Arbitrarily Shaped Rigid and Smart Objects Using Stretchable Interconnections

In this paper, we present the developments of a new technology to realize 2.5-/3-D electronic circuits. The starting point is an electronic circuit embedded in a hard thermoplastic material, which is transformed into a 2.5-/3-D shape using a thermoforming process. The electronic circuit consists of stretchable copper interconnections in combination with electronic components. The circuit is produced using conventional printed circuit board process steps: a copper sheet is laminated on a thermoplastic material, such as Polycarbonate (PC) or Polyethylene Terephthalate Glycol (PET-G). The copper is structured using lithography and wet etching, and the electronic components are assembled using conductive adhesives. We investigated the use of different designs for stretchable interconnects to overcome the stretching of the electronic circuit by thermoforming it. A semispherical-shaped mold was used for this purpose where the local strain of the substrate can reach 100% with a sharp bending. It was shown that meander-shaped interconnects could accommodate the gradual deformation during the forming of the substrate either for PC and PET-G. However, at the mold's edge, the copper-meandered tracks showed a delamination of the copper from the substrate. In the meanwhile, the electrical conductivity of the copper tracks did not show any significant variation. Another meander design based on an elliptical shape (in order to increase the length of the track) is proposed and compared with the first design.