An Additive Approach to Embed Chips in a Metallic Matrix Infused PCB

As microelectronics become more highly integrated, there is a considerable interest in embedding chips inside a printed circuit board (PCB). Embedding can reduce system footprint because the top and bottom sides of the embedded chips are still usable for other devices or interposers. If heat-generating chips are embedded in a good thermal conductor, external heat sinks/spreaders which occupy significant space and add weight may not be needed anymore. In this work, an additive approach to embed chips in metallic matrix inside a PCB is described. The metallic matrix, which is a commercial printable ink, is hardened by thermal curing to provide mechanical support for the chips. Simultaneously, the high thermal conductivity of the metallic matrix provides thermal diffusion pathways for heat-generating chips. Since the bottom and the side areas of the chips have direct contact with the metallic matrix the heat transfer is more efficient. The additive approach has been developed using two commercial printers: a penta head nScrypt and an Optomec aerosol-jet. The first step is to mount the chips face down on the UV-curable adhesive coating of glass carrier substrate. Pre-cut (1"x 1") FR4 with Cu clad on one side is used as the test PCB. The PCB, which has a circular cavity for hosting the chips, is then pressed against the carrier substrate and they are bonded together by the UVcurable adhesive coating. The metallic matrix is then printed to backfill the PCB cavity that encloses the chips. After it is thermally cured, the carrier substrate is detached from the PCB and thus exposes the active side of the chips that has the contact pads. The active side of the PCB (without the Cu clad) is then coated with an SU8 insulating layer. Prior to UV-curing the SU8 layer, graphene dots are printed to serve as a UV mask for the contact pads. This allows selective etching of the SU8 that covers the contact pads to create vias for electrical access to each contact pad. Interconnects to the contact pads are aerosol-jet printed using a particle-free silver ink. The SU8 coating serves as isolation between the printed interconnects and the metallic matrix, which serves as a ground. For demonstration, a positive fixed 5V voltage regulator in bare die form was employed as a test chip.