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Selecting a High-Speed Interconnect & Keys to a Successful Implementation

by Mike Rock; Engineering Manager — Signal Integrity Products | Feb 21, 2017

         When designing applications utilizing high-speed interconnects, it’s vital to navigate around all potential “speed bumps” along the signal’s path.  Factors such as stack ups, tolerances, via design, trace widths, plating, and copper etching will have to be understood and mastered to achieve an optimal signal path.  Any design check list should also include the connector, which often times is overlooked.  Connectors must be scrutinized or it may wreak havoc on the system’s signal integrity. With this in mind, what should we be looking for when choosing the connector and how do we determine what we’ll actually need?  A proper connector should provide a few key elements:

  • Matched impedance over the bandwidth of interest
  • Low insertion loss over the bandwidth of interest
  • Reliable connectivity to the PCB
  • Reliable connectivity to the cable system

         In order to ensure that the connector provides low loss and matched impedance to your signal, we need to look at scattering parameters or s-parameters.  S-parameters specify the nature of linear electrical networks and will identify the bandwidth and the loss of the circuit, providing a peek into performance potential.  S-parameters data are provided by manufacturers as a way of characterizing their connectors and should be the first criteria when considering a high-speed one.  Designers should also transform and observe s-parameters in the time domain, as a TDR (Time Domain Reflectometry) plot, and see what the impedance profile looks like internally.

         It is important to note too that connectors have a good many “flavors” (termination style, internal signal length, materials, etc.) and the designer needs to understand how the s-parameter files were created.  It’s important to ask the vendor several questions:

  1. Does your intended pinout match the simulated one?
  2. How were the other pins terminated?
  3. What layout was used in the provided data?
  4. Are there any stubs?
  5. What was the termination style used for characterization? (Plated Thru Hole, Surface Mount, Press-fit)
  6. How was the fixture de-embedded or what’s included in the measurement?
  7. Is this the exact part number that you need?

Answers should be ascertained before blindly placing a touchstone file into a simulation and trusting the results.  Here at AirBorn, we provide simulated and measurement-correlated data for each customer’s intended use. This provides customers a more accurate assessment of connector impact to the design and bolsters confidence levels in the simulated data.

         Another important criterion in selecting the connector is PCB termination options. Connectors typically come in several termination styles including (but not limited to) surface mount, press-fit, and PIH (paste in hole); each having distinct strengths and weaknesses.  Press-fit terminations are very rugged and provide maximum retention and connectivity to the circuit board, but also present a tough challenge for high-speed applications.  Routing a signal out of a high-density press-fit connector may require a high layer count PCB, presenting the longest path up through the plated holes to get to the connector.  The elongated route and fixed diameter of the drill holes can produce significant discontinuity for higher frequency signals and may hinder high data rates.

         A surface mount termination is more amenable to high speed design and allows for maximum flexibility when matching impedances at the connector launch point.  Designers can route directly into the pad of the connector or use a drill hole of choice to provide a path up through the PCB material.  In addition, the vias can be buried within the PCB material or back drilled to reduce the unused via stub and improve the frequency response over the press-fit termination style.  This method of termination offers the most benefits toward high-frequency design but is not necessarily robust. Surface mount connectors normally require some additional reinforcement to ensure a strong connection to the PCB, such as mounting hardware.

         The paste-in-hole (PIH) style of termination is somewhat of a hybrid between the previous two styles. Similar to press-fit, PIH termination has a shorter, non-press-fit pin that is inserted into a plated through-hole footprint and soldered in place.  The main difference being the pin is much shorter and the holes can be back drilled to remove the excess stub on the signal.  High-density connectors of this sort can still be a challenge to break out into the PCB.  AirBorn offers these pins as short as 10mils which provide improved high-frequency response as well as a rugged connection to the PCB.  Each termination style will have some discontinuity in impedance to the signal as it transitions from the PCB into the connector, all while allowing the designer varying degrees of freedom to deal with this discontinuity and minimize the impact to signal integrity.

         The fourth criteria when selecting a connector for your high-speed design is how it contacts to the mating connector.  There are several methods of mating contact, all with their unique set of benefits and limitations to the designer.  Each must be judged how they will affect the signal integrity of the overall design.  One very popular mating contact is the edge connector.  This method relies on the edge of the PCB, or a mechanical component attached to the PCB, to slide into the connector.  The connector has spring loaded fingers that will slide along the PCB pad to make electrical contact at a single point.  This allows for minimal discontinuity in the signal but it also provides only a single point of contact and may prove to be detrimental for designs exposed to high-vibration or shock (requiring additional retention methods).  Many of the MSA standard designs (SFP, SFP+, QSFP) use this type of electrical interface.

         A more widely-used rugged contact region is the press-fit.  This fairly basic system relies on the female side to have a socket and the male to have a spring pin that inserts into the socket.  This mating category has two or more points-of-contact and can offer several advantages over the single point-of-contact offered in the edge mount mating system.  The multiple contact points in Figure 1 below present a lower contact resistance and inductance to the signal.  It is also a very reliable and rugged mate that is sure to maintain contact under extreme vibrations.  AirBorn uses this pin and socket design in several of our high-reliability, high-speed products (such as our HD4 and verSI families) for this exact reason.  This pin and socket contact can also be physically longer than the edge contact, presenting a larger discontinuity if not designed properly.  Therefore it’s crucial to study the s-parameters and impedance plots provided by the manufacturer.

High Speed Figures verSI

Figure 1 - Press-fit Pin and Socket (verSI)

         Finally, when designing for speed, it is important to consider the throughput relative to the footprint.  Figure 2 below compares two basic systems: one using a 25G per lane QSFP connectors and the other using a 10G per lane HD4 connectors.  The resulting output (4,320 Gbps vs. 3,600 Gbps) shows that superior density achieved through connector design can mean greater data throughput utilizing 10G HD4 connectors vs. the higher-speed 25G QSFP connectors within the same rack system.  This ultimately provides savings on components and energy costs and can save you data center square footage too.

High Speed Figures HD4

Figure 2 – Greater throughput through density (HD4 vs QSFP)

         At the end of the day, designers need to get a strong, undisturbed signal from point A to B and a proper connector is a critical element along that journey.  After all, even if you design in the highest-speed connector, if you experience intermittent opens, what good is the speed?  At AirBorn, we offer a wide selection of connectors for high-speed and high-reliability solutions as well as the end-to-end solution for your channel.  We work closely with every customer to assess their needs and get them into the correct interconnect solution to ensure that their links work optimally with minimal signal degradation.  As a designer, you have a number of connector options from which to choose, so pay close attention to the associative data and choose wisely as the uninterrupted flow of information is critical.

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