The flip-chip bump interconnection for millimeter-wave GaAs MMIC

The flip-chip bump interconnection structure has become popular for microwave and millimeter-wave package applications. This structure is expected to provide higher performance and a low cost packaging method. This paper presents the results of an evaluation of flip-chip assembled radio frequency (RF) devices. A coplaner transmission line type GaAs monolithic microwave integrated circuit (MMIC) was mounted on an aluminum oxide (Al₂O₃) substrate using the flip-chip thermal compression method. The electrical performance (S-parameter and noise figure) was measured and the reliability of the interconnection was tested. The changing rate of the characteristic impedance (Zo) of transmission line on bare-chip caused by bare-chip surface proximity to a substrate was simulated by finite element method (FEM) analysis. Flip-chip bonding conditions were fixed to keep the gap that less than 1% of Zo changing rate and sufficient bonding strength for reliability of interconnection. The DC characteristics of a 30 GHz, 60 GHz and 77 GHz band low noise amplifier (LNA) were the same before and after mounting, and the RF performance of the assembled MMIC was the same as the bare-chip without packaging. However, the influence of underfilling was observed. When epoxy resin was injected into the gap between the bare-chip and the substrate, the frequency band of the MMIC shifted to the low side. The reliability of the bump interconnection was excellent. The interconnection resistance did not change in a temperature cycle (-55°C to +125°C until 1500 cycle) test