Application of trace impedance monitoring on production quality control

Impedance control has been widely applied to PCB and IC packaging substrate manufacturing for better interconnect signal quality. In the IC packaging manufacturing using organic dielectric materials for the substrate, we have already found that a post-simulation with cross section geometry can give us an ¿effective dielectric constant¿ to bridge the gap between the simulation and the measurement of the characteristic impedance. We also found that the impedance monitoring not only can be a tool to control the electrical property of the products, it can also help non-destructively to control the quality of the production such as the uniformity of the dielectric thickness. In this report we show an example of the impedance-monitored production. The dielectric thickness-induced impedance variation has been investigated. The difference between two units in a same panel was pointed out. The impedance values of a specific trace set in all the units of the production panel were shown to have a clear correlation with the dielectric thickness variation. The impedance monitoring has proven to be an effective non-destructive method to provide direct information for the quality control of substrate or PCB production.