Z-Axis Interconnection: A Versatile Technology Solution for High Performance Electronics

Abstract

This paper presents a novel Z-axis interconnect approach for extending performance beyond the limits imposed by traditional approaches. Specifically, metal-to-metal z-axis electrical interconnection among substrates (subcomposites) of the same or varying size, or among flexible and rigid elements (rigid-flex), to form a single structure is described. The structure employs an electrically conductive medium to interconnect thin coreless substrates. The substrates are built in parallel, aligned, and laminated to form a variety of multilayer high density structures including rigid, rigid-rigid, rigid-flex, stacked packages, or RF substrates. The z-interconnect based structures offer many advantages over more conventional build-up technologies. For example, it enables designs having increased wiring density, leading to greatly reduced layer counts. When an increase in metal layer counts is required, z-interconnect avoids the cumulative yield loss of sequential (build up) processing. The parallel processing of cores and/or subcomposites leads to reduced fabrication cycle time. Avoidance of through hole drilling allows for reduction or elimination of via stubs that cause signal attenuation at high frequencies. In addition, multilayer rigid-flex packages for a variety of applications are being developed. For these applications, z-interconnect allows for placement of flex elements into any layer of the substrate, the opportunity for multiple flex layers within a rigid-flex substrate, the ability to connect multiple multilayer substrates of varying size, and the ability to connect between any two arbitrary metal layers within the rigid region without the use of plated through holes (PTHs). The process allows fabrication of z-interconnect conductive joints having diameters in the range of 55–500 microns. Via or component pitches down to 150 microns have been demonstrated. A number of RF structures have been designed and built with z-interconnect technology, affording the flexibility to place wide signals, narrow signals and grounds and clearances only where needed. Electrically, S-parameter measurements revealed low loss at multi-gigahertz frequencies and the insertion loss for narrow, short lines and wide, long lines are similar. The electrically conductive adhesive used to form z-interconnects shows good signal transmission to 25GHz. Z-interconnect technology provides unique solutions for next generation complex packaging products.