Enabling high-performance RF designs using Z-interconnect: Materials, manufacturing and electrical performance

More and more board and chip package RF designs require high-signal density while delivering frequencies in the tens of gigahertz. The challenges for organic substrate in meeting these electrical requirements include using high-speed, low-loss materials, manufacturing precise structures, and making a reliable finished product. In addition, many RF systems have mechanical and environmental requirements like lightweight and low- moisture absorption. This paper describes a procedure used to fabricate RF circuits in a package composed of laminated substrates. A highly versatile laminated structure is enabled by the presence of Z-interconnects that are included between the layers. These interconnects are normal to the surfaces of the substrates which is considered to be the 'Z' direction. Such Z-interconnects hold promise for increasing signal density at microwave frequencies in a given package. The manufacture of Z- interconnect structures involves building mini-substrates of 2 or 3 layers each, then assembling several mini- substrates together to make the finished product. Final lamination includes connecting metal layers vertically, using a conductive paste. Designing and manufacturing the mini-substrates separately makes it possible to reliably manufacture substrates with no via stubs, very low-loss materials, nearly arbitrary transmission line structures and versatility in optimizing features to reduce signal loss. Examples of circuits, that were fabricated using the Z- interconnect process, are given and test results shown. These circuits are tested for electrical performance consisting of S-parameter measurements. Equivalent low-loss performance to typical ceramic substrates will be shown up to 25 GHz. These results compare the use of the technique with liquid crystal polymer (LCP) substrate material with the use of Teflon-based material. Teflon- based circuits are compared to low temperature co-fired ceramic (LTCC) substrates. Due to its design flexibility, lighter weight and hermeticity, the organic-material based Z-interconnect is shown as an improvement over ceramic substrates.