Concept and Process Development Using Ex Situ Fabricated High-Density Interconnect Plugs for Circuit-Board Embedded Magnetic Components

Magnetic circuit components have long been the largest and most difficult components to miniaturize. Printed circuit board (PCB) fabrication methods cannot achieve small diameter, high aspect ratio vias for high winding turn numbers to achieve high inductance, and microelectronics methods cannot achieve the high-quality and thick ferrite core materials. In this article, we present an alternative breakthrough method of printed circuit fabrication to significantly reduce the effective via diameter for embedded magnetic devices, boosting the inductance per unit area. To reduce the effective via diameter, high-density via plugs are ex situ fabricated and shaped, and then embedded into PCB substrate in the same process as standard magnetic core embedding. The structure of the plug is such that precise placement within the substrate, or alignment to the plug with the winding pattern, is not necessary. Devices presented in this article use via plugs with an effective diameter of
$40 \mu \text{m}$
and a theoretically unlimited aspect ratio, though refinements in the plug fabrication process are expected to reduce the via diameter down to
$10 \mu \text{m}$
. The process is experimentally demonstrated with a 14-turn racetrack-shaped transformer; novel diagnostic methods using magnetooptic garnet films to image current paths are also demonstrated.