Investigation and characterization of millimeter-wave transmission lines and bandpass filters on liquid crystal polymer

As frequencies tend to increase for the next generation of wireless applications, the materials and integration techniques in RF system are experiencing more demanding performance constraints. Many of the materials commonly used for system-on-package (SoP) microwave circuit construction have shortfalls that limit their implementation for higher frequency applications, such as in the millimeter-wave applications. The industry standard for circuit boards, i.e., FR4, becomes dysfunctional due to prohibitively large losses in the high Gigahertz range. Low temperature co-fired ceramic (LTCC) has attractive electrical characteristics, dense multilayer circuit integration, and very good package hermeticity, but the cost is relatively high. One potential material that could address the needs for wireless systems built across a very wide frequency range is liquid crystal polymer (LCP). The low loss (tan delta = 0.002-0.004), near hermetic nature (water absorption 0.04%), and low cost (~$5/ft² for 2-mil single-clad low-melt LCP) make it appealing for high-frequency designs where excellent performance is required for minimal cost.