Which PCB substrate to use for 2 - 8 Ghz circuits?

I will design Microwave Mixer and Microstrip Filters in the 2-8 Ghz domain. Which substrate is most widely used for such designs? Taconic, Rogers??

Hi!

Rogers RT duroid 5880 is a very commonly used substrate. It has very uniform properties and low loss tangent.

Good luck!

It, as always can depend on your requirements.

There are a wide variety of materials out there and they are not created equal.

Here are the things to think about:

Cost : Materials for RF tend to start at 2x the base cost of FR4. Plus different materials can cost different to process.

Dielectric Constant: Since the dielectric constant effects the width of microstrip lines it will have a big effect on the abilty for a design to be buildable. Thinner lines and small spaces of resonant structures, paricularly filters, can cause more variation then you can accept. It also detmines how big the circuits are.

Loss Tangent: I have not had to worry about material loss that much but in some applications it can be imporant.

Thermal Expansion: If you want to mate different materials this can be a big deal. Say if you put a layer of a material and use FR4 on a different layer and the expasion does not match the board could delaminate.

Humidity: PTFE based materials have the ability to hold quite a bit of water and that will effect the electrical performance.

Fire Retardant: some RF materials do not meet some fire retardance requirements.

Given the circuits I would suggest you look at Rogers 4000 or Taconic RF-35.

Teflon based substrates like Rogers RT duroid 5880 mentioned above are classic teflon based microwave substrates. The material is expensive and only a few specialized PCB manufacturers can process this material.

For circuits that are cost sensitive, I like the Rogers series 4000 materials (RO4003). These have good high frequency properties (low loss) and compared to teflon based substrates, they are less expensive and easier to process. Many PCB manufacturers are able to process these materials because they are compatible with the "normal" PCB via process.

High Frequency Laminates

I recommend RO4350B.