Nano-micro filled conductive adhesive based 3D micro arrays for Z-axis interconnections

This paper discusses nano-micro filled electrically conductive adhesive (ECA) based 3D micro array interconnects to connect multiple electronic layers. Adhesives formulated using controlled-sized particles, ranging from nanometer scale to micrometer scale, were used to form micro arrays of contact pads having diameters ranging from 5 mum to 300 mum for internal and external interconnect applications. SEM and optical microscopy were used to investigate the micro-structure, and conducting and sintering mechanisms. This work also discusses micro-filled epoxy-based conducting adhesives modified with nanoparticles for low temperature sintering. A variety of nanoparticles ranging from 10-15 nm was used to modify micro adhesive composites. Addition of in-situ nanoparticles reduces sintering temperature without compromising electrical conductivity. Adhesives exhibited volume resistivity ranging from 10^-4 ohm-cm to 10^-6 ohm-cm, depending on composition, particle size, and loading of the adhesives. It was found that with increasing curing temperature of the adhesive, the volume resistivity decreased due to sintering of metal particles. Adhesives modified with nanoparticles or in-situ nanoparticles showed 30-90% resistance drop when cured at 240-265 degC instead of 200 degC. A few optimized metal-epoxy adhesives 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 mum to 300 mum, were filled with an optimized conducting adhesive. The adhesive-filled joining cores were laminated with circuitized subcomposites to produce a composite structure. High temperature/pressure lamination was used to cure the adhesive in the composite and provide Z-interconnection among the circuitized subcomposites. A variety of joining core- and subcomposite structures such as OS/IP, 0S/2P, 2S/1P, 2S/2P were used for hole fill applications. As a case study, an example of nano-micro filled conductive adhesives used in a z-axis interconnect construction of a 26 metal layer package with a 300 mum pad pitch is given. The present work describes processing of conductive adhesives used to achieve smaller feature dimensions, satisfy stringent registration requirements, and achieve robust electrical interconnections.