Design of Microstrip-to-Microstrip Transition for High-Density Flip-Chip Interconnects

In this paper, the challenge of the microstrip-to-microstrip (MS-to-MS) transition for high-density flip-chip interconnects was addressed, along with a solution. When it uses a Ms-to-Ms transition, it will produce strong resonances between the on-chip and off-chip metal grounds, resulting in subpar transmission performance. In addition, electromagnetic compatibility problems will arise from coupling between RF and IF interconnects. To solve these issues, a U-shaped slot on the off-chip ground in conjunction with shorting bump balls is proposed. 18 flip-chip interconnects were used in a 2x2 mm RF chip, with 4 for RF, 4 for IF, and the remaining 10 for supply and control voltages. Good transmission performance is demonstrated with the proposed structure at RF (55–65 GHz) and IF (1–5 GHz) bands. In the meantime, there is a significant suppression of coupling between RF and IF interconnects.