Key Research Areas
Fraunhofer Institute for Reliability and Microintegration
Department System Integration & Interconnection Technologies
In the past years a number or approaches to realize stretchable electronic circuits have been reported. In the frame of the European Project STELLA a particular fabrication technology for stretchable electronic systems has been developed at Fraunhofer IZM in cooperation with TU Berlin. The technology is derived from conventional printed circuit board manufacturing processes. Stretchability of the boards is enabled by (i) using polyurethane instead of FR4 or polyimide as a carrier material of the copper structures and (ii) a meandering design of the copper interconnects between commercial (rigid) electronic components comprising the system. Such boards can be extended by up to 300 % before rupture of the copper interconnections. For repeated elongation/relaxation cycles, elongations with of a few percent will require in a couple of ten thousands of cycles prior to fatigue rupture.
The electronic components (off the shelf components) are assembled onto the boards, which are equipped with a local solder mask and appropriate surface finish. The electronic interconnection of components to the board established using a low temperature solder alloy (SnBi, Tm=142 °C). For protection and enhanced system robustness all components are subsequently encapsulated in with polyurethane capping.
Systems thus realized can be readily attached to different kinds of surfaces. Most interesting for various application cases is the attachment to textiles by a simple lamination process.
The field use case studies of stretchable systems in the frame of the STELLA are sensor applications in the field of medical electronics like a breathing frequency monitor for babies, a shoe insole pressure sensor for diabetes patients, or a band aid inlay to measure pressure and humidity of an acute wound when pressure therapy is applied. The in subsequent projects besides medical devices the application of stretchable systems to textiles was successfully demonstrated having consumer use in mind. On the other hand industrial sensor devices were realized where both high flexibility and stretchability of the distributed sensor device were required.