Dielectric constant of PCB material - Page 1
Forum section: electronics / general

author: geggi1
author_rank: Frequent Contributor
timestamp: January 31, 2015, 01:41:23 pm
content: Is there a simpel way to determine the dielectric constant of PCB materials?I'm planning to make some microstrip RF pcb.From different information i have found on the net i know that it is frewuency dependant.I have acess to different kinds rf testgear.

author: Howardlong
author_rank: Super Contributor
timestamp: January 31, 2015, 02:38:08 pm
content: You could make a capacitor out of a piece of double sided copper clad board made form the substrate, and knowing the area of the "plates" and the thickness of the substrate, calculate its dielectric from measuring the capacitance.

author: nctnico
author_rank: Super Contributor
timestamp: January 31, 2015, 05:15:05 pm
content: You can look up the specs of the material at the PCB producer. The dielectric constant doesn't depend on the frequency but the loss in the material does. If the application has to be very precise go for Rogers material otherwise FR4 is good until several GHz (assume er=4.5). When working with microstrips also consider the tolerances of the trace widths. If you get close to the minimum trace width the manufacturing tolerances will screw things up.

author: geggi1
author_rank: Frequent Contributor
timestamp: January 31, 2015, 05:59:23 pm
content: I got a big pile of PCB material FR4.From different sources on the internet i have read that the constant is different from whom have produced the PCB material.Other sources also indicate that the constant will change with frequency.

author: tggzzz
author_rank: Super Contributor
timestamp: January 31, 2015, 07:32:10 pm
content: Don't forget that for 4-layer boards you also need to know the thickness and er of the prepreg.

author: free_electron
author_rank: Super Contributor
timestamp: January 31, 2015, 08:15:49 pm
content: there is other factors at play. the 'weave' of the material is important . the higher you go in frequency the biggger the impact of this is.the fiberglass strands create discontinuities int he dielectric constant.there are two kinds of fibers ( i forgot the name) one has abigger impact than the other.how they are bundled, how thichk the strands are and how many strands per linear meter all has an impact.also the wall of the trace is never vertical but sits under an angle (due to the etching). this also creates problems in the far rf.

author: xygor
author_rank: Regular Contributor
timestamp: February 07, 2015, 12:42:47 am
content: See"PCB Stackup Design Considerations for Altera FPGAs"http://www.altera.com/literature/an/an613.pdf"PCB Dielectric Material Selection and Fiber Weave Effect on High-Speed Channel Routing"http://www.altera.com/literature/an/an528.pdf

author: IconicPCB
author_rank: Super Contributor
timestamp: February 07, 2015, 08:16:18 am
content: We have fabricated various RF designs in Rogers ceramic loaded material.Planar strip line to multilayer cavity structures.Mechanical milling with a resolution of 5 micron offers 100GHz upper limit .

author: steve_w
author_rank: Regular Contributor
timestamp: February 10, 2015, 10:09:14 am
content: Have a read of these papers:http://www.wetterlin.org/sam/SA/Operation/PCB_Diel_Const.pdfhttp://www.wetterlin.org/sam/SA/Operation/FringeEffects.pdfregardsSteve W

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: February 11, 2015, 07:49:35 am
content: I think it is easier to use substrates with known loss parameters such as Rogers Duroid 5880. Sometimes you can get it on Ebay for about $50 for single Letter size sheet - search for "duroid" or "rogers laminate". It is soft PTFE -based substrate that can be cut with strong scissors. What you need is an aluminium/copper plate to which you can bolt your prototype board and SMA connectors also called launch blocks. See attached PDF for more info.Which design tool you are using? RF Design tools such as Agilent ADS or Microwave Office already come with libraries that have all dielectric constants for commercially available materials.I can imagine that if you know the exact size and thickness of copper of your microstrip you can calculate parameters of dielectric in reverse if you know impedance (real and complex) of your microstrip. To measure that at RF frequencies you will need access to network analyzer.

author: steve_w
author_rank: Regular Contributor
timestamp: February 11, 2015, 11:18:30 am
content: Alexei,Good advice, I liked the PDF, brings back memories of my student days in the 90's, the only thing missing is PUFF :-)regardsSteve W

author: nctnico
author_rank: Super Contributor
timestamp: February 11, 2015, 05:12:08 pm
content: The accuracy of the information largely depends on the frequencies of interest and manufacturing tolerances. IMHO the best way is to create a design which can deal with tolerances and some loss instead of making a circuit difficult (=expensive) to produce.