Choice of PCB material and thickness @ 5GHz

[pauloau]

rf pcb 5ghz

Hi,
I am designing an evaluation board to test an integrated RF power amplifier. This amplifier will work at 5GHz. The possibilities that I have at hand for the substrate of this board is FR-4 with the following thicknesses: 0.1mm, 0.2mm, 0.5mm, 0.8mm, 1.2mm... I know that thinner substrates will provide lower electrical field leakage but will also increase the capacitance of the transmission lines placed in the top of the board (with regard to the ground plane in the bottom of the board).
Does anybody has experience of designing PCB at this frequency and could tell me in practice which thickness would work better? Or should I forget about it and go directly for other kinds of laminate with tighter tolerances and lower losses?
Thanks a lot.

Paulo

 

[House_Cat]

5ghz pcb material

You don't choose the thickness of the material to control loss - you choose the thickness of the material to control the impedance of the transmission path for a particular trace width.

The losses in the dielectric are a function of the material, and not the thickness. High Tg FR4 resin is available with an Er of about 3.7. That is about as good as it gets with FR4. If you need lower dielectric loss, you can go to Rogers, Taconic, or GE for lower loss material.

FR4 will work OK for your application if the traces are short. Find the loss tangent, or Er, of the FR4 you will use on your board. Then you should compute how much additional signal loss you can tolerate from skin effect over the distances you expect on your board. Using the allowable total loss, calculate the necessary trace width to stay within your loss budget. Using the trace width, calculate the thickness of dielectric you need to give the required trace impedance. Iterate the calculation as necessary to get the trace width and dielectric thicknesses adjusted to fit the board space and material available.

 

[actra]

pcb material choice

FR4 will work ..but with higher losses...but whats good its very low cost...

try to choose fr4 with less thickness than 0.8 mm in order to reduce the microstrip line width...
0.8 for fr4 will give about 1.4 mm track to be matched with 50 ohm charactristic impedance..

 

[Element_115]

If this is just for a PA I would go with Rogers. They also have an online
impedance calculator which is pretty good
www.rogers-corp.com/mwu/mwi_java/mwij_vp.html

Cheers

 

[trigger74]

If your $$$ is enough... layout a set of microstrip lines and test the characteristics of the PCB using a network analyzer.