Switchback Routing vs. Serpentine Routing for Maximum Density
Ever drive through a winding mountain pass? When I was a student, there was one back road that would wind back and forth as you ascended a small mountain. This style of paving roads is also used to route traces in PCBs. This routing style, called switchback routing, can be used for length matching/delay matching in replacement of, or alongside, other length tuning styles.
With all the different length tuning and routing styles available in PCB design tools, it may not be obvious when switchback routing is a better choice than other length tuning styles. In fact, some component manufacturers explicitly recommend using switchback routing or trombone routing with their components. Let’s take a look at switchback routing and layout styles and some of the benefits they provide.
What is Switchback Routing?
Switchback routing is a PCB trace layout and length tuning style that incorporates a number of U-turns in the trace as it is routed towards its destination. Switchback routing and trombone routing are similar, and the two terms are sometimes used interchangeably. If you look at some switchback routing styles, you will see some interesting back-and-forth trace patterns that do not resemble the typical trombone routing style.
The closest substitute for switchback routing is the typical serpentine routing style (a.k.a. accordion routing), where traces are meandered transverse (perpendicular) to the trace direction. In switchback routing, successive U-turns are used to meander the trace length in the longitudinal direction, rather than in the transverse direction as is the case in accordion routing. High-quality CAD tools will allow you to use either style of routing to increase trace density, or for delay/length tuning parallel signal nets.
Each routing style provides some particular benefits in terms of coupling, trace density, and signal integrity. For highly complex boards, it is not uncommon to find a mix of serpentine, trombone, and switchback routing styles in an effort to pack traces into the smallest possible space. The image below shows just such a mix of these different routing styles for an LPDDR3 memory interface.
When to Use Switchback Routing vs. Serpentine Routing
If you look through some application notes for different components, or some design rules, you will find some guidelines for sizing a switchback routing pattern. Some of these application notes will explicitly state that you must use switchback routing to ensure signal integrity. For application notes specific to components, it is usually a good idea to follow these guidelines. Outside of these specific applications, you should refrain from applying guidelines in application notes to other situations, and rules defined for switchback routing are no exception.
One primary advantage of switchback routing compared to serpentine routing is that it allows compensation of skew with a shorter required length match. The example image below shows length-matched switchback and serpentine routing patterns. These two patterns are designed to compensate for the same amount of skew while ensuring maximum routing density by staggering the length matching segments. The switchback pattern requires a shorter total length than the serpentine pattern for a given level of skew.
Note that the switchback design shown above is not the best choice for differential pairs. The required coupling switches between differential mode and common mode as the signal traverses the switchback region. If any noise is induced in the switchback region, it may not be seen as common mode noise at the receiver. Even in the case where the switchback regions are not staggered, tight coupling is difficult to maintain unless the switchback regions are routed closely together and track each other throughout the length of the pair. Using a serpentine length tuning scheme to match multiple differential pairs is a better choice. You’ll sacrifice density for better common mode noise suppression.
Crosstalk in Switchback Structures
Obviously, both serpentine and switchback routing styles include the same U-turn structure, and they both experience some near-end crosstalk (NEXT) as a signal enters the delay structure. NEXT in these traces can distort signals in certain cases. Like many signal integrity problems in PCBs, the effect of NEXT in switchback routing will depend on the rise time of your signal and the distance over which the signal must traverse. Furthermore, when the switchback structure is tighter (i.e., there is less spacing between portions of the structure), any crosstalk will be more intense; this is why some application notes and technical papers state that switchback structures should be spaced by at least 3x the distance to the nearest ground plane.
Whether NEXT distorts the signal seen at the receiver depends on the amount of delay being added with the switchback pattern in comparison to the rise time of the signal. If the delay created by the switchback is much less than the signal rise time, then any NEXT will be superimposed on the rising edge of the signal. Rather than distorting the shape of the edge, it simply causes the rising edge to arrive at its destination slightly earlier. In other words, the delay created by the switchback structure is slightly less than what would be calculated by simply looking at the trace length.
In contrast, when the rise time is much shorter than the delay created by the switchback structure, the rising edge in one portion of the structure generates NEXT in a different portion of the structure. This crosstalk signal then arrives at the receiver before the intended signal, and the receiver sees a distorted version of the desired signal.
Either of these effects can be seen in the time domain using a time-domain reflectometry (TDR) trace or in simulation results. Any field solver that works directly from your layout data can show you this effect. In a TDR trace, a capacitive impedance discontinuity can be seen at the structure due to reflections. Decreasing the spacing between each switchback section will increase this capacitive impedance discontinuity. This point, as well as distortion produced in switchbacks, should illustrate the complex nature of these structures and the various tradeoffs involved in switchback routing:
- Shorter delay structures reduce distortion, but there is a larger impedance discontinuity when the pitch between each U-turn is smaller.
- A delay structure can be made longer to reduce an impedance discontinuity, but there may be some level of distortion.
The routing and crosstalk tools in Altium Designer® are an excellent set of features for implementing switchback routing and examining any effects of NEXT in your switchback designs. The time-based length tuning features are ideal for delay matching multiple signals and ensuring they stay synchronized throughout your board. You’ll also have access to a powerful set of schematic design tools, CAD features, and production planning tools.