Many PCBs are specified to have a controlled impedance value. There are several circuit and material properties that impact the impedance performance of a circuit. Some of these properties are obvious to engineers who have worked with controlled impedance boards over the years. However, even experienced engineers are sometimes surprised to find the level of impact these properties have when looking at all of the things that influence PCB impedance performance. Additionally, there are several issues to consider when making impedance measurements to ensure the values are accurately captured.
As a reference, consider a microstrip transmission line circuit. When an impedance model is generated for a PCB microstrip construction, the different variables that influence impedance can easily be changed to see the magnitude each variable has for altering the impedance value of the circuit. There is a hierarchy of the influences of these variables on PCB impedance. For a microstrip transmission-line circuit, the most to least influential variables include substrate thickness, conductor width, copper thickness, and dielectric constant (Dk).
As a material supplier, we are often asked to investigate the Dk of our material when a circuit has an issue with incorrect impedance. In reality, the Dk is the least impactful variable for impedance. It is much more common for substrate thickness and conductor width to play a significant role in impedance variation. The substrate thickness concern can be an issue related to the copper-clad laminate and/or the PCB fabricator, depending on how the circuit construction. In general, the substrate thickness control is better for a microstrip circuit using the copper-clad laminate compared to a microstrip that a PCB fabricator might construct using a prepreg and copper foil for a foil lamination process. Due to the thickness control issue—assuming a multilayer circuit with a microstrip layer on the top or bottom of the structure—a circuit with a demanding impedance specification is usually best when made with a laminate from the material supplier as opposed to a foil lamination at the PCB fabricator.
Controlling the signal conductor width is more critical for circuits that use thin substrates because thinner substrates usually have narrower conductors. A normal conductor width tolerance is ± .5 mil, but this can depend on the copper thickness and circuit density. For a circuit using a thick substrate and a wide signal conductor, the common conductor width tolerance has less influence on impedance variation than a thin circuit with a narrow conductor. The impedance of a circuit using a narrower conductor is impacted more by the conductor width tolerance than a circuit using a wider conductor.
To read this entire column, which appeared in the October 2018 issue of Design007 Magazine, click here.