# Effect of surrounding material on microstrip impedance?

If you were to have a microstrip transmission line PCB trace, which then had a dielectric material run completely on top of the PCB track coverlayer. Would this surrounding material affect the impedance of the line, causing a need to change the width to accommodate? I see that by adding this dielectric material, it starts to look more like a strip line in that it is sandwiched by dielectric on either side.

This would be for a high frequency (GHz) signal.

Yes, any dielectric above will have an effect, although not a large one since most of the field is below the microstrip. Lots of calculators can do embedded microstrips, so you can see for yourself. For example, to pick one at random:

Edit: this calculator may not be accurate, check the one suggested below.

https://www.eeweb.com/tools/embedded-microstrip-impedance/

Adding 20 microns of dielectric (same as substrate) above the microstrip will change the impedance by about 3.5 ohms. Usually not significant, but not unmeasurable either.

• Thanks for answering. As this would be on top of the cover layer, I'm not sure this would exactly apply. As I imagine the distance would also play a factor Commented Oct 12, 2022 at 17:08
• @AdamMakin Height above does matter since the field is strongest closest to the conductor. Try punching in different height values into that calculator and you can get a rough idea of how much. Commented Oct 12, 2022 at 17:36
• I don't trust this calculator. Adding dielectric material above the trace should increase the capacitance and thus lower the characteristic impedance. But this calculator is showing the opposite effect. It also doesn't complain if you give it invalid inputs (like H2 < H1) Commented Oct 12, 2022 at 18:44
• Here's a calculator that gives the expected dependence between upper dielectric thickness and impedance: cecas.clemson.edu/cvel/emc/calculators/PCB-TL_Calculator/… Commented Oct 12, 2022 at 18:51
• Need to keep in mind that the dielectric constant of materials decrease with increasing frequencies, especially when you're in the GHz or higher region. So for these impedance calculators to give good results, you 1) have to adjust the Dk based on your expected frequency or 2) use a tool that asks you for the frequency of interest. Commented Oct 13, 2022 at 0:24