Quadra Solutions’ guide to PCB track length matching in high-speed design

To maximise the performance of a circuit design, the implementation of track length matching is key. In any circuit design, an electronic signal takes a certain amount of time to travel along the conductor to reach its end destination. It is crucial to identify the sections of a printed circuit board (PCB) design that requires length matching to guarantee that the signals arrive with accurate timing.

 

Track length matching is normally thought of as in pairs, but it also applies to buses with single-ended signals and differentially-driven buses. As CPU peripherals and other digital systems require faster-operating speeds, the propagation delays in a high-speed system, places tight tolerances on the allowed track length in a conductor carrying digital signals.

This article explores Quadra’s best practices for applying track length matching.

PCB track widths

So, what is Track length matching?

As the name suggests this is the laying out of a design that matches the lengths of two or more PCB tracks, also known as traces. These groups could be one of the following:

  • Single-ended tracks routed in parallel.
  • Differential pairs.
  • Multiple differential pairs in parallel.
  • Clock signals in either single-ended or differential pairs in parallel.

 

PCB tracks carrying digital signals do not necessarily need to be perfectly length matched. With copper surface layer tracking there will always be an amount of jitter on the rising edge, preventing signals routed in parallel to be perfectly length matched. The end goal is to reduce the length or timing mismatch below limiting values. The allowed length mismatch and timing mismatch are related to the signal velocity.

 

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How do you know what track length is optimum?

Component datasheets, signalling standards and interface standards are key. There is so much standardisation these days that most high-speed components use one of the many signalling standards. Manufacturers should always have impedance and routing specifications available.

A length mismatch can also be converted to a timing mismatch using signal velocity, although extreme care must be exercised when selecting the velocity of a digital signal. Modern digital signals, generally run with edge rates much less than 1 ns, will have bandwidths reaching into the high GHz, and will only tolerate very small mismatches. Dispersion in the PCB substrate causes the signal velocity to vary with frequency. For example, FR4 exhibits normal dispersion below ~1 GHz, so lower frequencies will arrive at the receiver earlier than higher frequencies.

 

Clock Signals

Using system clocks to trigger every device in a chain of components is extremely difficult in large designs. This is, in part, due to the fact that each integrated circuit has different logic gate delays and rise timing. Typically, modern digital components use source synchronisation or embedded clocks. For embedded clocks signals it is best practice to route a track alongside the parallel data track. The clock tracks also need to be length matched to other data tracks.

 

Track length matching to prevent skew on parallel data buses

pcb tracks

Skew simply refers to a timing mismatch between the rising edges of two or more digital signals in a parallel bus. The signal propagating on the shortest trace will arrive earliest, triggering a downstream gate before the other signals on the bus. eCADSTAR allows you to define buses and differential pairs at the schematic stage, enabling you to enforce track length matching your layout which will keep any skew within tolerable limits.

 

Length matching in multiple single-ended nets is straightforward. Adding tuning structures to all tracks on the bus ensures they are the same end length.

If you have multiple differential pairs carrying parallel data, each differential pair must be matched, and the pairs then need to be matched to each other.

 

What About Analogue Differential Signals?

Analogue signals can also be routed as differential pairs; however, it is less common even at very high frequencies. Some manufacturers offer differential op-amps for high GHz bandwidth allowing for easier routing of analogue signals around a board.

Analogue differential signals require accurate length matching, just like digital differential signals. The difference is that analogue differential signals resemble 3-phase AC wiring, where an adjacent analogue ground plane is used as a reference for both ends of the pair. While digital signals do not explicitly need a ground plane, placing a ground plane adjacent to the digital differential pair is beneficial for the reasons outlined above.

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Effective PCB design | How to ensure your PCB Trace Widths are Effective

Sarah Santangelli takes us through the importance of Trace Widths and what they mean for your PCB design.

What is Trace Width?

Trace widths

Trace width is an important design factor in PCB design. PCB trace could be analog, digital or power, from one junction to another.

The junction can be the pin of a component, a branch off of a larger trace or plane, or an empty pad or test-point intended for probing. Trace widths are often measured in mils, or thousands of an inch. A standard trace width for an ordinary signal (no special requirements) may be in the 7-12 mil range and be as long as a few inches, but there are many things that should be considered when defining the width and length of a trace.

Efficient Trace Width

To ensure effective operation in transmitting signal and power the importance of PCB Trace Width and resistance is crucial. So, what steps do you need to take?

Determine the standard track width to be used

  • It is important to balance the standard track size to be used within the design. Too narrow and too close means there is a higher chance of it shorting. If they are too wide and far apart, it can restrict the number of tracks in a given area and this may force the use of additional planes in the boards to ensure the PCB design can be routed. It is a balancing act that needs to be right.

 

Consider track size for lines carrying current

  • The thin tracks used in today’s printed circuit boards can only carry a limited current. Consideration needs to be given to the size of the trace for any that carry power rails rather than low level signals. The table below gives some track widths or a 10degree C temperature rise for different thickness copper boards.

 

RECOMMENDED MAXIMUM CURRENT FOR PCB TRACE
CURRENT
(AMPS)
WIDTH FOR 1 OZ BOARD
(THOUS)
WIDTH FOR 2 OZ BOARD
(THOUS)
1105
22015
35025

Fix the printed circuit board pad to hole ratio and size

  • At the beginning of the PCB design it will be necessary to determine the pad and hole dimensions. Typically a ratio of about 1.8: 1 (pad : hole) is used, although sometimes a pad 0..5 mm larger than the hole is used as the measure. This allows for hole drilling tolerances, etc. The manufacturer of the bare PCB will be able to advise on the standards that are required for their process. The ratio becomes more important as the size of the pads and holes reduces, and it is particularly important for via holes.

Determine PCB pad shapes

  • Component libraries associated with PCB CAD systems will have libraries for the schematic and PCB footprints for the different components. However these may vary according to the manufacturing process. Typically they need to be large for wave soldering than for infra-red reflow soldering. Thus the manufacturing process needs to be determined before the design starts so that the optimum pad sizes can be chosen and used on the PCB CAD system and hence on the printed circuit board itself.

 

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