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Z-Axis Interconnections for Next Generation Packaging
Keywords: Z-axis interconnection, rigid-flex, Package-Interposer-Package
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. One substrate/PWB strategy allows for metal-to-metal z-axis electrical interconnection of sub-composites during lamination to form a composite structure. Conductive joints are formed during lamination using an electrically conductive adhesive (ECA). As a result, one is able to fabricate structures with vertically-terminated vias of arbitrary depth. This paper describes a novel Z-axis interconnection approach for the fabrication of next generation packaging. Specifically, metal-to-metal z-axis electrical interconnection among substrates (sub-composites) of varying size, or among flexible and rigid elements (rigid-flex), to form a single structure is described. The z-interconnect based structures offer many advantages over more conventional construction methodologies, 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. In addition, multilayer rigid-flexible 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. The processes and materials used to achieve smaller feature dimensions, satisfy stringent registration requirements, and achieve robust electrical interconnections are discussed. Performance characteristics, including both electrical and mechanical behavior of circuits with various levels of complexity will also be presented.
Rabindra Das, Principal Engineer
Endicott Interconnect Technologies, Inc.
Endicott, NY

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