Stretchability : the metric for stretchable electrical interconnects

Stretchable circuit technology, as the name implies, allows an electronic circuit to adapt to its surroundings by elongating when an external force is applied. Based on this, early authors proposed a straightforward metric: stretchability—the percentage length increase the circuit can survive while remaining functional. However, when comparing technologies, this metric is often unreliable as it is heavily design dependent. This paper aims to demonstrate this shortcoming and proposes a series of alternate methods to evaluate the performance of a stretchable interconnect. These methods consider circuit volume, material usage, and the reliability of the technology. This analysis is then expanded to the direct current (DC) resistance measurement performed on these stretchable interconnects. A simple dead reckoning approach is demonstrated to estimate the magnitude of these measurement errors on the final measurement

Free-form 2.5D thermoplastic circuits using one-time stretchable interconnections

A technology is presented for the production of soft and rigid circuits with an arbitrary 2.5D fixed shape. The base of this technology is our proprietary technology for elastic circuits with a random shape, in which the elastic thermoset (mostly PDMS) polymer is now replaced by soft or rigid thermoplastic variants. An additional thermoforming step is required to transform the circuit from its initial flat to its final fixed 2.5D shape, but for rigid fixed shape circuits only one-time stretchability of the extensible interconnects is required, relieving the reliability requirements

Stretchable mould interconnect optimization : peeling automation and carrierless techniques

The primary bottleneck of the stretchable mold interconnect (SMI) technology is its reliance on carrier boards. These are necessary to handle the meandered circuit during production and to ensure dimensional stability of the flexible circuit board before encapsulation. However, for all the problems it solves, it also introduces a new major problem by requiring a peeling step – which is difficult to automate. This manuscript aims to present some of the work that went into eliminating this problem, discussing both unsuccessful and functioning methods to tackle this conundrum and some of the experimental work that went into verifying these techniques. First, alterations to the design to simplify peeling are considered, followed by adhesivebased peeling processes and mechanical pin-based systems. Next, masking and structuring of the carrier board adhesive are considered. Finally, two carrierless methods which circumvent the problems are discussed, a two-step process – which cuts temporary support structures after partial encapsulation – and a technique whereby the frame is designed to fail in a controlled manner during the first use of the circuit, creating a carrierless process feasible for high-volume production

One-time deformable thermoplastic devices based on flexible circuit board technology

This contribution describes an efficient process flow for production of one-time deformable electronic devices based on standard circuit board technology and demonstrates multiple devices fabricated using this technique. The described technology has the potential to streamline and simplify the production of complex user interfaces which typically require extensive mechanical design and many components. The employed technique allows for the production of complex 3D shapes without the need to modify existing circuit board manufacturing equipment or processes significantly. To achieve this the device is manufactured in a flat state, encapsulated in a thermoplastic polymer laminate and deformed afterwards. This allows the usage of standard electronic components in surface mount packages, which are assembled using lead-free high-temperature solder. The circuit is deformed using a high-volume cost-effective thermoforming approach, where the encapsulating polymer is heated above its glass transition temperature and forced against a mold where it is allowed to cool down again. To enable significant out-of-plane deformations stretchable meandering interconnects are used, which were traditionally developed for dynamically stretchable devices. Fabrication of the circuit starts using a standard flexible copper clad laminate which is processed using the default techniques, the resulting circuit is then attached to a carrier board coated with a reusable high-temperature pressure sensitive adhesive. The interconnect and circuit outline is then defined using laser routing or punching, cutting the flexible circuit without damaging the carrier. The residuals not part of the circuit are removed, in a process akin to protective film removal, and solder paste is stencil printed on the circuit. Afterwards components are placed using a pick-and-place machine and the boards are reflow soldered. After functional testing and repair (if necessary) the circuits are placed in a vacuum press with a thermoplastic laminate, consisting of a thermoplastic elastomer and a rigid thermoplastic sheet. During this lamination the components are protected by a highly conforming press pad. Because the adhesion between the elastomer and the circuit far exceeds that between the circuit and the carrier the circuit is released readily as the thermoplastic laminate is peeled away. The resulting laminate is built up further using thermoplastic films and sheets, and finally deformed using a vacuum forming machine. The resulting device, which is trimmed to remove the clamping edges, can then be mounted in the final assembly. The advantages of this approach are demonstrated using a series of test vehicles, demonstrating the integration of complex circuits, connectors, and power circuitry. Finally, a series of design considerations that became apparent after initial reliability testing are discussed, together with the resulting design rules

Thermoplastic electronic circuits : design, technology and characterisation

Elektronica en polymeren behuizingen gaan hand in hand. Jammer genoeg is het vaak de elektronica die de vorm van de behuizing bepaalt of de vorm van de behuizing die de functionaliteit van de elektronica beperkt. De afgelopen jaren zijn er meerdere mogelijke oplossingen voorgesteld om dit probleem aan te kaarten, maar vaak met grote beperkingen of een exuberant prijskaartje. Om dit op te lossen werd een radicale aanpak voorgesteld waarbij het elektronische circuit geïntegreerd wordt in de polymeren behuizing. Hierdoor ontstaat een grotere vrijheid om elektronische apparaten te ontwerpen en kan de interne ruimte benut worden voor andere functionele aspecten van het toestel. In dit onderzoek werd een technologie ontwikkeld die in staat is om een gewone flexibele printplaat te verwerken binnen in een thermoplastisch polymeer, waarna het volledig circuit door middel van thermovormen zijn uiteindelijke vorm krijgt. Hiervoor wordt het polymeer opgewarmd tot het een rubberachtige vloeistof is geworden, waarna het een nieuwe vorm krijgt door aangedrukt te worden tegen een matrijs. De ontwikkeling van deze integratietechnologie wordt toegelicht in detail, waarna de nodige stappen voor industrialisatie en karakterisering besproken worden

Design automation of meandered interconnects for stretchable circuits

Many of today’s stretchable interconnection technologies make extensive use of meandered interconnections, these provide two-dimensional in-plane springs capable of extending and compressing as the need arises. While a significant quantity of research work exists on these interconnects, few generic methodologies exist to design circuits in a consistent manner. This shortfall leads to questionable design practices, which can negatively affect reliability and performance. Presented within this paper is a method to quickly determine which meander fits within a given area based on common geometric boundary conditions