A broadband 3D vertical Microstrip to Stripline transition in LTCC using a quasi-coaxial structure for millimetre-wave SOP applications

This paper presents a broadband microwave vertical transition structure suitable for System on Package (SOP) applications at millimeter-wave frequencies (mmW). The designed structure includes a Microstrip (MS) to Stripline (SL) back to back vertical transition optimized for 60 GHz applications but also suitable for E-band frequencies as well. The mmW transition presented here was fabricated in a Low-Temperature Co-fired Ceramic (LTCC) process using commercially available Ferro A6M tape layers. Design and optimization of this microwave transition was carried out in order to maintain a constant characteristic impedance over the entire bandwidth. A bandwidth improvement of 30% is observed by optimizing the ground via fence of the vertical transition. A maximum of 12 surrounding vias were used due to limitations in the LTCC process. Measurements of this vertical transition show a Return Loss of better than 10 dB for frequencies as high as 90 GHz while maintaining an insertion loss of less than 0.9 dB. A grounded coplanar waveguide (CPWG) to MS planar transition with good electrical performance for up to 110 GHz was also designed and implemented to characterize the vertical transition fabricated.