I have a doubt about something:
Considering a 4 layers PCB where the configuration is as follow:
Toplayer / Prepreg / Ground Layer / Core / Power Layer / Prepreg/ Bottom Layer
Do I understand well that when performing impedance matching and width strip calculation on a micro strip on top layer (for an antenna strip for example), the calculation of W should be done using the dielectric constant of prepreg and H of prepreg being just below the top layer ?
When I ask our supplier what is the material used for prepreg and its related dielectric constant, they seems to be confused: one time they say FR4 and send the datasheet, another time they say 2116 but seems uncapable to give datasheet (hence I have no dielectric constan available and prepreg thickness calculation).
Best Answer
The height is the distance between the trace and the ground plane.
If you have a signal on the top layer, and a ground plane after the prepreg layer, it will be the thickness of the prepreg layer.
If the signal is on one side of the core, and ground plane on the other side of the core, then it would be the thickness of the core.
If you have a signal on one layer, and then the ground plane two layers away (e.g. Sig|prepreg|core|plane), then it will be the combined thickness of both.
As a side note, 2116 is an FR4 prepreg material. According to slide 6 of this presentation, it has a dielectric constant of roughly 3.6-3.8, although it seems to vary depending on the number of layers. For two layers, the next slide shows it goes up to about 3.9-4.3 which is about the same as an FR4 core.
I had a quick look, and several PCB companies I found use two layers of 2116 to build up the required thickness. So you can probably work on the theory of it being similar to the FR4 core. However in practice if impedance is that critical, you would make test pieces to measure and characterise the impedance.