Over-molding of flexible polyimide-based electronic circuits

With the increasingly high demand for flexible microsystems, their embedding technologies have become more and more important. In this paper, we present the developments of over-molding technology to realize polyimide-copper (PI/Cu) electronic circuits embedded in plastics. The circuit consists of Cu interconnections on a PI substrate with assembled electronic components, which is inserted in a flat mold of a tensile specimen shape in an injection molding machine. This substrate is over-molded on one side with polyamide-6 (PA6). We have investigated two main properties of the circuits: the adhesion with the over-molded plastic and the effect of the over-molding on the electronics, measured after the molding process. Different Cu cladding foils were used in this study to optimize the adhesion between the foil and PA6. It was shown that the adhesion between the PA6 and the PI-Cu foil with acrylic as an adhesive layer is superior compared to the adhesive-less and the epoxy-based foils. Moreover, to check the electronics, 0 omega resistors were assembled to evaluate the influence of over-molding on the electronics interconnections. The measurements of the acrylic foil were taken in three steps: before over-molding, after partial over-molding process without plastic injected, and finally after a complete over-molding cycle with plastic injection. The data presented showed that the fabrication process is suitable to integrate electronic components via over-molding and still have a functional electronic circuit on PI-Cu foil. Additionally, simulations were done to prove the same concept on a theoretical base, showing that the resistors should be functional after over-molding. Finally, we demonstrated our findings with a simple functional circuit where Near-field communication (NFC) etched antenna and a chip can wirelessly power assembled LEDs. Moreover, a long-term reliability test was performed and proved that such test structures could be used in long-term applications.