Overcoming PCB-based Interconnect Design Limitations

By Contributed Article | August 28, 2018

Cabled interconnects could help system designers configure their architectures to achieve greater flexibility, performance, and cost savings.

By Lucas Benson, Senior Business Development Manager and Manager of Global Product Management, TE Connectivity

Global data consumption is driving faster silicon I/O speeds, from PCIe Gen 3 (8Gb/s) and Gen 4 (16Gb/s) to PCIe Gen 5 (32Gb/s).  Similarly, Ethernet I/O speeds have moved from 10Gb/s to 56Gb/s and are looking forward to 112Gb/s. Printed circuit board (PCB) technologies have improved to support high speed data transport but can constrain the end-to-end channel performance. Current PCB technologies have limited signal integrity (SI) performance (mainly due to insertion loss) which limits the reach of a clean data signal. Cabled interconnects offer a lower-loss solution to PCB routing due to their increased channel reach and cable flexibility — cables can be routed virtually anywhere in the design — to other PCBs, outside of the path of airflow, etc.

In this article we’ll look at common design challenges related to PCBs and cabled interconnects, discuss the advantages of cabled interconnects, and outline some design considerations.

Design Challenges and Cabled Interconnect Advantages

The three main attributes system designers require when designing internal circuits in next-generation equipment are design flexibility, high performance, and low cost.  As connector and silicon speeds have moved into multi-dozen gigabit ranges, designers are finding that standard PCB traces constrain their ability to address all three factors. By using internal cabled interconnect solutions, designers can more freely configure their architectures and achieve the flexibility, performance and cost required to be successful and competitive.

Flexibility – Cabled interconnects are being used in switches, routers, wireless controllers, servers and storage appliances. They are giving designers flexibility of architecture as more and more connections are being packed into the same equipment. For example, customers can run high-speed pairs from a NIC card to next to the SAS or PCIe controller, instead of through two connector junctions and three different PCBs. Wireless equipment designers can separate the heat generators within a remote radio head unit (RRU) farther away from each other to balance the heat conduction to the RRU shell, among many other examples.

Performance – A rule of thumb is that cabled interconnects enable signal to travel up to seven times the distance of PCB materials at 12.5GHz (28G NRZ and 56G PAM-4), when holding insertion loss (IL) constant at -8dB.  As speed requirements continue to increase to 112G PAM-4, the distance advantage is expected to be exponential over using traditional technologies.

PCB Performance

Cost Control – Cabled interconnects can reduce the need to use expensive PCB materials, reduce the layer count needed in a PCB, and avoid the need for expensive re-timers.  Furthermore, in systems where multiple PCBs are used, cabled interconnects eliminate the expense of traditional board-to-board connectors.

Design Considerations

Cabled interconnects are a newer technology in the market, and with new technologies there are always some unknowns. There are many internal cabled solutions in the market, yet not all solutions are created equal, so the designer must consider a few major factors when choosing the solution that will work for them.

One of the most important requirements that has surfaced is the need for robustness and good ergonomics while maintaining a low profile. System integrators are conditioned to stress test interconnects, and where there is a potential failure mode, it will be found. Internal cable vendors are now releasing cabled interconnects with guide rails for applications where blind mating is required, or where an undue amount of torque may be applied to the interconnect. For example, when two connection interfaces are perpendicular to each other and connected with a rigid cable, or when two parallel PCBs are connected via cable, and the connection needs to blind-mate.

Another differentiating factor is the need for a solution that delivers excellent SI performance at both 100 Ohm and 85 Ohm impedances, and is forward compatible (i.e., good enough for more than one generation, whether it be PCIe Gen 5 or 56G PAM-4). Not all cabled interconnects deliver the same level of SI, and this is increasingly a factor as speeds move to 32Gb/s and beyond and the next-generation system refresh cycle continues to shorten.

Design reuse is another factor. In previous equipment designs, designers would have to route and learn a different connector for each internal connection, whether it be a PCIe CEM connector footprint and breakout, a riser card, the HDD interface or the cabled connection between the PCIe or SAS controller and the storage midplane. There has been an effort in the industry to standardize the receptacle connector between internal cable and internal card edge connections. For example, a cabled connector that can also accept a card edge, and be leveraged as the interface for NICs, riser cards, HDDs, as well as the cabled connection between the controller and storage midplane.  The benefit is that the designer can learn just one technology and copy and paste the footprint and PCB stackup across the server, switch, or router. Ideally, the interconnect solution should deliver a common platform with the necessary pin counts to support many different types of connections.

Finally, sourcing and standardization are important considerations. Fortunately, the designs of the best-performing cabled interconnect solutions are being created and adopted by standards bodies, which ensures a broad supply base and assurance of supply. Major partners in the industry have also established dual sourcing relationships, guaranteeing true interoperability.  This dual sourcing takes aim at solving common latching and tolerance stackup issues when standard products are actually not interoperable but made to be within the spec tolerance limits.

As internal equipment density and speeds inevitably migrate higher, standard PCB technologies are posing a number of problems for designers. By moving to cabled interconnect products, designers can achieve the key goals of high performance, design flexibility, cost control and assurance of supply to deliver the next generation of data center products.

Lucas Benson is the Global Product Manager of next-generation high-speed networking interconnects for TE Connectivity’s Data and Devices business unit. 

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