Z-axis Interconnections for Next Generation Packaging

Abstract

In this paper, the use of electrically conducting adhesives (ECA) to form z-axis interconnections for next generation packaging is discussed. In particular, current efforts related to Z-axis interconnections for device level fabrication, integration, and electrical performance are highlighted. A few optimized ECAs were used for hole fill applications to fabricate Z-axis interconnections in laminates. Conductive joints were formed during composite lamination using the ECA. Around 5,000 to 200,000 through holes in the joining cores, formed by laser or mechanical drilling, and having diameters ranging from 50 μm to 750 μm, were filled with an optimized conducting adhesive. The adhesive-filled joining cores/layers were laminated with circuitized subcomposites to produce a composite structure. As a case study, a variety of z-axis interconnect constructions for a flip-chip plastic ball grid array package, rigid-flex, rigid-rigid, package-interposer-package (PIP), RF structures, and PWBs were fabricated and evaluated at both the subcomposite and composite levels to understand structural and electrical integrity. Electrically, S-parameter measurements showed very low loss at multi-gigahertz frequencies. The losses were low enough to support typical SERDES up to 15 Gbps over 750 mm. The present process allows fabrication of z-interconnect conductive joints having diameters in the range of 55 to 500 μm. The processes and materials used to achieve smaller feature dimensions, satisfy stringent registration requirements, and achieve robust electrical interconnections are discussed.