A chassis has to move high-speed signals between cards. A PCB backplane does this with copper traces, a cable backplane replaces the high-speed traces with twinax cable, and a midplane or orthogonal design shortens the path by changing where the cards meet. As speeds reach 112G and 224G, trace loss is what pushes designers toward cable and orthogonal layouts.
In short:
- PCB backplane traces lose signal quickly at high speed and need low-loss material or very high layer counts.
- Cable backplanes move the high-speed signals onto twinax cable, and keep power and slow signals on a standard backplane.
- Midplanes put the backplane in the middle of the chassis with cards on both sides. Orthogonal designs connect cards directly to one another at 90 degrees.
- Cable backplanes cost more and are mostly custom, but are used where performance matters more than cost.
For the PCB connector side, see our high-speed backplane connector guide and backplane systems engineering guide.
Why the PCB Backplane Runs Out of Room
Signals on PCB traces suffer insertion loss and distortion, and compensating for it uses more power. One industry article says this degradation becomes a design-limiting factor in high-performance computing systems. The usual yardstick is the IEEE 40 dB insertion loss limit for high-speed Ethernet channels, which covers the PCB traces and cable together.
A TE comparison for its cabled backplane puts numbers on it: 0.11 dB per inch for the cable against 0.75 dB per inch for a Meg 6 PCB at 12.5 GHz, and signal integrity maintained over distances 2 to 4 times greater. The same document says high-layer-count PCB backplanes using HDI structures need 20 to 30 fabrication steps and cost 5 to 10 times more than traditional PCB substrates. These are one manufacturer's figures for its own product.
Cable Backplane
A cable backplane takes the high-speed I/O off the board and runs it through twinaxial cable, which has better impedance control and isolation. Low-speed signals and power stay on a standard copper backplane. The cable is one cable per point-to-point connection, with a typical characteristic impedance of 85 to 100 ohms.
- Speed: connector families from Amphenol, Molex, TE and Samtec are described as rated to 224 Gb/s.
- Where used: very large systems with long point-to-point circuits, where performance matters more than cost.
- Serviceability: early designs were masses of loose cables that were hard to build and repair. Modular cassettes now handle this, but large cassettes are heavy and need strong guidance, float between mating connectors and tight rack tolerances. Field repair is limited, so manufacturers test 100 percent before shipping.
- Supply: cable backplanes are largely custom assemblies, though the connectors can be second sourced.
Midplane
A midplane moves the backplane to the middle of the chassis, so cards plug in from both the front and the back. The signal path between a card and the midplane is short. This is a common layout in switches and modular chassis.
Orthogonal Architecture
An orthogonal midplane removes the backplane. Front cards connect directly to rear cards at 90 degrees, so each front card can reach every rear card. One article notes that orthogonal layouts create challenges for card access and cooling airflow, while a TE document says cards at 90 degrees can improve thermal management. Check which applies to your chassis.
The Scale Example
One article reports that the Nvidia DGX GB200 NVL72 uses about 5,000 NVLink copper cables, roughly two miles in total, in four vertical cartridges. The same article says optical transceivers for the system's 2.4 million links would have added about 40 MW, so copper was chosen. It also notes that doubling bandwidth halves the effective reach of copper, so future systems will likely limit copper to about half a rack. These are reported figures, not from the manufacturer.
Choosing Between Them
- PCB backplane: lower speeds and lower cost. Check insertion loss against the channel budget.
- Cable backplane: 112G and 224G channels with long reach, where the budget allows custom assemblies.
- Midplane or orthogonal: shorter paths and a different cooling layout, useful when you can design the chassis around it.
- Mezzanine: stacking boards, covered in our mezzanine vs backplane comparison.
Selection Checklist
- Channel budget: total insertion loss from chip to chip, including board, connectors and cable.
- Speed grade: 112G or 224G rating of the connector family, with tested data.
- Serviceability: how a failed cable or cassette is replaced.
- Guidance and float: cassette designs need alignment and float at the mating interface.
- Airflow: cable bundles and orthogonal layouts change the cooling path.
- Supply: custom assembly lead time and second sourcing.
Frequently Asked Questions
What is a cable backplane? A backplane where high-speed signals travel on twinax cable instead of PCB traces.
What is the difference between a midplane and a backplane? A backplane sits at the rear of the chassis. A midplane sits in the middle, with cards on both sides.
Why not just use better PCB material? Low-loss laminate helps but cable still has far lower loss per inch at these speeds.
Is copper or optical better at 224G? Copper reach shrinks as bandwidth rises. See our copper vs optical guide.
Where to Go Next
Sources
- Cable Backplanes Come of Age (Connector Supplier): why cable, architectures, serviceability, NVL72 figures.
- TE cabled backplane systems: loss per inch, cost, cable type and layouts. Manufacturer figures.