Development of Flexible Hybrid Electronics Using Reflow Assembly with Stretchable Film

Abstract

Flexible hybrid electronics (FHE) are manufactured by combining traditional circuit board fabrication and assembly processes with emerging printed electronics technology. By integrating surface mounted electronic components with printed stretchable conductive circuits and compliant/stretchable substrates these hybrid constructions have potential to revolutionize electronic assemblies used for Internet of Things (IOT), wearable, medical, wellness, automotive and aerospace markets. By employing FHE principles, designers can create heterogeneous electronic systems with unique form factors and functionality. These devices can conform to the curves of a human body or even be applied to the surface of or molded within an irregularly shaped mechanical structure. FHEs also offer the promise of light-weight and cost-effectiveness, scalable manufacturing. The FHE industry remains in the early stages of development. A variety of design, material, assembly and reliability issues remain to be addressed. For example, the typical polymer based conductive pastes used for forming FHE circuit structures are not as conductive as the etched copper on traditional printed circuit boards (PCBs.) Additionally, most of these polymer-based conductive pastes are not readily solderable and the electrical interconnections formed with conductive adhesives in current FHE designs may not be as conductive or reliable as those formed with solder. Additionally, commercially available stretchable thermoplastic film substrates have relatively low thermal resistance and cannot withstand the current lead-free surface mount technology (SMT) reflow temperatures. This paper discusses these challenges and presents an FHE manufacturing process utilizing a stretchable thermosetting polymer substrate, a combination of both screen-printed stretchable conductive paste and etched copper structure, and the conventional SMT processes to create a functional proof of concept double-sided device integrating both active and passive components.