impedance - When to use a special high frequency PCB material and not Fr-4? Rogers

Based on this article, fr-4 can usually be used up to 7 GHz. Above 10 GHz losses become too high and a special high frequency (HF) material should be used (e.g. Rogers).

Advantages of a special material:

• control of Er over a broad range of frequencies and from batch to batch,


• lower dielectric losses at HF.

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1. Can anyone please share experience (give a broad guidance) at what frequencies an HF material should be used and when it is a must? Rogers, as a most common, is of a particular interest.


2. How much is approximately lost due to dielectric losses (leakage) in Fr-4?

Appendix 1
Figure: Comparison of losses FR-4 vs Rogers. Source - the same article.

Appendix 2

My particular situation is a PCB with a 1 GHz square signal (thus, 5 GHz harmonics and higher should be present) and I am contemplating whether to buy an expensive Rogers or not. I can tolerate an insertion loss not greater than 10% (~0.5 dB) per inch within the 1-5 GHz range. The FR-4 the foundry provides does not have specs above 100 MHz - so, I can either use it at my own risk or use Rogers (which has specs in the needed range).

Appendix 3

Answer like "see the datasheet" are not very welcome. I've already done.

Answer

FR-4 really only means the material is flame retardent; that may seem an oversimplification, but there are many varieties of FR-4 that are specified to 10GHz.

As a couple of examples, see Isola 370HR; this is a good general purpose laminate that I have successfully used with 5Gb/sec signalling rates. Above that, the variation of dielectric constant with frequency starts to make ISI troublesome over any reasonable distance.

For higher speeds, I have used 408HR as it has a very flat dielectric constant across frequency (to say nothing of a much lower loss tangent); there are numerous FR-4 type laminates with quite high performance for the simple reason you cite: Cost. I have used this particular laminate with signals (differential) of 10Gb/sec.

There are numerous suppliers, and all have high speed offerings.

Interestingly, many flexible materials have superior performance in terms of loss tangent.

Exotic materials have their place, but more usually they are found in microwave equipment.