# Calculation of Microstrip Width

I am confused by the "substrate height" and "trace thickness" when I am calculating my microstrip width on here . From manufacturer, I got Does the "out layer copper thickness" refer to "trace thickness" and "inner layer copper thickness" refer to "substrate height"? The board is FR4 with thickness is 0.8mm. I am trying to get 50 ohm impedance. Thank you!

• this should align with the manufacturers plating up outer layer
– user16222
Jun 22, 2018 at 17:26
• @JonRB Thanks for your fast reply. But I am not quite sure what you mean. :(
– LFJY
Jun 22, 2018 at 17:27
• Substrate height is the thickness of the fibreglass, 0.8 mm. Jun 22, 2018 at 17:30
• @LeonHeller So that is the height of my PCB. Is that correct?
– LFJY
Jun 22, 2018 at 17:31
• If you add on the copper top and bottom. Jun 22, 2018 at 17:32

Inner copper width is applicable only to multilayer boards (more than two layers). The outer copper thickness is applicable to the two outermost layers. If you have a trace running inside the board on an inner layer, you would need to use the inner-layer copper thickness for the calculation. If your microstrip is running on either the top or bottom layer, then use the outer layer thickness. The substrate is the FR-4, and is its own thickness (core).

Here is an example of a 4-layer PCB stackup:

The "Top Layer" and "Bottom Layer" are your outer copper layers, so they would have the "Outer Layer Thickness". The "Prepreg" layers in the above image are a fiberglass weave that separates the "Top Layer" or "Bottom Layer" copper from the "Internal Ground Plane" or "Internal Power Plane" copper. The "Internal Ground Plane" and "Internal Power Plane" layers are the inner copper layers, so they have the thickness specified for that. The "Core" is the rigid fiberglass FR-4 that gives the PCB its strength.

You have not yet mentioned whether your board is 2-layer or multi-layer (more than 2). If it is only 2-layer then your "substrate height" in the calculator would be the thickness of the core itself (specified by the manufacturer). If you have a multilayer board your "substrate height" would likely be the thickness of the prepreg layer between the "Top Layer" and the "Internal Ground Plane" layer.

• Thank you! If I am using 2 layer FR-4 board, I do not need to consider the inner copper. In my case, "trace thickness" is 35um and the "substrate height" is 0.8mm. Is that correct?
– LFJY
Jun 22, 2018 at 17:36
• That sounds correct, assuming you are using a 0.8mm core. But if you're making a 2-layer board I would expect the overall board thickness to be 0.062" (1.67 mm) so I would think your core should be around 0.059" (1.5 mm) Jun 22, 2018 at 17:41
• Let me say that if you're working on a board that requires controlled impedance, it would really be best if you use a multilayer board with a full ground plane on an inner layer. It is much more reliable that way. It's much better than just having a poured polygon on the bottom layer, which gets broken up by tracks and component pads/holes. Jun 22, 2018 at 17:43
• Thank you for your advice. It is clear. For my 2 layer board, I actually only need one microstrip. It is tiny, so I am trying to make it 2-layer.
– LFJY
Jun 22, 2018 at 17:50
• Ok, just make sure there is a good copper ground pour on the bottom layer, directly beneath the microstrip Jun 22, 2018 at 17:51