Versatile Z-Axis Interconnection for High Performance Electronics
Authors: Das Rabindra , Lauffer J. M. , Egitto F.D. 
Published: October 2015

The demand for high-performance, lightweight, portable computing power for next generation packaging is driving the industry toward miniaturization at a rate not seen before. Electronic packaging is evolving to meet the demands of higher functionality in ever smaller packages. To accomplish this, new packaging needs to be able to integrate more dies with greater function, higher I/O counts, smaller die pad pitches, and greater heat densities, while being pushed into smaller and smaller footprints. New packaging designs are emerging that require joining (stacking) of multiple packages, joining of different size packages, and flexibility and/or rigidity to accommodate requirements related to size, weight, and complexity. 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 (sub-composites) 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, an increase in metal layer counts without the cumulative yield loss of sequential (build up) processing, 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), allowing for increased wiring density, and reduction or elimination of via stubs that cause signal attenuation, In addition, multilayer rigid-flex packages for a variety of applications are being developed. Several classes of flexible materials that can be used to form high-performance flexible packaging are discussed. Materials, including polyimides, PTFE, liquid crystal polymer (LCP), have been used to develop multilayer rigid-flex packages. 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. The processes and materials used to achieve smaller feature dimensions, satisfy stringent registration requirements, and achieve robust electrical interconnections are discussed. 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-interconnect shows good signal transmission to 25GHz. Z-interconnect substrates provide unique solutions for next generation complex packaging. Collectively, the results suggest that Z-interconnect technology may be attractive for a range of applications, not only where miniaturization is required, such as consumer products, but also in high performance large-area microelectronics such as supercomputers, radio frequency structures, etc.