Elastic interconnects stretch flexible circuit possibilities

Elastic interconnects stretch flexible circuit possibilities

Researchers at the Interuniversity Microelectronics Centre (Leuven, Belgium) and Ghent University (Ghent, Belgium) recently designed and fabricated elastic interconnections that can be stretched between 50% and 100% without influencing resistance values. The research is in accordance with the recently formed SWEET (Stretchable and Washable Electronics for Embedding in Textiles) Project, and scientists hope the development of stretchable interconnects will enable flexible, washable electronic circuits and devices that can be implanted into skin or integrated into clothing and other fabrics.

Stretchable interconnects can be achieved by embedding horseshoe-shaped metal wires in an elastic base material.

“For biomedical and textile applications, the comfort of the user will be enhanced if the electronic circuits are not only flexible, but also elastic,” state researchers in the study. “Biomedical applications include implantable devices and electronics on skin.” The technology behind elastic point-to-point interconnections is basically a molded interconnect device based on two-dimensional spring-shaped metallic tracks and embedded in a highly elastic silicone film.

How they did it

A photoresist film was spin-coated on a copper foil and patterned through a photomask in a horseshoe shape, which can reduce stress by about 46% compared with other approaches, such as elliptical shapes. Splitting each conductor track into four separate parallel tracks also helped to increase elasticity without limiting performance.

A 4-µm-thick gold layer was electroplated, and then followed by a 2-µm nickel layer to allow termination wires to be soldered on the pads of each end of the interconnect. The interconnects were then encapsulated in a 0.5-mm-thick elastic polymer, and the result was a 3-cm-long interconnect.

Results

Stretching the interconnects to electrical failure, which occurs due to a rupture in the metallic track, researchers tested the elasticity of the interconnects at two angles—0° (H0) and 30° (H30). The best results came from one of the H30 samples able to stretch from 3 to 6 cm before it failed at 100% elongation.

Stretchabilities of 50% and more have been obtained and also the possibility to embed components has been demonstrated.

In general, the H30 samples (the horseshoe pattern) were better performers than the H0 tests, achieving an average maximum elongation of 72% versus 52% for H0 samples. Notable is the fact that in all cases of failure, conductance was recovered when elongation returned to 0%.

The SWEET Project, formed in March 2007, expects the research will take about three years and 675,000 Euro. Researchers include Dominique Brosteaux, Fabrice Axisa, Eva De Leersnyder, Frederick Bossuyt, Mario Gonzalez, and Jan Vanfleteren. For more information on the research project, go to http://tfcg.elis.ugent.be/projects/stretchable.html

Ralph Raiola