What ultra-low-loss PCB material do you recommend for PCIe 5.0 technology?

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PCIe 5.0 technology pushes the envelope on what is possible regarding data rates between applications and processors. The industry is evolving to increase data rates at higher bandwidth. The PCIe 5.0 doubles the data transfer rate compared to its predecessor, the PCIe 4. It delivers a whopping 32GT/s for superior performance across all channels.

The system bandwidth is dictated by various factors, including connectors, cables, PCB traces, and IC packages. They all require high data rate designs, which remains a significant challenge for the industry.

In the past, system designers for PCIe used FR4 PCB materials that delivered transfer rates of 8GT/s. However, designers are now pivoting to ultra low loss PCB materials to minimize channel loss at speeds exceeding 16GT/s.

The PCIe 5.0 standard increases transfer speeds to 12GB/s bandwidth over an x16 configuration. This indicates just how far the industry has evolved.

The go-to ultra-low-loss PCB material for designers is MEGTRON 6. The only problem is that the material is up to 2.5 times more expensive than FR4 generic PCB materials. Moreover, PCB traces need to be spaced further apart in order to maximize performance. This results in a larger, and, consequently, costlier circuit.

Designers have to spend more time with extra processes such as back drilling via stubs which can add to the overall cost of the HDI PCB. The use of custom surface mount connectors also increases the costs to manufacturers.

Using re-timers occupies space on the board, which can drive up assembly coasts of the board and increase the amount spent on materials. It also increases the requirement for system power.

Megtron 6 is a popular choice of PCB material used in making PCIe 5.0 server systems. In these systems, the distance from the CEM connector and Root Complex pin is over 10.

Challenges Associated with PCIe 5.0

Like most high-speed interfaces, the PCIe 5.0 lane introduces several challenges to designers in terms of layout planning and routing. The traces, connectors, I/Os on an IC, as well as substrate materials restricts the bandwidth on the PCBs.

The section below analyzes several loss sources in PCIe 5.0 lanes.

Dielectric losses: Signal attenuation in high speed PCBs due to relaxation and excitation is a common occurrence as the electric field passes through the lane.

Conductor losses: These losses occur due to DC resistance because of the cross section of the conductor.

Resonant power loss: Skew and losses due to the fiber weave effect across the PCB substrate that results in loss of power at certain frequencies.

Copper roughness loss: The PCB  needs to be modeled correctly to perform impedance calculations to account for etched copper on PCB laminates (both rolled and electrodeposited).

Due to the thick substrates, the radiation loss in the microstrips are larger. These losses may be ignored in stripline routing and closed waveguides. The same applies in micro via PCBs with a high layer count for microstrips on thin laminates.

This indicates that the FR4 material is no longer suitable for use with PCIe 5.0 lanes where the data rates exceed 32 Gbps. Low loss laminates like MEGTRON 6 need to be used for superior performance.

Roughness losses and dielectric losses in PCI 5.0 lanes can be minimized by selecting a low loss PCB material such as MEGTRON 6. Note that vias and mating connectors will also lead to losses due to reflection and impedance mismatch with the signal bandwidth. Losses may be limited in the PCIe 5.0 spec and are restricted to -37 dB at data rates of 32 Gbps.

The arrangement of components on PCBs will dictate the number of connectors, vias, and the interconnect length to be used in PCIe 5.0 routing and the length used in the lane.

Other Low Loss Materials for Use with PCIe 5.0 Lanes

If your budget does not allow the use of Megtron 6, there are other low loss materials available. In fact, there is no industry standard for specific materials that guarantee signal integrity. The goal is to select a material that allows you to stay within your loss budget for the lane length.

FR4 laminates are simply not suitable regardless of your loss budget. Viable materials other than MEGTRON 6 include laminates by Rogers and iSpeed.

Rogers 4350, in particular, is a popular laminate of choice because it is the oldest option on the market. In any case, Samsung's MEGTRON family leads the industry in high speed, low loss PCB applications.

By all accounts, MEGTRON 6 offers the best ratio of price to performance thanks to superior properties such as dissipation factor, thermal conductivity, and relative permittivity.

Other Considerations in PCI 5.0 Lane Design

Outside of the argument of choosing a suitable PCB material, like Megtron 6, there are factors to consider. The industry standard is to lay out host processors and daughterboards in point to point topology (where two hosts are connected in a dedicated fashion).

PCIe devices and processors can be placed on the same circuit or separated with the help of a connector.

Coupling Capacitor

Another factor is the placement of coupling capacitors. There is a difference of opinion among vendors for the value of coupling capacitors to be used. Regardless of the values of the coupling capacitor, the main idea is to clear DC offsets along all ends of the link while allowing data to flow between components.

Connector and Trace Impedance

There are certain rules to be followed for differential signals. It is common to have larger deviations in impedance (nearly 15 Ohms) for connectors that are placed between longer sections of traces on connectors. Length matching can affect various S parameters, such as return loss, which should be configured correctly to improve performance.

Final Thoughts

The electronic industry is transitioning to PCIe 5.0 technology. In fact, the most demanding GPUs and CPUs will follow the PCIe 5.0 standard. The best ultra low loss PCB material to stay competitive is MEGTRON 6.

For more information and guidance, get in touch with the experts at Hemeixin HDI PCB.

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