This question comes up in almost every advanced class I teach: what are the limits to FR4?
Like almost every important question in signal integrity, the answer always stars with “…it depends.”
The next step is to “put in the numbers” using analysis tools, based on all the assumptions and conditions for the specifics that should be considered.
In a long differential channel, if we do everything right, like no stubs anywhere, no asymmetry anywhere, no cross talk and no impedance discontinuities anywhere, the limitation for the highest data rate a channel can support is set by the signal to noise ratio (SNR) at the receiver.
While there are theoretical evaluations based on Shannon’s Information Theory about a channel’s information carrying capacity, its – 3dB bandwidth and the SNR at the receiver, there is an alternative analysis based on practical considerations.
If everything is done right in the channel, the fundamental limit to the data rate it will support will be set by the frequency dependent attenuation and how much signal is required for an acceptable eye . It’s not just the attenuation, it’s the frequency dependent attenuation. If we have roughly a linearly decreasing attenuation with frequency, much of this can be compensated using equalization techniques such as CTLE (continuous time linear equalization), FFE (feed forward equalization) or DFE (decision feedback equalization).
Typical high performance specs offer a limit of about –25 dB attenuation at the Nyquist frequency as the practical limit to what can be recovered in a usable eye. However, my buddies who work with optimized TRX equalization techniques tell me that if all the more than 10,000 coefficients available for the three equalization techniques are optimized perfectly, it may be possible to recover a usable eye with –40 dB attenuation at the Nyquist frequency.
Now we can ask, how far and at what frequency can you go in an FR4 interconnect and still have less than –40 dB attenuation? This is a simple analysis, which we go through in our S-parameters for SI (SPSI) class and Advanced Gigabit-differential Channel Design (AGCD) class. The result is a simple relationship between the length of the interconnect, in inches and the highest data rate, in Gbps, below which the attenuation will be less than –40 dB and an acceptable eye can be recovered. This relationship is:
This assumes the attenuation is limited to just dielectric loss and no conductor loss, which is the ultimate best that can be done. There is a distance-bandwidth trade off. This is fundamental and is the driver for transitioning to fiber optic connections at either high data rates or long distances. The boundary of when photons are more cost effective than electrons is set by this relationship.
Generally, the closer you get to this fundamental limits, the more expensive it becomes to implement a solution in copper and the more cost effective the solution may be in optical interconnect.
For example, in a 40 inch backplane, the ultimate limit to copper is about 20 Gbps. It is probably not practical to achieve 28 Gbps in a 40 inch backplane using a pulse amplitude modulation of two levels (PAM2), with an FR4 type material even with wide copper traces. Data rates above 20 Gbps using copper interconnects will require a lower loss laminate.
This estimate is not so far off from what is actually measured. Here are examples of the measured insertion loss for different length transmission lines in FR4 interconnects using wide conductors.
For the 40 inch interconnect, the frequency at which the insertion loss is larger than – 40 dB is about 10 GHz. This suggests the possibility of sending data at about 20 Gbps through this interconnect, close to what we estimated.
What’s the limit to copper interconnects in backplane applications? It depends. As a rough starting place, doing everything right, FR4 interconnects will limit out at about 20 Gbps in 40 inch backplanes. For higher data rates, and to have better margins, lower loss laminates will be in your future. It may be a possible to implement 40 Gbps backplanes in copper using suitable low loss materials.
There will be a limit to copper where it becomes more cost effective to switch to optical interconnects. I remember the days when folks suggested this limit was 2.5 Gbps. Then practical solutions in copper were developed. Then the limit was touted as 5 Gbps. But this was overcome. Then I heard the limit was 10 Gbps, but cost effective solutions were found.
As the cost of higher data rate copper channels goes up and the cost of optical channels comes down, they will cross and optical interconnects for 40 inches will be cost effective. I think this day is still in the future. To paraphrase Mark Twain, “the reports of copper’s death are exaggerated.”