By Eric Bogatin, Mike Justice, Todd DeRego and Steve Zimmer, published in the proceedings of the IPC Expo, Long Beach, CA, April 1998
Controlled impedance boards are proliferating in the industry. To help meet the more stringent design demands of these boards, the most versatile tool for the fab engineer is a field solver. With this tool, the design rules can be established, second order effects can be identified or ruled out, and a tolerance analysis can be performed. It is critical to the fabrication engineers to have confidence in the tools they use to establish the geometrical design rules for high-speed boards. We report here general techniques for using a field solver and methods to verify their accuracy using exact analytic expressions and test coupons. We find that the absolute accuracy can be better than 0.5% against analytic expressions and better than 6.5% against actual boards.  This limit is more dependent on the uncertainty in the knowledge of the geometry and material properties of the test structures than the accuracy of the tool.