Novel multilayering technique using folded flexible circuits

Multilayer circuit board technology offers the only feasible way to route high-density conductors through a narrow space. This technology has two limitations: the rigid multilayer circuit board typically used is not always compact enough for the application, and it is expensive. In this paper, we present a less expensive alternative that additionally achieves the required compactness: flexible circuits that are folded in a unique and organized pattern. Flexible circuits have the unique property of being three-dimensional (3-D), that can be shaped in multiplanar configurations and reinforced in specific areas for specific applications. Rearranging the two-dimensional layout of conductors on rigid circuit boards to the 3-D configuration of flexible circuits results in significant compactness. The compactness results from a structured folding of several parts in complementary directions, which allows intermediate portions to be stacked on each other. This technique also makes it possible to increase the conductor density on multilayer circuits by eliminating the need for electroplated holes called "vias." The final folded multilayer circuit is fabricated from a single flexible circuit comprising a series of foldable strips, rigid and flexible, which are interconnected, folded, and bonded into a composite structure. This folding greatly reduces the physical area occupied by the conductors associated with a planar circuit board by slightly increasing the thickness. The slightly increased thickness does not impinge on space needs for other purposes because ample space is available in the thickness dimension for heat to dissipate from the circuit board. In our application, we are able to save 67% of the unfolded area and additional saving is possible, depending on the application. The folded circuit can be bonded after the first fold, or folded subsequently to reduce its area further. It can then be bonded with glue as a composite multilayer structure.