Optimized Castellated Hole Interconnects for Ceramic-Based Modular Millimeter-Wave Applications Up to 85 GHz

This paper investigates the application of castellated holes, or Plated Half Holes (PHH), as RF interconnects for modular millimeter-wave designs by integrating them on low-temperature co-fired ceramics (LTCC). This work explores the electromagnetic capabilities of such transitions through a combined approach of full-wave simulation, simplified modeling, and measurement. A key objective was to investigate how much a detailed HFSS model could be simplified, both geometrically and conceptually, while still providing reliable results about the original structure's high-frequency performance. Capacitance values were estimated and calculated from simplified geometries and compared to simulation data. While no precise quantitative agreement is expected or required, the simplified models were sufficient to identify dominant effects, understand design tradeoffs, and guide parameter optimization. Empirical correction factors were introduced to improve correlation and to create a tool for rapid estimation for design optimization. An example LTCC application achieves insertion loss above −0.5 dB up to 85 GHz and above, as confirmed by full-wave simulations. Test structures were designed and fabricated, showing a general proof of concept of manufacturing castellated holes on LTCC technology in the desired geometrical dimensions to ensure efficient hybrid stack-up integration and high-performance RF interconnects.