The PCB consists of layers of metal and insulator and can consist of several layers. Examining some common elements of a PCB will help the reader understand what many people believe is “Black Magic”.
Copper traces are utilized to connect one element node to another node. The shape of these traces determine one very important aspect of a PCB – the characteristic inductance, capacitance, and ultimately the characteristic impedance. Resistance is generally ignored as most designs do not carry more than several mA of current and the results can often be negligible.
Characteristic impedance (Z0) was covered previously, so this will not be discussed here. But what is important is the inductance and capacitance as determined by the trace dimensions and the PCB dielectric (εr). FR-4, probably the most common PCB material used by manufacturers today and has a permeability range normally from 4.0 to 5.0, but 4.5 is often used as a typical permeability. Check with the PCB manufacturer to determine what material they utilize and the associated permeability.
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