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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).

Similarly as in PCB 17 or 35 μm 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.

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.

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.

The purpose of this paper is to present an update on the progress of the design and reliability of stretchable interconnections for electronic circuits.

Finite element modelling (FEM) is used to analyse the physical behaviour of stretchable interconnects under different loading conditions. The fatigue life of a copper interconnect embedded into a silicone matrix has been evaluated using the Coffin‐Manson relation and FEM.

The mechanical properties of the substrate and the design of the metal interconnection play an important role on the fatigue lifetime of circuit. In the case of copper embedded into a PDMS Sylgard 186, more than 2,500 tensile cycles have been observed for a periodic deformation of 10 per cent.

Reliability results are limited and need further work to create a more accurate empirical model to estimate the lifetime of stretchable interconnections.

The combined use of FEM and experimental analysis enable a more reliable design of the stretchable metal interconnections. The proposed horseshoe design offers the benefit of reduced permanent damage during elongation.

Ultra-thin chip packaging (UTCP) is one of the flexible assembly technologies, by which thinned dies are encapsulated inside spin-coated dielectric films. For sake of higher density integration and bending stress suppression, two UTCPs can be stacked vertically. The purpose of this paper is to present an improved UTCP process flow to embed thinned chip in a symmetric dielectric sandwich for a flat topography. The UTCP flat top surface is suitable for metallization and further 3D stacking.

In the new process, a central photosensitive polyimide film is introduced, in which a cavity is made for the embedded chip. The cavity is defined by lithography using the chip itself as a photo-mask. In this way, the cavity size and position is self-aligned to the chip. The chip thickness is compensated by the surrounding central layer, and a UTCP with flat topography (flat UTCP) is realized after top dielectric deposition.

A batch of daisy chain test vehicles was produced. The feasibility of the process flow is verified by optical and electrical measurements. The result shows 100 percent yield, which is much better than previous work. A thermal humidity test showed no significant degradation of the flat UTCPs after 1,000 hours.

High yield fabrication of flat UTCP is first shown. An innovative self-alignment lithography step is introduced to make a cavity in dielectric for chip thickness compensation by using the chip itself as a photo-mask.

The purpose of this paper is to demonstrate electromechanical properties of a new stretchable interconnect design for “fine pitch” applications in stretchable electronics.

A patterned metal interconnect with a zigzag shape is adhered on an elastomeric substrate. In situ home‐built electromechanical measurement is carried out by the four‐probe technique. Finite element method is used to analyze the deformation behavior of a zigzag shape interconnect under uniaxial tensile loading.

The electrical resistance remains constant until metal breakdown at elongations beyond 40 percent. There is no significant local necking in either the transverse or the thickness direction at the metal breakdown area as shown by both scanning electron microscopy micrographs and resistance measurements. Micrographs and simulation results show that a debonding occurs due to the local twisting of a metal interconnect, out‐of‐plane peeling, and strain localized at the crest of a zigzag structure.

In this paper, the zigzag shape is, for the first time, proven as a promising design for stretchable interconnects, especially for fine pitch applications.

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