12. Route a Differential Pair

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WidthNetLayer

Problem 12

Route a 0.20 mm pair from the transmitter to the receiver on L1 over L2 ground.

Both polarities are 0.20 mm on L1 over L2 ground. Skew at most 0.50 mm.
A via that leaves the ground reference is not a free way to match length.

Context

A differential pair is two traces used as one signal. Skew is the length difference. This challenge grades width, gap, skew, and a continuous ground reference. It does not ask you to hit a calculated impedance.

Open the lesson Component guide

Components on this challenge

Differential pair. Two traces used as one signal, compared with each other.

Full brief

Route the two polarities together on the layer that faces ground, inside the published width, gap, and skew. This challenge does not grade a calculated impedance.

How PCBGrade measures this

J1 and J2 are locked on a 70 × 30 mm four-layer board. J1 pin 1 is P and pin 2 is N. J2 matches. Route both on L1 (F.Cu). L2 (In1.Cu) is the GND reference. L4 looks at L3, and L3 is not GND, so a layer change is not a GND reference. The reference route is all on L1 with no vias. A via barrel, if you use one, is 1.565 mm and is added to that polarity’s length and to skew. It is not added to uncoupled length. This challenge does not calculate impedance. The pair net class sets width 0.20 mm and intra-pair clearance 0.18 mm. Every other clearance stays 0.20 mm. Every P and N segment must be within 0.005 mm of 0.20 mm (15 points). A parallel run is two same-layer segments parallel within 1° whose edge gap is at most 1.0 mm. Edge gap is rounded to the nearest 0.001 mm, the same quantum as every other length, and that rounded gap is what the window uses. Every such run must have an edge gap from 0.18 mm to 0.22 mm (10 points each side of the window). There is no separate neck-down. Coupled length is the overlap of runs inside that window. Uncoupled length is the worse polarity’s trace length minus its coupled length, and must be ≤ 5.0 mm (8 points). Each polarity’s length, traces plus barrels, must be from 40.0 mm to 60.0 mm (5 points each edge). Skew is the absolute difference and must be ≤ 0.50 mm (12 points). One uncompensated 90° turn is about 0.66 mm to 0.80 mm of skew. GND reference is the centreline fraction over L2 GND, sampled every 0.5 mm, worse polarity ≥ 0.95 (12 points). Continuous means the centreline does not leave L2 GND and re-enter it, except within 0.5 mm of that net’s own via or in the first or last 0.05 mm of a segment (8 points). Each polarity may have at most 2 vias (4 points). The two polarities must have the same via count (2 points). When both have vias, each matched pair of centres must be from 0.80 mm to 1.00 mm (1 point each edge). Zero vias passes that check because there is no pair to measure. DRC is 4 and connectivity is 3. There is no extra score cap.

pair geometry · 35 pts
skew/length · 30 pts
reference plane · 20 pts
DRC/via compliance · 15 pts
Cap 69: Copper outside the outline cannot pass, and neither can a short between nets.

Trace · pour outline · ratsnest · Drag a part or via. Drag a trace end to edit it. Drag empty board to pan.

Pass at 70. Check previews. Submit records.

USB4

Gap minimumNot checked
Scored · 10 pts
Coupled copper stays at least 0.18 mm apart.
Gap maximumNot checked
Scored · 10 pts
Coupled copper stays at most 0.22 mm apart.
SkewNot checked
Scored · 12 pts
The two polarities stay within 0.50 mm, including via barrels.
Uncoupled lengthNot checked
Scored · 8 pts
Breakouts and tuning stay within 5.0 mm on the worse polarity.

High-speed2

Via pair minimumNot checked
Scored · 1 pt
Paired vias need room for the 0.60 mm pads and the 0.18 mm pair clearance.
Via pair maximumNot checked
Scored · 1 pt
Paired vias stay within 1.00 mm centre to centre.

Routing8

Trace widthNot checked
Scored · 15 pts
The pair is drawn at 0.20 mm, within 0.005 mm.
Length minimumNot checked
Scored · 5 pts
Each polarity is at least 40.0 mm.
Length maximumNot checked
Scored · 5 pts
Each polarity is at most 60.0 mm.
GND referenceNot checked
Scored · 12 pts
The pair is referenced to L2 ground. L4 over L3 power does not count.
Continuous referenceNot checked
Scored · 8 pts
A slot the centreline crosses and then leaves is a split return, even when the length fraction stays high.
Vias per polarityNot checked
Scored · 4 pts
Each polarity can change layers twice. On this stack the GND rule makes that change unused.
Via count matchNot checked
Scored · 2 pts
A via on one polarity has to have a partner on the other.
ConnectivityNot checked
Scored · 3 pts
P and N have to reach the receiver.

DRC1

DRCNot checked
Scored · 4 pts
The pair class allows 0.20 mm width and 0.18 mm between P and N. Everything else stays at 0.20 mm.

PCBGrade

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