Full Chip-Package-Board Co-Design of Broadband QFN Bonding Transition Using Backside via and Defected Ground Structure

Abstract

A complete chip-package-board co-design of bonding transition for a quad flat pack nonlead (QFN) package was conducted from dc to millimeter wave frequencies. First, two ground paths in parallel were used to improve the operating frequency of the commercially available QFN to 50 GHz. Owing to its rectangular cross section, ribbon bond has a better form factor than the corresponding round wire bond with the same dc resistance; it is therefore more effective in impedance matching, and can carry more current at high frequencies. Ribbon bonds were utilized to improve incrementally the frequency performance. Applying the -1.5-dB rule for |S 21 | and the -10-dB rule for |S 11 |, improvements are found to be 1.7 and 3.2 GHz, respectively. Second, QFN frequency performance was significantly improved by using an embedded DGS on the PCB. At 50 GHz, the transition was found to be excessively capacitive. A high-impedance DGS, being inductive itself, was used to compensate for the capacitive nature of the transition. The lumped element approach was taken to provide the background rationale how a DGS on the PCB ground can be adequately used to reduce the capacitive nature of the transition around 50 GHz. Indeed, from simulation at 50 GHz, utilizing the DGS helped to improve the impedance matching, and reduce the insertion loss, further extending the range of operating frequencies. Later, a full wave simulation of the DGS-compensated transition was conducted and the transition was experimentally validated. The full-wave simulated and experimentally obtained results are in good match. From measurements, when the DGS is used, the -1.5-dB rule for |S 21 | and the -10-dB rule for |S 11 | enable the QFN package to achieve the bandwidth up to 62.3 and 66 GHz, respectively.