The flip-chip package has been widely developed and used in cutting-edge system-in-packages (SiPs). It might provide better electrical stability and performance for DC, AC, and even thermal connections. RF transition is often accomplished using coplanar waveguide (CPW) lines with G-S-G connecting structures. Bump balls are used in the G-S-G interconnection structure to connect the CPW lines on and off the chip. The pitch distance between two bump balls must, however, typically be at least twice as large as their diameter in flip-chip packages for a G-S-G interconnection arrangement to fit.  Applying this CPW-to-CPW transition to sophisticated and quickly evolving phased array and MIMO scenarios is problematic. These scenarios call for the integration of numerous RF and IF connections in a compact device. The MS-to-MS transition is a viable technique for achieving high-density flip-chip interconnection, because it only utilizes one bump ball for each interconnection. As a result, compared to CPW-to-CPW neighbor, the distance between two interconnects may be significantly shortened and more interconnects could be integrated in a single die. Correspondingly, this structure has been investigated and some fresh issues have also been identified, including a resonance that impedes signal transmission. Therefore, we studied the problem and challenges of the MS-to-MS transition in a millimeter-wave RFIC. Then, a solution of the MS-to-MS transition for high-density flip-chip interconnects was presented. More details…