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34 The PCB Magazine • June 2015 aged integrated circuits (ICs) and passive com- ponents are assembled onto the rigid PCB by soldering. For many applications, especially for mobile, portable, wearable and implant- able 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 miniaturisa- tion of the circuit reduces the presence of the system. A second approach is to transform the flat rigid circuit into a three-dimensional, con- formable variant, following the random shape of the object or body part onto which it is inte- grated. In this contribution, two original technolo- gies developed at imec-CMST are presented. The ultra-thin chip package (UTCP) technol- ogy embeds 20–30 µm thick chips in a stack of spin-on polyimide (PI) layers. Adding thin- film, fan-out metallization results in an ex- tremely miniaturized, lightweight and flexible chip package with a total thickness below 100 µm. Next to flexible electronics, a number of technologies for dynamically or one-time de- formable stretchable circuits are under develop- ment. 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. Although these technologies were not ex- plicitly developed for space applications, their unique features create the potential for use in this new application domain. Miniaturization through UTCP use and 3D integration through circuit random deformability significantly re- duces system size and weight, which is an im- portant advantage for space applications. An interesting point of view, further discussed in this paper, is the possible improvement in inter- connect reliability that these new technologies offer. Thanks to the embedding in elastic mate- rials, stretchable circuits could show a decreased sensitivity to vibration. UTCPs can be embed- ded in flexible or rigid PCBs using lamination, through-hole drilling, and via metallization. UTCP production and PCB embedding is com- pletely solderless, thus avoiding associated reli- ability problems, usually encountered in harsh environments. The following two sections describe the pro- cess flow and application examples for flexible chip packaging and stretchable electronics. Sec- tion 4 discusses the advantages these technolo- gies can offer for space applications. 2. Flexible Chip Packaging One of the main drivers in packaging re- search is to integrate as much functionality into a single package as possible, without increasing the overall size of that package. The ultimate goal is a system-in-package (SiP), where both active and passive components are integrated, realizing a standalone (sub)system with a given functionality. One of the current challenges is to match this increase in functional density with improved flexibility and reduced overall thick- ness. A key aspect herein is the use of thinned bare-die Si chips to minimize the package form factor. Looking purely at the functional density, several SiP approaches can be applied to real- ize the high-density modules. Fan-out wafer- level packaging (FOWLP), where the package is reduced to its absolute minimum, and direct chip embedding are two examples of tech- nologies that offer increased functionality in a reduced form factor. A European representa- tive of the latter technology is described in the next section. The overall thickness of these packages, however, remains in the order of hundreds of FLExIBLE AND STRETCHABLE CIRCUIT TECHNOLOGIES FOR SPACE APPLICATIONS continues FeAtuRe Figure 1: Overall view of the ultra-thin chip pack- age (left) and close-up of the fan-out circuitry (right).

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