Investigation on Impedance Optimization for the BGA-Bonding Wire Transition Structure

Ball grid arrays (BGAs) and bonding wires (BW) are widely used in electronic packages, which may result in the impedance discontinuity. In this work, an optimization strategy, using the impedance conversion method for the transition structure codesign, was proposed. On that basis, a mathematical model for the transmission characteristics of a BGA-BW transition structure was developed, which predicts the impact of the various structural and material parameters on the electromagnetic wave reflection and transmission characteristic, such as the size of the solder ball, the type and number of the BW, and the frequency and the length of the transmission line (TL). A definite relationship was found between the frequency behavior and the length of the TL between solder balls and BWs, which can be used to determine the minimum value of the reflection coefficient. In addition, a 3-D electromagnetic field model was developed to verify the signal transmission characteristics of a BGA-TL-BW transition structure. Simulation results from both the mathematical prediction model and the electromagnetic field simulation model were validated experimentally. These results provide theoretical understanding and support for the application of impedance optimization to microwave circuits up to 35 GHz using BWs and BGAs.