Wednesday, September 3, 2008

Trace Bends for Differential Pairs

The use of bends should be kept to a minimum, since a bend can introduce common mode noise into the system, which will affect the signal integrity and EMI of the differential pair.
Bends on traces should be ≥ 135°. Tighter bends should be avoided because they impact the loss and jitter budgets.

If bends are used, the following guidelines are recommended to avoid tight bends. See below picture.

  1. Keep all angles between traces (α) ≥ 135°.
  2. Maintain an air gap (A) of ≥ 20 mils.
  3. Segments such as B and C, which flank a bend, should have a length ≥ 1.5 times the width of the trace.

The number of left and right bends should be as close to equal as possible, to minimize the length mismatch.

When a serpentine section is used to match one length to another, as shown below, the length of each jog must be at least 15 mils (three times the 5 mil trace width). The maximum distance between traces in a serpentine section should be less than two times the distance between traces in a non-serpentine section.

An uncoupled section of trace routing into a pin or a ball should be ≤ 45 mils when using multiple bends, as shown below.

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