Effective dielectric constant method for trace impedance control

Impedance mismatch induces signal reflections along with degradations in transmission lines. Those distortions of signals can result in electrical function failures in high frequency or high speed devices. Impedance control thus has long been developed in PCB and IC packaging substrate manufacturing for better interconnect signal integrity. During our PBGA substrate impedance controlled production, an intrinsic gap between measured impedance and impedance from post-simulation with measured geometry was found. An engineering approach to define ldquoeffective dielectric constantsrdquo and to bridge the gap between the post-simulations and the measurements has been reported. This engineering procedure has been proved to be convenient and useful in practices. The statistics of the effective Dk distribution is better than direct impedance compensation. The deviations were found to be approximately the same for different stacks and transmission line types. The processing-related factors, for example, the conductor-dielectric interface roughness deviated from the ideal 2D simulation geometry, have been discussed as the possible causes.