Dear Naveen,
Although this is a little late in the day, and you may have already decided a
PCB material for your RF design, but I was curious as to what kind of RF
application were you looking to use the PCB material for.
My understanding is that there are two kinds of resistivity - Bulk (Volume) and
Surface.
For a thin metallic sheet, we would get Sheet resistance (opposition to the
flow of surface current parallel to the surface) by dividing the surface
resistivity by the thickness of the sheet. This has units of "Ohm-Square"
although it practically is Ohms (the length dimensions cancel out), so as to
maintain discrepancy from Bulk resistance which also has units of Ohms.
The Volumetric (Bulk) resistivity is the opposition to the flow of current
perpendicular to the surface, through the volume of the conducting material.
For all practical purposes,
Volume Resistivity (Ohms cm) = Volume Resistance (Ohms) * Thickness of material
Surface Resistivity (Ohms cm) = Sheet Resistance (Ohms square) * Thickness of
material
For a thin material, the values of Volume and Surface Resistivity will be the
same.
Please note, the resistivity is the electric property of the material, while
the resistance is the derived property and depends on the length and
cross-section of the material considered.
PCB resistivity is seldom used when factoring out what material is to be used
for a particular application. Resistivity of the copper and thickness of the
copper on the other hand are more relevant variables - they will decide what
the Skin Depth is.
Yuriy Shlepnev has a fantastic Design Con presentation from 2016 that goes
through the Debye and Double Debye modelling of dielectrics.
https://www.simberian.com/Presentations/Tutorial_A_Material_World_Final.pdf
Yours sincerely,
Soumojit
Soumojit Bose
Hardware Engineer - Signal Integrity
Dell EMC | Purpose Built Technologies
Cube location: WS 2-503, 171 Hop 1, 2nd floor, pole B3
Phone (Office) +1 774 803 2100
Phone (Cell) +1 970 310 1458
Soumojit.Bose@xxxxxxxx
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx <si-list-bounce@xxxxxxxxxxxxx> On Behalf Of
Naveenchandra V
Sent: Monday, September 9, 2019 2:21 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Surface Resistivity-PCB materials for RF design
[EXTERNAL EMAIL]
Hi All,
I have a small doubt on selecting the PCB material for one of our RF design,
What would be the impact on Surface Resistivity when we designing a RF board? I
got two different resistivity values (such as 4.2 X 10^9 and 1.03 X 10^7) from
PCB vendor datasheet. However, DK and Df values almost similar. What values
should we consider? Higher or lower?. Could anyone please help me on this. It
would be very helpful.
Thanks,
Naveen
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