Analysis of high-speed package substrate without plating bars

Abstract

Plating bars must be designed to plate the copper on the wall of via-holes and the gold on the surface of wire-bonding pad. However, they provide the discontinuities and decrease the transmission efficiency, and so multi-GHz clock lines with them on package substrate can suffer reflection and cross-talk noise. In this paper, we demonstrate that package substrate without plating bars (WoPBs) is superior to package substrate with plating bars (WPBs) for the noise and transmission properties studied with S-parameter and eye- diagram. In order to minimize reflection noise, the structure of two coupled transmission lines is optimized using 3D full-wave electromagnetic field simulation and it is verified with time domain reflectometry (TDR). Quantitatively analyzing the noise of the worst-case coupling mode and the relation of the length between the signal lines and plating bars, the simulation is carried out for high-speed differential clock lines running parallel to another signal line. S-parameter and resistance-inductance-capacitance (RLC) from 3D model of three signal lines divided into the unit blocks such as bonding pads, the signal lines, via-holes and plating bars are extracted. The cut-off frequency, the noises and transmission properties are investigated as the length of the signal lines and plating bars by using extracted S-parameter from the unit blocks and time domain waveform, after the lumped circuit model is constructed with RLC. As the length of plating bar and the signal line is varied, the far-end cross-talk (FEXT) and the near-end cross-talk (NEXT) of low-speed signal line and high-speed differential clock lines is larger over the decuple. Reflection noise can be decreased over 10dB and WoPBs without plating bars improves output voltage and frequency bandwidth.