Right Angle Routing and Differential Traces

Routing is the part of the layout that most theoretical work eventually depends on. A perfect stackup and a well-considered placement still produce a poor result if the traces themselves are drawn without regard for how signals behave along them. Three geometries cause most of the avoidable problems: the right angle corner, the acute angle, and the differential pair routed without discipline.

What a Right Angle Actually Does

A trace has a characteristic impedance determined by its width, its thickness and its distance from the reference plane. At a corner, the geometry changes. A right angle corner presents a wider effective conductor than the straight section, which appears as a small capacitance, and the transition into and out of that capacitance is an impedance discontinuity.

Three consequences follow. The added capacitance slows the rising edge slightly, which may or may not matter depending on the timing budget. The impedance discontinuity reflects part of the signal, producing ringing and, on a bus, reducing the margin at the receiver. And the outer tip of the corner concentrates the field, so the corner behaves as a small radiator at the highest frequencies present in the signal.

None of those effects is catastrophic for a single corner on a slow net. The problem is that a real board contains hundreds of corners, and the effects accumulate along a long route. The 45 degree corner avoids all three, because the transition is gradual and the conductor does not widen abruptly at a point.

Acute and Obtuse Angles

The same argument applies to angles other than 90 degrees. An acute angle, where the trace turns back on itself at less than 90 degrees, produces a narrow tip that is difficult to etch consistently and that can trap etchant during manufacture. The result is a trace that is narrower at the corner than the designer drew, which is a worse impedance discontinuity than a right angle. In addition, an acute angle in a manufacturing context is where solder or chemical residue can collect.

Obtuse angles are less problematic electrically but are still unusual in practice because they consume more routing area than a 45 degree corner. The accepted convention is to route with 45 and 135 degree segments, which satisfies both the electrical and the manufacturing requirements.

Differential Pairs

A differential pair carries a signal as the difference between two traces. The receiver responds to the difference and rejects what is common to both, which is why the structure is used for fast serial links and for any interface that must survive a noisy environment. That rejection only works while the pair remains symmetric.

Three properties must be maintained. The spacing between the two traces must be constant, because the differential impedance is a function of that spacing. The two traces must be the same length, because any difference in propagation time converts part of the differential signal into a common-mode component, which the receiver cannot reject. And both traces must reference the same plane along their whole route, because a change of reference changes the impedance of one trace relative to the other.

Where the pair must be tuned, the tuning should preserve symmetry, and the amplitude and spacing of any compensatory structure should be chosen so that the sections do not couple to each other. The rules for that tuning are set out in this article on serpentine routing and length matching.

Length Matching Within a Group

Length matching extends beyond differential pairs. Any group of signals that must arrive together, such as a parallel data bus or a set of address lines, has a length tolerance derived from the timing budget of the receiver. The tolerance is normally small, and achieving it requires the routing to be planned rather than adjusted afterwards.

The practical approach is to route the group as a bundle, keeping the traces in the same order from source to destination, and to allocate the extra length where the geometry permits rather than at the end of the route. Where a trace must detour around an obstacle, the compensation belongs beside the detour, so that the tuning structure is not concentrated in a single region of the board.

Length is only a proxy for delay. Two traces of equal physical length on different layers have different electrical lengths, because the propagation velocity depends on the dielectric environment. Where a group of nets changes layers, the length report from the layout tool is not sufficient, and the group should be checked with an extraction tool that accounts for the differences. The general behaviour of fast buses is described in this discussion of high frequency traces and data buses.

Spacing, Coupling and Reference

The other routing decision that affects signal quality is the distance between unrelated traces. Parallel runs that are long and closely spaced couple into each other, and the coupling is controlled by spacing and by the height of the traces above their reference plane. The rule of thumb that governs the spacing is described in this article on the 3W rule for crosstalk.

Where a sensitive trace must run beside a noisy one, the options are to increase the spacing, to route the two on different layers with a plane between them, or to insert a grounded guard trace between them. The guard trace must be stitched to the plane, or it becomes a coupling path rather than a barrier, and the stitching should be dense enough that the guard remains a low-impedance conductor along its whole length.

It is worth noting that these spacing decisions interact with the layer arrangement. The coupling between two traces depends on how strongly each is tied to its reference plane, so the same spacing that is sufficient on a thin dielectric may be inadequate on a thicker one, where both traces are less tightly bound to the plane and more tightly bound to each other.

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A final consideration is the reference itself. Every routing decision assumes that the plane beneath the trace is continuous. A trace that crosses a plane boundary has a return current that must divert, and the diversion affects the impedance of the route for the length of the crossing. Checking the plane continuity under every fast net is as important as checking the length and the spacing.

FAQ

How bad is a single right angle corner? On a slow net with a generous timing budget, its effect is negligible. The three mechanisms it introduces, corner capacitance, impedance discontinuity and tip radiation, become significant when a route contains many corners or when the edge rate is fast.

Why is an acute angle worse than a right angle? Because the narrow tip is difficult to etch consistently, so the trace is often narrower at the corner than the design intended. That is a larger impedance discontinuity than a right angle produces, and the feature also tends to trap etchant and residue during manufacture.

What breaks a differential pair? Unequal spacing, unequal length and a change of reference plane along one trace only. Any of those converts part of the differential signal into a common-mode component, and common-mode noise is exactly what the pair was chosen to reject.