Guide on PCB Trace Length Matching vs Frequency

August 13, 2019 Cadence PCB Solutions

Diagram of circuit and routing connections

 

When I first started out baking, I was extremely haphazard with my measurements and materials. So long as it tasted great, I figured, so what if it looks a mess? It took years of this mindset before I finally admitted to myself to try adhering to recipes 100% of the way, and let me be the first to say: I was wrong. A cake might taste great with a little extra butter and a little less sugar, but my god does it taste amazing with the exact proportion of materials. And that’s not even discussing how pristine and perfect it looks.

If you go to a bakery, you most likely wouldn’t pick up a cake with shoddy icing detail and a lopsided sponge. The same goes for when you mismatch certain PCB traces in your design: you’re inviting nasty problems that will haunt you post-deployment. But when should PCB traces be of equal length? And is there tolerance to the difference in length? 

What Is PCB Trace Length Matching 

In simple words, PCB trace length matching is the process of ensuring two or more signal traces are of the same length. The question that begs an answer is: what types of traces need to be matched? 

To understand PCB trace matching, you’ll need to forget the ideal concept of electronics, where the receiver immediately receives any waveform initiated by a transmitter. Theoretically, we have assumed that clocked signals have zero time gap and will be processed concurrently at the receiver.  

But in reality, the pulses on a bus may not arrive concurrently at the receiving pins, if there is a degree of mismatch on the length of the PCB traces. PCB trace length matching ensures that these traces, where the timing of the arrival of the pulses is critical, are matched to equal length.  

Why PCB Trace Length Matching Matters 

If your idea of PCB routing neglects PCB trace length matching, you’re putting the entire design at risk. It doesn’t matter if the DRC check tells you that all your nets are routed, but if buses or differential signals are taking different paths, you will have some cleanups to do later. 

While different paths may lead to the same destination, this doesn’t guarantee the best experience for the receiving components. This is specifically true for synchronous protocols like SPI, I2C CAN, ISA, ATA, and PCI. 

Synchronous protocols require precise timing between the clock and data signals. The data needs to be stable within a timeframe, and the transitioning of the clock signal will latch the data to the receiver. When the clock and data traces have different lengths, a timing mismatch called clock skew happens. This may lead to the wrong state of data being latched and affecting the functionality of the electronics. 

 

Capture of lit-up, high speed routing on a circuit

PCB trace length mismatch can result in clock skew.

 

Besides synchronous signals, PCB trace length matching is also a best practice in differential signals. Differential signals work on the assumption that both traces are at the complete opposite of each other in amplitude. While having similar trace length keeps timing difference to a minimal, it also prevents EMI from unanticipated ground current flow from the mismatch.

At What Frequency Should You Worry About PCB Trace Length Matching 

You shouldn’t freak out just yet, just because you’ve forgotten to match data buses or differential pairs. Depending on the difference between the PCB trace lengths and the speed of the traveling pulse, you’ll be able to estimate if there are going to be any issues with signal integrity.

 

Sheep figuring standing at the center of routed traces on a circuit

PCB trace length matching is crucial for high frequency synchronous signals.

 

As you’re probably aware, signals travel on PCB traces with a certain speed. This parameter is termed as the propagation delay. Communication signals operate at different frequencies, and you’re able to get the clock period by inverting the frequency value. 

Propagation delay is expressed in time per unit of length. By estimating the total mismatch length, you’re able to get the resulting skew duration. You’ll then refer to the datasheet on the receiver component, and acquire the tolerance in the timing for the data setup, latch, and hold for a valid cycle.

As long as the skew duration falls within the tolerance window, you’re unlikely to bounce into any complications. I would make it a best practice to match PCB trace lengths wherever they are recommended.

With the right PCB software, matching PCB trace length is an easy process. You can make use of the length matching feature in OrCAD PCB Designer to keep PCB traces length within a tolerance value. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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