of a growing tin whisker in very small steps, and a sensitive detector very precisely recorded the local stresses at each probed position.

"We showed, for the first time, that tin whiskers are formed on tin layers which clearly exhibit mechanical stress differences," says Eric J. Mittemeijer, Director at the Max Planck Institute for Metals Research in Stuttgart. "And that these stress differences are the driving force for the growth of the whiskers." He and his colleagues demonstrated this in experiments by measuring local stress distributions in films that exhibit whisker growth and in films that do not exhibit whisker growth.

Looking closely at the state of stress in a material can be helpful in many investigations. It reveals a lot about the forces that operate in electronic or mechanical microsystems, which might have a detrimental effect on their function - in the same way as the tin whiskers cause short circuits in electronic components. "We hope that our findings will help to protect electronic components from the growth of tin whiskers," says Matthias Sobiech. "Because we now know that it is important to remove the stress differences in the film." The researchers are now setting out to investigate how that can be done successfully.

More information: Matthias Sobiech, Markus Wohlschlögel, Udo Welzel, Eric J. Mittemeijer, Werner Hügel, Andrea Seekamp, Wenjun Liu, and Gene E. Ice, Local, submicron, strain gradients as the cause of Sn whisker growth, 94 (2009), 221901

Provided by Max-Planck-Gesellschaft (news : web)

Explore further: Researchers develop scalable methods for manufacturing metamaterials

add to favorites email to friend print save as pdf

Related Stories

New research shows why metal alloys degrade

Sep 24, 2008

(PhysOrg.com) -- Metal alloys can fail unexpectedly in a wide range of applications -- from jet engines to satellites to cell phones—and new research from the University of Michigan helps to explain why.

Robotic whiskers can sense 3D environment

Oct 08, 2006

Many mammals use their whiskers to explore their environment and to construct a three-dimensional image of their world. Rodents, for example, use their whiskers to determine the size, shape and texture of objects, and seals ...

Rats' senses a whisker away from humans

Feb 15, 2007

The sophisticated way in which rats use their whiskers in their surrounding environments show significant parallels with how humans use their fingertips, according to new research carried out at the University ...

Will carbon nanotubes replace indium tin oxide?

Mar 09, 2009

(PhysOrg.com) -- Up until now, George Grüner tells PhysOrg.com, most of the studies regarding the properties - and uses - of carbon nanotubes have been restricted to the visible spectral range. “We, however, were intere ...

Researchers catch rats' twitchy whiskers in action

Feb 27, 2008

Rats use their whiskers in a way that is closely related to the human sense of touch: Just as humans move their fingertips across a surface to perceive shapes and textures, rats twitch their whiskers to achieve ...

Recommended for you

Novel technique opens door to better solar cells

Apr 14, 2014

A team of scientists, led by Assistant Professor Andrivo Rusydi from the Department of Physics at the National University of Singapore's (NUS) Faculty of Science, has successfully developed a technique to ...

User comments : 2

Adjust slider to filter visible comments by rank

Display comments: newest first

Pkunk_
3.7 / 5 (3) Sep 29, 2009
It would be interesting to know if this outburst of whiskering is due to the ROHS restrictions in effect for last 2 years.
Before ROHS they used to dope lead with tin to control the tin whisker problem.
PPihkala
5 / 5 (1) Sep 30, 2009
Before RoHS lead was added to soldering tin mix because 63% tin and 37% lead compound is the lowest melting point mixture one can make. But it also has been shown in previous research that lead is one of the best additives to tin to prevent tin whiskers from growing. It seems that 200-300 ppm of lead in solder should be enough to prevent whiskers and it would still make it RoHS compatible, as the limit for lead is 1000 ppm. So current problem might be that they make too pure (too few ppm of lead) solders for use in equipment.

More news stories

NASA's space station Robonaut finally getting legs

Robonaut, the first out-of-this-world humanoid, is finally getting its space legs. For three years, Robonaut has had to manage from the waist up. This new pair of legs means the experimental robot—now stuck ...

Ex-Apple chief plans mobile phone for India

Former Apple chief executive John Sculley, whose marketing skills helped bring the personal computer to desktops worldwide, says he plans to launch a mobile phone in India to exploit its still largely untapped ...

Filipino tests negative for Middle East virus

A Filipino nurse who tested positive for the Middle East virus has been found free of infection in a subsequent examination after he returned home, Philippine health officials said Saturday.

Egypt archaeologists find ancient writer's tomb

Egypt's minister of antiquities says a team of Spanish archaeologists has discovered two tombs in the southern part of the country, one of them belonging to a writer and containing a trove of artifacts including reed pens ...