Flexible and Stretchable Circuit Technologies for Space Applications

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Flexible and stretchable circuit technologies offer reduced volume and weight, increased electrical performance, larger design freedom and improved interconnect reliability. All of these advantages are appealing for space applications. In this paper, two example technologies, the ultra-thin chip package (UTCP) and stretchable moulded interconnect (SMI), are described. The UTCP technology results in a 60μm thick chip package, including the embedding of a 20 μm thick chip, laser or protolithic via definition to the chip contacts and application of fan-out metallization. Imec’s stretchable interconnect technology is inspired by conventional rigid and flexible printed circuit board (PCB) technology. Stretchable interconnects are realized by copper meanders supported by a flexible material (e.g., polyimide). Elastic materials, predominantly silicone rubbers, are used to embed the conductors and the components, thus serving as circuit carrier. The possible advantages of these technologies with respect to space applications are discussed.

The driving application for flexible and stretchable circuit technologies is consumer electronics, especially handheld and mobile devices, which benefit the most from the reduction in form factor, the increased functional density and enlarged user comfort that is made possible with these technologies. Reduced volume and weight, increased electrical performance, larger design freedom and improved interconnect reliability are benefits that are also appealing for space applications.

Traditionally, electronics and sensor circuits are fabricated on flat rigid substrates, like FR-4 PCBs. In this conventional technology, packaged integrated circuits (ICs) and passive components are assembled onto the rigid PCB by soldering. For many applications, especially for mobile, portable, wearable and implantable electronics, the circuit should preferably be seamlessly integrated into the object that is used for transportation, is carried along, or worn on or inside the body. The electronics should be comfortable and unnoticeable to the user. In general, standard circuits do not fulfil these requirements. The user comfort can be increased in two ways. Extreme miniaturisation of the circuit reduces the presence of the system. A second approach is to transform the flat rigid circuit into a three-dimensional, conformable variant, following the random shape of the object or body part onto which it is integrated.

In this contribution, two original technologies developed at imec-CMST are presented. The ultra-thin chip package (UTCP) technology embeds 20–30 μm thick chips in a stack of spin-on polyimide (PI) layers. Adding thinfilm, fan-out metallization results in an extremely miniaturized, lightweight and flexible chip package with a total thickness below 100μm. Next to flexible electronics, a number of technologies for dynamically or one-time deformable stretchable circuits are under development. The stretchable concept is based on the interconnection of individual components or component islands with meander shaped metal wirings and embedding in elastic polymers like silicone rubbers (PDMS), polyurethanes (PU) or other plastics.

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Editor's Note: This article originally appeared in the June 2015 issue of The PCB Magazine.

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