By Eric Bogatin, presented at DesignCon 1999 Controlled impedance circuit boards are the norm. Establishing the design rules for the PCB stack up is often hit or miss. Whatever rule of thumb a designer uses, fab vendors have their own fudge factors to add. Often these are based on empirical results based on building test boards.  The IPC tried to assist in the design process by publishing the IPC-2141 Controlled Impedance Circuit Boards and High-Speed Logic Design guide. They offer analytic approximations for common PCB stack-ups such as stripline and microstrip. However, they beg the question, under what conditions can these equations be accurately used? In this paper, these recommended approximations are introduced and compared to the results from field solver calculations that have been verified to be accurate to better than 1%.