Humidity-Dependent Loss in PCB Substrates

摘要

Increasing operating frequency of IO busses for computing has highlighted the importance of transmission line loss. For FR4 PCB mother boards computing at speeds above 1 GHz this is dominated by dielectric losses. FR4 dielectrics are epoxy based materials which absorb moisture and are subject to dielectric loss variation due to absorption of ambient moisture. Resonator measurements of the substrate alone are useful for modeling but they do not take into account PCB design variables that influence absorption rates. Transmission line loss data and analysis obtained from two PCB designs subjected to a program of drying and moisture absorption are presented. A model to predict absorption rates and transmission line loss is presented. FR4 materials will be compared and contrasted with a low-loss substrate material.