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I am confused by the "substrate height" and "trace thickness" when I am calculating my microstrip width on here . From manufacturer, I got enter image description hereDoes 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!

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  • \$\begingroup\$ this should align with the manufacturers plating up outer layer \$\endgroup\$ – JonRB Jun 22 '18 at 17:26
  • \$\begingroup\$ @JonRB Thanks for your fast reply. But I am not quite sure what you mean. :( \$\endgroup\$ – LFJY Jun 22 '18 at 17:27
  • \$\begingroup\$ Substrate height is the thickness of the fibreglass, 0.8 mm. \$\endgroup\$ – Leon Heller Jun 22 '18 at 17:30
  • \$\begingroup\$ @LeonHeller So that is the height of my PCB. Is that correct? \$\endgroup\$ – LFJY Jun 22 '18 at 17:31
  • \$\begingroup\$ If you add on the copper top and bottom. \$\endgroup\$ – Leon Heller Jun 22 '18 at 17:32
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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:

enter image description here

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.

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  • \$\begingroup\$ 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? \$\endgroup\$ – LFJY Jun 22 '18 at 17:36
  • \$\begingroup\$ 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) \$\endgroup\$ – DerStrom8 Jun 22 '18 at 17:41
  • \$\begingroup\$ 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. \$\endgroup\$ – DerStrom8 Jun 22 '18 at 17:43
  • \$\begingroup\$ 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. \$\endgroup\$ – LFJY Jun 22 '18 at 17:50
  • \$\begingroup\$ Ok, just make sure there is a good copper ground pour on the bottom layer, directly beneath the microstrip \$\endgroup\$ – DerStrom8 Jun 22 '18 at 17:51

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