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38 The PCB Magazine • June 2015 technology, combined with the fabrication of the interconnections between the layers using via drilling and metallization by electroplating. Such a stacked UTCP is not flexible anymore, but an extreme degree of miniaturization is re- alized: in a package of about 300 µm thick, four dies can be embedded and interconnected [4] . 3. Stretchable Electronics The overall concept of a stretchable circuit is illustrated in Figure 5. The circuit is comprised of a number of rigid (or moder- ately flexible in some cases) component islands, where each island holds a single component or a limited number of components. The individual rigid compo- nents or component islands are interconnected, not by straight Cu lines, as in con- ventionally designed PCBs, but by means of meander shaped Cu conductors in- stead. Once the meanders have been embedded in a mechanically elastic circuit carrier, they can dynami- cally elongate (stretch) un- der mechanical forces and come back to their original state, without losing their electrical interconnection function during this stretch- ing. The meander shape in combination with the elastic carrier makes the conductor func- tion as a two-dimensional spring. Realizing the planar meanders using the stretchable mould- ed interconnect (SMI) technology developed by imec and Ghent University, requires only a minimal deviation from conventional PCB processing. The first steps in the SMI process flow are shown in Figure 6. A rigid carrier substrate is covered with a temporary adhesive, onto which a copper foil or polyimide-copper flexible lami- nate is mounted by lamination. The optional polyimide layer serves as support material for the copper meanders and for the component islands. Supporting the meanders with a PI car- rier has proven to greatly improve the mechani- cal reliability and operational lifetime of the copper meander circuits. Next, the samples go through a number of standard PCB manufacturing and assembly steps. Copper meanders, along with the stan- dard design interconnections on the compo- nent islands, are patterned by photolithography and wet etching. The polyimide support layer is structured by laser cutting. If necessary, sol- der mask is locally applied by screen printing, FLExIBLE AND STRETCHABLE CIRCUIT TECHNOLOGIES FOR SPACE APPLICATIONS continues FeAtuRe Figure 5: Overall concept of a stretchable circuit. Figure 6: First part of the SMI process flow.