Traditional connector challenges are magnified as speeds increase, but new design tools, mezzanine approaches, and thermal management strategies help high-speed connector designs morph to meet challenging data demands.
Jairo Guerrero, Director of Marketing, Enterprise Business Unit, Molex, LLC
The continuous proliferation of IP services and access speeds has led to exponential growth in bandwidth and a steady demand for higher speed router and switch interfaces. This has resulted in the realization of interconnect technologies and standards for 40Gb/s and 100Gb/s interfaces, and plans for 400Gb/s speeds are already materializing.
The need to transmit ever more data at increasingly higher speeds has encouraged system design strategies that include connectors specifically designed for high-speed operations. Such connectors can maintain signal integrity at high speeds and employ new protocols.
PAM4 will play a growing role in this transition. Non-return-to-zero (NRZ) signaling, an industry standard, is giving way to PAM4 modulation in many applications due to the latter’s ability to process data rates of 56Gb/s, 100Gb/s, and higher.
While PAM4 offers important speed improvements over NRZ, its data must be encoded prior to transmission and then decoded upon receipt. This requires additional processing capability and makes PAM4 more challenging to implement. However, in applications with critical high-speed demands, the additional capabilities of PAM4 balance out the higher processing costs.
Further, NRZ is still appropriate for certain high-speed applications. New backplane connectors can provide data rates above 50Gb/s in PAM4 and NRZ systems. Compared to in-line beams, these backplanes optimize signal integrity performance and improve insertion loss, enabling interface resonance frequency that exceeds 30GHz. They can also deliver enhanced signal integrity by optimizing geometries and employing differential shielding that minimizes impedance discontinuities and reduces crosstalk.
Traditional connector challenges are magnified as speeds increase. Higher data speed channels typically involve increased electromagnetic interference, higher crosstalk, and impedance discontinuities; so, protection against these issues must be designed in. The backplane connectors described above typically work with existing headers to ensure backwards compatibility and enable integration into existing designs.
Another issue with increased system speeds is maintaining appropriate signal integrity. One way to accomplish this is to remove high-speed signals from the PCB by applying high-speed copper cable. This alternative can be used with 50Gb/s NRZ and 50Gb/s PAM4 live, encoded serial traffic using QSFP cable assemblies, and connector interfaces.
Tools to Expedite Design
With so many new designs required for high-speed connectors, tools that help simulate system performance can significantly reduce design cycle time. In traditional manual system simulation, each component is simulated independently, so it can take a week or more to simulate individual system designs. When multiple design iterations are needed, this can slow the design process down to a crawl.
Alternately, new software-based design tools employ libraries of pre-simulated models based on typical designs, materials, traces, and vias, allowing designers to select the models they want and get results almost immediately, hastening market readiness. Such software allows first-order system approximation, which provides crucial insight into critical parameters for developing new systems and delivers considerable value to designers using more high-speed interconnects.
High-speed mezzanine systems offer another route to ramp up data speeds. With tunable differential pairs, enabling matched impedance configurations, single-ended lines and power, and a range of stack heights and compliant-pin terminations, high-speed mezzanine connectors are enabling data rates up to 56Gb/s, which are appropriate for high-speed infotech and telecom applications, among others.
The most popular attachments for mezzanine connectors are either press-fit or SMT, each of which presents advantages and disadvantages. Press-fit mezzanine connectors deliver process ease, while SMT connectors typically enhance performance by enabling optimized footprints and removing stub effects from the compliant pin. SMT connectors often require rework, though, making them potentially more challenging than those with press-fit attachments.
New compliant-pin technology, which allows system designers to rework the board and maximize system utility while achieving the necessary signal integrity, effectively reduces this performance gap between SMT and press-fit, rendering the difference null in a real channel. This allows designers to select the attachment method they prefer based on variables including layout, routing, and board thickness rather than signal integrity.
Further, by employing a triad wafer design, high-speed mezzanine connectors can offer high-speed differential pairs that can be tuned to 85–100Ω impedances, single-ended triads for low-speed options, and power triads. This allows designers to employ a single connector for different signal speeds, freeing up critical PCB space.
Thermal Management Strategies
As speeds increase and new modules enter the market, enhanced thermal management solutions are becoming a key element for next-gen systems. For example, stacked connectors deliver higher speeds, but use about 4.5–5W more power and produce more heat in 100Gb/s QSFP modules than standard interconnects, which isn’t desirable since module temperatures in enterprise systems must generally be kept below 70°C, with ambient enclosure temperature below 45°C to avoid decreased reliability and an overall decline in performance.
One successful new heat management approach is to employ internal riding heat sinks and high-flow cages designed to optimize air movement. This thermal management strategy can reduce the overall temperature in an emulated 5W optical QSFP module by 9°C, so this and others like it will be vital for the next-gen modules required to support 7W or more.
The demand for fast data is already overwhelming, and will only continue to grow as each novel new solution quickly becomes the latest benchmark for standard. So, future interconnect solutions must continue to enable both advanced technology and increased network bandwidth. Successful products will need to be capable of supporting a wide range of data rates using multiple connector shapes and sizes, and new designs must meet demanding high-speed performance requirements while simultaneously optimizing both efficiency and reliability.