In this paper, Dr. Eric Bogatin reviews the need for controlled impedance boards and how to use the combination of 2D field solvers and TDR measurements to efficiently design the stack-up for controlled impedance. An important ingredient to accurate design is an accurate value of the dielectric constant. He shows how to measure the Dk of any board using a simple transmission line test structure, a TDR and a 2D field solver. This technique can be applied to any board.