Crackling wires and microchip packaging

A recent research advance in the mechanics of crystalline materials was sensationally reported by the BBC as paving the way to handheld supercomputers. That may be a rather extreme interpretation, but the discovery by scientists at the University of Edinburgh is nevertheless of major importance for electronics engineering at the nanoscale.

The researchers, led by Michael Zaiser, are interested in what happens to crystalline materials under stress, and specifically how large strain fluctuations make it difficult to control shape in a plastic forming process.

Plastic bending is often used in microchip packaging when interconnects are attached to the chips. “Current bonding wires have diameters down to 10 μm,” says Zaiser. “Our research could help to prevent some nasty surprises when wire diameters reach the micron range, and provide guidance on how to achieve a defined loop shape even in this “crackling plasticity” regime.”

Zaiser and his colleagues have found that wires bend through a series of tiny avalanches as defects move through the material. The mechanical yielding that causes permanent deformation can be thought of as an abrupt phase transition in the material.

The study shows that avalanche strains increase as sample size decreases, and with single crystals the largest dislocation avalanches extend across an entire cross-section of the material. In the case of polycrystals, the avalanches are limited by grain size, and this could help smooth the deformation process and make it more easily controllable.

Peter Gumbsch, who heads the Fraunhofer Institute for the Mechanics of Materials in Freiburg, stresses that the presence of avalanche strains does not mean nanowires will not deform. “They do, but in a much more jerky way,” he says. “The challenge then is to find ways to get them back to the smooth way of forming.” Gumbsch says that one way of doing this would be to confine the avalanches to volumes smaller than that of the wire by using nanocrystalline material.

Further reading: Dislocation Avalanches, Strain Bursts, and the Problem of Plastic Forming at the Micrometer Scale, Csikor et al., Science 318, 5848 (2007).

Figure: Molecular dynamics simulations show that bending a 0.1 μm aluminium wire will be an irregular, jerky process dominated by dislocation avalanches that span the width of the wire (© 2007 American Association for the Advancement of Science).

Article first published in Nanomaterials News.