What is propagation delay on PCB?

Latency is frustrating, both in our daily lives and in the devices we conveniently use. Propagation delay/time delay inhibits the intended functionality of the device. I say “I am responsible for your product” when I am responsible for the basic functionality of your product. These components are arranged in a specific pattern based on the circuitry etched on the printed circuit board.

PCB circuits are the signal carriers in most devices used in today’s electronics industry. Good PCB design must ensure timely transmission of signals from the source to the load by adjusting trace impedance and trace length to control propagation delay.

High-speed interfaces such as DDR3, HDMI, Ethernet, SATA, PCI ExpressUSB, etc. are used for fast data transmission and are prone to problems related to time delays. This is why synchronization between high-speed signal clock and data signals and between source and destination components is critical when designing PCBs. If they don’t sync, the data will be corrupted.

In this article, we will discuss and let you understand the delay effects of signals in PCB.

1.What is propagation delay on PCB?

The propagation delay on the PCB tracks the one-way (source to load) time it takes for a signal to travel on that trace. It is expressed in units of time. Propagation delay is a function of dielectric constant (Er) and trace geometry. For a given PCB laminate and a given dielectric constant, the delay for different impedance lines is fixed. Learn more reading How to Select PCB Materials and Laminates .

Why does propagation delay occur?

Stray capacitance is the cause of delay in any circuit. We don’t intentionally use capacitors in circuits; however, there are capacitors. The total capacitance depends on their relative sizes and the geometry of the traces. Stray capacitance includes line-to-line capacitance, capacitance between lines and adjacent ground planes, and capacitance between traces and free space.

The effect of stray capacitance causes signal delay. The bottom line is that when a digital signal changes state, it takes some time for the conversion to occur at the destination due to stray capacitances that must be charged and discharged.

2. What is the difference between transmission delay and delay?

Transmission delay and propagation delay can be used interchangeably, but they need to be treated differently when a system needs to be analyzed from the analog/digital side.

The timing of logic signals in PCB traces can be calculated using the power transmission line method:

Method 1: Terminate the transmission line through its characteristic impedance when the delay on one side of the PCB trace is equal to or greater than half the rise/fall time of the applied signal.

Example: A 2-inch microstrip line will exhibit a delay of approximately 270ps at Er of 4. Using the above method, the terminal will be accurate when the signal rise time is less than 500ps.

Method 2: Use PCB trace length 2 inches/ns (rise/fall time) method. If signal traces exceed trace length standards, termination must be used.

Example: PCB traces designed for high-speed logic with 5ns rise/fall times should be terminated by their characteristic impedance if the trace length is 10 inches or gre

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3. How does propagation delay affect signal speed on PCB traces?

When a PCB operates at high frequencies, its trace paths need to be treated as transmission lines. Controlled impedance calculations are important for these transmission lines so signal reflections, crosstalk, noise, and ground bounce may be mitigated. These issues require special attention as they may pose a threat to the signal quality, causing the entire system to malfunction. This is why we need to know how fast a signal propagates through a transmission line and the propagation delay associated with it.

How to calculate signal speed?

Electromagnetic signals travel in vacuum/air at the same speed as light, i.e.:

Vc= 3 x 10^8m/s = 186,000 mils/s = 11.8in/ns

For microstrip/stripline PCB designs, the signal speed is calculated as follows:

Where? Vc is the speed of light in vacuum/air.


Signal speed in microstrip/stripline PCB designs.

Ereff is the effective dielectric constant of the microstrip line, and its value is between 1 and Er.

Ereff= (0.64 Er + 0.36)

It means if the signal speed on the PCB is lower than the signal speed in the air. If the PCB material has an Er of 4, then the signal speed on the stripline design is half that in air, about 6in/ns.

How to measure propagation delay?

Let’s calculate propagation delay/time delay using trace length and vice versa. Mathematically, the delay is tpd:1/v. where v is the speed of the signal in the PCB transmission line. In vacuum/air, equal to 85ps/In.

On a PCB transmission line, tpd is given by:

Propagation delays in microstrip/stripline PCB designs.

Signal speed on the interconnect

When a voltage is applied between the signal and the return path of a transmission line, the signal propagates down the transmission line. There is always a voltage distribution between the signal and the loop. The voltage distribution on the transmission line is shown in the figure below.


The voltage distribution between signals and loops on a transmission line.

4. How to reduce/control propagation delay?

We cannot change the speed of the clock signal, but we can change the time the signal arrives by changing the PCB trace length. Small changes in trace length can help PCB components simultaneously synchronize with the clock signal and thus achieve their fixed state.

Clock skew calculations for different PCB components also help compensate for propagation delay/latency issues. The serpentine is another method that gives the signal the time it needs to reach full level before the next clock pulse arrives. It delivers clock pulses that are just the perfect amount of delay. In best practice, differential traces are best bent together and must remain tightly coupled. So, how do you decide which traces have to be serpentine?

Identify the longest signal trace and snake the remaining traces through the network to synchronize the signal on all traces.

Adjust the length of clock traces connecting components in a given network.

Delay the timing of the clock pulse until the integrated circuit can ramp up to full voltage application.

Note: In signal nets, traces with mismatched lengths always match the trace with the largest length. Add meanders to shorter traces to increase their length.

Trace length adjustment/matching

Due to timing mismatch, signals traveling through different traces in the PCB arrive at the load at different times. Delay tuning is critical to ensure that data arrives at the load at the exact time across multiple parallel interconnects, traces in differential pairs, and clock signals. Perform trace length adjustments to account for signal latency.

Delay and trace length adjustments reflect the same idea. For length adjustment, you can set the length of the signal trace in the matching net group. As we all know, timing delays are inevitable. We can only take steps to control it. By placing traces with the same length in matching groups, we try to ensure that all signals crossing these traces arrive within a fixed time delay. Now the question is, what if there are two unmatched signal traces in a matched group? Signals traversing such traces can be synchronized by adding delays to shorter signal traces, called serpentines.

In theory, the receiver will pick up any waveform/signal sent by the **** immediately. We also assume that the clock signals have zero time gaps and are processed simultaneously at the receiver. But the reality is different. If there is a slight mismatch in the length of the PCB traces, the signals may not arrive at the receive pins at the same time. PCB trace tuning ensures equal trace lengths with matching arrival times of critical signals.

Control propagation delays with controllable impedance traces
Different trace geometries/structures are available for controlled impedance traces. The tracking configuration we discuss here is based on IPC standard 2141A.

Microstrip: It consists of a discrete insulated wire separated by a fixed distance above the ground plane. The dielectric can be the insulating walls of the wire, or it can be a combination of this insulation and air.

image.png

Surface Microstrip: In a double-sided PCB design with a ground plane on one side, the signal trace on the other side can be designed with controlled impedance. This tracking structure is called a surface microstrip or simply a microstrip.

One side is the ground plane and the other side is the surface microstrip line for the signal traces.

Embedded/Symmetrical Stripline: In multilayer PCBs, this arrangement enables signal tracking between power and ground planes. The return current path of the high frequency signal trace is above and below the signal trace on the plane. Therefore, high-frequency signals remain inside the PCB, reducing interference and shielding other stray signals.

Symmetric stripline enables signal tracking between power and ground planes.

Note: The ground layer is at zero potential, and the low-impedance reference plane has a larger area.

Coplanar Structure: Coplanar waveguide structures have signal traces and return path conductors on the same layer of the PCB. The signal trace is in the center and surrounded by two adjacent external ground planes; it is called “coplanar” because these three planar structures are on the same plane. The PCB dielectric is underneath. Both microstrip and stripline may have coplanar structures

A coplanar structure has a signal trace surrounded by two adjacent external ground planes.

High-speed PCB design rules for reducing transmission delays
Accurate time delay management requires trace length calculations to implement high-speed PCB routing therefore. The distance between components on a PCB is very short. We don’t even consider the time it takes for the signal to travel across the PCB. But in high-speed printed circuit boards, the switching occurs at an edge rate of less than 1ns. Such short distances between components are obviously of paramount importance if you want your system to operate without interference.

The designer must know the specific timing tolerances of the expected PCB signals.

Different interfaces operating at different data rates using different signaling standards. Specify different allowed length or time mismatches. When planning I/O channels in a PCB, designers should look for acceptable length mismatches and translate them into allowable timing mismatches.

Avoid substrates with larger dielectric constants: Typically, substrates with larger dielectric constants result in lower signal speeds that exceed the allowed specified propagation delays, especially between data lines and clocks.

Pay attention to pin package delay: when a signal reaches a pin/pad of a specific component, it still needs to traverse the exposed conductor path along the inside of the component package. As it travels through signal traces and connections associated with internal circuitry, signals experience parasitic inductances and capacitances that affect how quickly the signal propagates in the connection compared to the trace. The connecting wires also have different geometries, adding to the signal delay on the different pins/pads. It is recommended to ask the manufacturer about pin package delays associated with a specific part. It is expressed in lengths in picoseconds (ps) or millimeters/micrometers. This length should be considered when doing any delay/length tuning on differential pair signals or single-ended signals.

Watch out for your “clock skew”: When the clock and signal traces are different lengths, it causes a timing mismatch called clock skew. Ignoring this issue may result in incorrect data being locked. The clock delay should be greater than the maximum delay in the data signal.

Check Tilt Duration: For clock and receiver datasheets, always refer to the datasheets to understand the settings and tolerances of the parts.

Material dispersion is a high-frequency phenomenon. Be sure to consider the switching frequency.

Consider these parameters before choosing high-speed PCB design software
If you use software that only considers length mismatch, you can easily calculate the correct length mismatch value for a given substrate. This length mismatch is equal to the timing mismatch times the signal speed in a given substrate (in/ns). A good PCB design software should provide the following for target propagation delays:

Interactive routing capabilities

Smart length adjustment

IPP enhancements (instant access to new and existing features)

Dynamic data model (accelerates network list creation)

User friendly

High speed printed circuit board materials

We all know that signal speed is not only affected by trace geometry and trace location, but also by substrate material. The dielectric constant of a trace determines its signal speed. Let’s look at an example; if we suspend a trajectory in a vacuum, only the dielectric constant of that trajectory affects the velocity. If the trace is on the PCB surface, the signal speed will also be affected by other nearby media; this is what happens in PCBs. The circuit runs on the PCB substrate material (which has a fixed dielectric constant value). The dielectric constant of the substrate, air, and solder mask are the main factors that determine signal speed. This is called the effective dielectric constant (erbium effective). Plate thickness also affects the speed of the signal. See more information on Learning High-Speed PCB Design.

FR4 can be used in high-speed PCB designs when the layers are laminated with high-speed laminates such as Rogers. In the high frequency band, FR4 experiences dispersion, thereby increasing transmission speed and reducing transmission delay. Electromagnetic absorption in FR4 produces more traces of signal attenuation associated with it. Therefore, high-speed laminates are used along with FR4 laminates underneath high-speed traces, especially at frequencies above 5GHz.

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5. Propagation delay of several PCB materials

The signal speed and propagation/delay of some PCB materials are shown in the table below:

The graph below depicts that there is a direct correlation between signal loss and frequency. At the same time, we can also see that some materials have less loss than others. This diagram shows which materials may have better electrical properties at higher speeds.

As frequency increases, some materials exhibit less lossy behavior.

The primary function of a printed circuit board is to transmit signals between devices accurately and without loss. Propagation delay or time delay plays an important role in the proper operation of high speed boards. It can’t be completely avoided, but it can be reduced by employing different trace configurations and then precise length matching and bending as needed. You should know the dielectrics in your design, the materials, and which traces require delay measurements. You also need to understand the relationship between signals such as clock and data to make the board work properly.

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