Low-Loss Impedance-Matched Sub-25-μm Vias in 3-D Millimeter-Wave Packages

This article presents, for the first time, low-loss small microvias in build-up layers for the next-generation high-density high-performance fifth-generation (5G) millimeter-wave (mm-wave) antenna-integrated packages. As the operating frequency increases, the signal losses in antenna packages become more critical and need to be mitigated to obtain desired performance in beamforming and massive multi-input multi-output. As electrical wavelengths in mm-wave spectra are short, the signal losses caused by microvias dominate the loss budget. To minimize the signal losses and identify the required microvia diameter in build-up layers, this article first focuses on the modeling of small microvias for impedance matching. Based on the models and simulated characteristic impedance, test vehicles with daisy chains are fabricated in build-up layers on a core package substrate. Ultraviolet (UV) laser is utilized to drill microvias, and targeted microvia diameters are obtained through a semiadditive patterning process. High-frequency measurements are also performed to correlate with the models and simulated results in the 28-GHz band. The characterization results exhibit good model-to-hardware correlation and indicate that small microvias (20 μm) provide impedance closer to $50~\Omega $ compared with the larger microvias. This matched microvia impedance lowers reflection and insertion loss, resulting in a 10% reduction in the signal losses caused by microvias in the 5G New-Radio n257 band around 28 GHz.