FR4 is a very popular material for PCBs. It is low cost and robust and is used for a multitude of applications. However, it does have some speed limitations when it comes to its use as a substrate material for high speed digital circuits ( as well as analog circuits. The analog and RF circuit limitations are presented in another post). An estimate for the upper limit of operation of logic clocks is presented in a very brief note in the Signal Processing Group Inc., website located at http://www.signalpro.biz. The note can be found in the “Engineer’s Corner”.