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The 5G Interconnect Design Challenges and their Solutions

Momentum around 5G is constantly on the build, bringing challenges and opportunities in equal measure. Avnet spoke with Molex Global Product Manager Mike Hansen for more information concerning the role copper interconnect plays within the growth and development of the following high-speed cellular network.

In the field of interconnect, there's RF, there's optical and there is copper. With this thirst for more data, there'd seem to be a general trend towards moving as much of the information traffic in to the optical domain, due in large part to the benefits it brings, like EMI resistance and high speeds.

However, anyone involved at any level using the manufacturing industry knows, cost plays most in the design decision.

Of the three modals mentioned above, just one can deliver both power and data and so most commonly it is necessary. But they are we seeing the end of copper interconnect for data?

The price of 5G performance

Mike Hansen offers the cellular context for this question. \”The old way was the passive antenna, with coax cables running down to a sizable baseband unit, where it might be demodulated and converted to fiber for the long backhaul. With 4G, operators started putting the remote radio manages to the pole, right underneath the antennas. This is when fiber right down to the baseband unit started becoming popular. With 5G there are a few major shifts. There are a lot more antennas with massive MIMO, you will find new frequency bands and it is all much faster. So in addition to bringing the remote radio head up in to the antenna and it becoming an active antenna unit, there's only a lot happening in a relatively small box.\”

These massive arrays of antennas now need to be demodulated, converted from analog to digital after which switched or routed until it gets to the fiber. \”Of course, the fiber is ideal for too long distance down the pole to the baseband unit, but we're still seeing quotes arriving for board-to-board and high-speed mezzanine connectors since there are a lot of boards being packed into the active antenna unit (AAU),\” Hansen explained.

All the traditional copper-versus-fiber arguments still apply, but based on Hansen, it's increasingly of a hybrid solution. \”Inside the box, we're seeing copper, and fiber away from box over long distances. It is a trade-off between speed, density and price,\” he said. In terms of density, copper still wins. Technologies like Molex's NearStack can support 16 differential pairs in a single connector in a very small footprint.

The trend for higher speed is now to route over twinax instead of use PCB traces because it avoids a lot of signal loss. A few of the companies designing and using high-speed switching chips, SERDES and ASICs don't want to route through a board and introduce a lot of loss. Using twinax is a great solution to that.

\”We might see a NearStack connector being right next to the chip around the board on and on out to some QSFP [quad small form factor] connector, so you have hardly any reduction in the board because the traces are extremely short,\” Hansen said.

While PCB manufacturers are constantly improving their own technologies, flame retardant 4 (FR4) continues to be dominant material. Although options including Mooring Equipment Guidelines 4 (MEG4), MEG6 and MEG7 are aimed at high-speed signalling, they're less common and much more expensive. Using twinax to bypass the PCB has become popular as it effectively creates an overpass for data that can travel inside a relatively straight line from point to point, rather than being routed using orthogonal PCB traces and perhaps moving between multiple layers through vias.

The high-speed interface

Moving from non-return to zero (NRZ) to Pulse Amplitude Modulation 4-level (PAM-4) and 56 Gbps to 112 Gbps can also be an important design consideration. The proceed to 5G will soon rely more on routing these high-speed signals. \”We're certainly designing knowing that. Even if manufacturers don't go full 56 or 112 Gbps PAM-4 today, a minimum of we'll have the connectors ready for it,\” Hansen said.

Hansen sees this as the second advantage of connectorizing a system, as it provides inherent modularity. \”You can swap in cards or better silicon a couple of years later very easily when you connectorize. Allow me to put it by doing this: Any new connector we develop for NearStack will be 112 Gbps, PAM-4 capable.\”

There isn't any single trick to designing a connector with this particular degree of performance. Hansen explained that Molex compares the entire channel. Including how it's inserted into the PCB, the geometry of the connector's beams, how the dielectric is positioned between the beams, the spacing and the mating interface. \”We view it from end to finish,\” he said. \”It helps that we have been doing this for 30 years. As speeds have gone up, we've evolved our geometries to meet the interest in speed while maintaining the density requirements.\”

Cable assembly innovations

There has also been innovation to cable assembly solutions. \”We have formulated a distinctive direct-to-contact weld termination in which the wires are directly welded to the signal contact. This makes it highly repeatable, which is great for SI. Another thing we do is use high-performance twinax.\” Molex also owns a twinax manufacturer, same with inside a good position to avoid supply chain issues associated with sourcing materials.

The connectors are modelled before going to production, with testing and adjustments made throughout the tactic to optimize for SI. Based on Hansen, this has be a core design tenet for Molex. The organization also does a lot of simulation around the front-end, making it simpler for an SI engineer to take a cable assembly model, place it into their own system model and see the way it performs. What this means is the whole high-speed channel can be simulated. According to Hansen, the experience for purchasers has been that the real-world results match up with the simulation.

Molex provides all its models and S-parameters because of its connectors to people to enable this degree of design simulation. Providing models is rapidly becoming the cost of entry when supplying manufacturers using high-speed signals. That extends to models for the cable assemblies at all lengths.

One of the unique connector solutions that Molex now offers may be the NearStack On-the-Substrate option. Because the name suggests, this puts the connector directly on the chip's substrate. This may sound really cool (because it is) it has a lot of unique design challenges. Because of this, it takes a lot of close collaboration with the customer which is really aimed at customers developing their own chips and chassis so they can implement that much cla of direct connection.

The performance from the On-the-Substrate connector is, understandably, really good. However, and also understandably, many customers are simply not in a position to mount the connector directly on the substrate. The reply to that, sampling soon, may be the NearStack HD. This provides the same performance but mounts on the PCB as opposed to the substrate. \”We're calling it 'near-chip placement' rather than 'on-chip placement,' \”Hansen said. This means now you can place the high-speed connector around the PCB, making it simpler to use. It will also help fill in the NearStack portfolio.