SCB and SMI: two stretchable circuit technologies, based on standard printed circuit board processes

Abstract

Purpose-In the past 15 years stretchable electronic circuits have emerged as a new technology in the domain of assembly, interconnections and sensor circuits and assembly technologies. In the meantime a wide variety of processes with the use of many different materials have been explored in this new field. The purpose of the current contribution is for the authors to present an approach for stretchable circuits which is inspired by conventional rigid and flexible printed circuit board (PCB) technology. Two variants of this technology are presented: stretchable circuit board (SCB) and stretchable mould interconnect (SMI). Design/methodology/approach-Similarly as in PCB 17 or 35 mm thick sheets of electrodeposited or rolled-annealed Cu are structured to form the conductive tracks, and off-the-shelf, standard packaged, rigid components are assembled on the Cu contact pads using lead-free solder materials and reflow processes. Stretchability is obtained by shaping the Cu tracks not as straight lines, like in normal PCB design, but as horseshoe shaped meanders. Instead of rigid or flexible board materials, elastic materials, predominantly PDMS (polydimethylsiloxane), are used to embed the conductors and the components, thus serving as circuit carrier. The authors include some mechanical modeling and design considerations, aimed at the optimization of the build-up and combination of elastic, flexible and rigid materials towards minimal stress and maximum mechanical reliability in the structures. Furthermore, details on the two production processes are given, reliability findings are summarised, and a number of functional demonstrators, realized with the technologies, are described. Findings-Key conclusions of the work are that: supporting the metal meanders with a flexible carrier prior to embedding in an elastic substrate substantially increases the reliability under mechanical stress (cyclic uniaxial stretching) of the stretchable interconnect and the transition areas between rigid components and stretchable interconnects are the zones which are most sensitive to failure under mechanical stress. Careful design and technology implementation is necessary, providing a gradual transition from rigid to flexible to stretchable parts of the circuit. Originality/value-Technologies for stretchable circuits, with the same level of similarity to standard PCB manufacturing and assembly, and thus with the same high potential for transfer to an industrial environment and for mass production, have not been shown before.

Arbitrarily Shaped 2.5D Circuits Using Stretchable Interconnections and Embedding in Thermoplastic Polymers

“…in lead free solder assembly), used in conventional PCB fabrication and assembly. Therefore currently 2 main approaches for production of PCB inspired stretchable circuits can be discriminated [1,2]: "polymer-first" approach: the base substrate, undergoing further processing, consists of the final elastic or thermoplastic polymer carrier, onto which the conductor for interconnecting the components is applied (e.g. by lamination) as a sheet.…”

Arbitrarily Shaped 2.5D Circuits using Stretchable Interconnects Embedded in Thermoplastic Polymers

“…[7] This was later extended to encapsulation in thermoplastic polyurethane with the aim of embedding electronics in textiles. [7] This technology offered an excellent starting point for the development of the one-time deformable SMI variant. The required modifications include, but are not limited to, a change of material portfolio and processing parameters.…”

“…The Stretchable Mold Interconnect (SMI) technology developed at imec in its current form was first described and optimized by Bossuyt et al [7,[24][25][26] This polymer-last technology was originally developed to integrate electronic circuits within polydimethylsiloxane (PDMS). [7] This was later extended to encapsulation in thermoplastic polyurethane with the aim of embedding electronics in textiles. [7] This technology offered an excellent starting point for the development of the one-time deformable SMI variant.…”

“…[6] This generally requires an encapsulant, which is also able to meet these deformations. [7] This typically leads to use of elastomers, such as silicone rubber (PDMS) or thermoplastic polyurethane (TPU), which can endure such deformations without incurring damage. If dynamic deformation and elasticity throughout the device's lifespan are unnecessary, the circuit can be freeform rigid.…”

Stretchable and deformable electronic systems in thermoplastic matrix materials

“…The diversity of approaches to realize such electronic systems is high [1][2][3][4][5][6][7]. The field is now commonly referred to as "stretchable electronics" [8].…”