Breakouts
Breakouts, the circuitry that interfaces the package or connector to the circuit board, are problematic. The realities of snaking a trace through a pin field, or attaching a connector to a pad, often force significant deviations from the ideal geometries and impedances desired for the traces.

At microwave frequencies, the first half inch or so of trace can easily account for the majority of the near-end crosstalk. This much trace can easily be entirely in the breakout region. The breakout region is best treated as a distinct entity when you do your modeling.

Figure 7.20. The Break-Out Under a BGA

Sometimes the electrical characteristics of the package or connector itself are significantly influenced by the details of the breakout. In such cases, it makes sense to include some or all of the breakout on the circuit board as part of the package or connector, including it in the package or connector model.

It makes little sense, for example, to characterize a connector that mandates use of a through-hole via of some size, without including that via in the characterization of the connector. The problem with this is that the model may then need to include a board-thickness parameter in some way.

For reasons of cost, packages are tending to finer pitches and closer spacings. At the same time, higher frequencies and the attendant greater losses call for wider traces. It is often found that traces in breakout regions simply cannot meet impedance, loss, and crosstalk characteristics desired for the rest of the board.

In simulations, it is necessary to optimize the breakouts and then choose the remaining interconnect to accommodate what is left of the interconnect budgets. That is, it is much easier to limit crosstalk in the long trace run across the board than it is to do so on the breakout region. It is much easier to hit the precise desired impedance out in that open space than it is in the very confined regions of the breakout.

Figure 7.21. A Typical Interconnect Design

Interconnects
The interconnect circuit is the entire assembly of features and traces that connect a transmitter to a receiver, as seen in Figure 7.21 above. This often involves numerous discontinuities and variations that are difficult to reliably deal with in hand calculations.

Up to now, the discussion has focused on how to calculate impedance as a function of distance from a discontinuity, how to calculate the cumulative effect of multiple discontinuities, and how to do all sorts of things by hand. SPICE simulators do an excellent job of dealing with all those things for you.

Having been told that, do not conclude that all the mathematical derivations have been for nothing. Without understanding the mathematics and physics behind what is happening, you would have no idea of how to make improvements when SPICE says that the interconnect link is broken.

You may have little interest in working with things like hyperbolic functions to determine the impedance at a position in the line, when SPICE can do it easily. But now you know how it works and will have ideas of what to do when SPICE says your link is busted.

In modeling the interconnect, it is important to recognize that, unless you take steps to overcome it, all simulations treat the world as ideal. The transmission line in the simulator does not randomly vary in width. The transmission line doesn't encounter regions of varied dielectric constant as traces on FR4 really do. In a simulation, unless you intentionally model the variations, everything is beautifully perfect—and not very realistic.

Connectors
Connectors are a real challenge for measurements and modeling. But that is starting to sound like a mantra by now. What isn't a real challenge? The dominant thing you need to know about connectors is that they often will be major locations of crosstalk in the link.

Assume you choose a connector that matches your line impedances. It is typical for the crosstalk of connectors to have a bigger impact on signal integrity at microwave frequencies than loss in the connector has. Never consider using a particular connector if its crosstalk is not well specified.

Don't settle for statements such as a connector has such-and-such percent crosstalk. Drill down and find out what that statement really means. It is fairly easy to get good crosstalk from a single aggressor signal or a slow rise time. But what is needed is the total sum of the contributions of all nearby signals at an appropriate rise time or frequency range.

Take a look at Figure 7.22 below. In some geometries there can be many more than just one or two aggressors coupling into a particular pin or pin-pair. You can't really blame a vendor if all they give you are accurate numbers, but not necessarily the numbers you need.

Figure 7.22. A Connector with Multiple Crosstalk Aggressors

If it is necessary to model this connector in a system simulation, who will provide the model and what type of model will it be? Every model for any device has a limited range of accuracy.

Questions you need to ask about connector models include over what frequency range is the model accurate and what level of accuracy does it provide in that frequency range? Also understand the conditions under which the model is characterized.

There have been cases where board features that were absolutely required for the connector were not included in the model because they made the connector performance look worse. A useful model is a model that accurately represents how the device will perform in a real application. Real applications often use board-to-board connectors actually mounted on boards.

Another aspect of connector selection you need to think about is the physical length of the path through the connector. Consider modeling an ideal lossless connector in SPICE. The only parameter you need to vary in this model is the length of the connector.

As an example, make the impedance of the path through the model exactly 50 ohms. In a real implementation, the circuits that go to this connector may target 50-ohm impedance too, but there will be a real-world tolerance. So model the line in and out of the connector as 45 ohms and terminate both ends at 45. Now run frequency sweeps at various physical lengths in the connector.

If you do this experiment, what you will see is that the connector, even with ideal lossless lines, acts something like a low-pass filter. And you will see that the knee frequency depends on the length of the connector.