Stretchable circuits developed for deformable devices

Electronic devices are becoming more flexible, as foldable mobile phones, rollable smart watches and more enter our lives. But can a display that folds like paper to put away in our pockets really become a reality? Researchers at Pohang University of Science and Technology (POSTECH) have now developed a deformable conductive film that connects flexible electronic devices, publishing their findings in the journal Science Advances.

For flexible devices such as stretchable displays, electronic skin and implantable devices, it is essential to make a deformable circuit board. Circuit boards that can be formed into different shapes require high extensibility of many materials and components like wirings, displays and sensors, as well as rechargeable energy supply devices such as batteries. Methods for connecting high-resolution circuits so far include soldering, wire bonding, anisotropic conductive film and flip-chip bonding, but there remains the issue of stably maintaining the physical and electrical properties even when their shape is altered.

To do this, the research team developed a stretchable anisotropic conductive film (S-ACF) that can connect other electrodes physically and electrically regardless of the rigidity, flexibility or elasticity of the circuit line. They achieved this by arranging metal particles at regular intervals in SEBS-g-MA, an extensible block copolymer, which maintains a strong interfacial adhesion while securing stable electrical connection even when its shape is changed via chemical bonding with the substrates. In particular, maleic anhydride present in SEBS-g-MA enables chemical bonding between substrates, creating strong adhesion at low temperatures. The researchers verified that the electrical and physical connection was effectively formed when the S-ACF was placed at the contact interface between the two substrates with mild temperature (80°C) treatment for about 10 min.

In addition, S-ACF can be selectively patterned so that particles are arranged in a desired part, which increases the polymer contact surface in an area that does not require electrical connection to increase bonding strength, and is economical by reducing the use of metal particles. Film produced in this way adds stretchability to the conventional anisotropic conductive films and enables high-resolution circuit connection (50 μm), low-temperature processing and production scalability.

“This film enables connecting devices with more complex structures in the future,” explained Professor Unyong Jeong, who led the study. “I hope that it will serve as a launchpad for integrating and manufacturing stretchable devices — which have been independently studied — into one substrate and integrated system.”

The research project has been recently selected as a Materials & Components Technology Development Program by the Korea Evaluation Institute of Industrial Technology (KEIT), and is now awaiting domestic production of high-precision anisotropic conductive films. Prof Jeong said, “We anticipate this research to be a prime example of a bold investment in basic research leading to commercialisation.”

Image caption: Stretchable anisotropic conductive film (S-ACF) and schematic diagram connecting the microchip and wiring.

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