Not exactly what you might think.
All PCB and Hardware Engineers have heard it at least once: 'Never use 90-degree bends when routing your PCB traces.'
When asked why (and rightly so), the majority of them say, 'Because when electrons reach the corner, they accelerate, causing radiations and reflections.'
The question at this point is:
Is it true that this is the cause of the reflections? But more importantly, should we care?
I aim to be as practical as possible and provide real answers:
The reflections are caused by the increase in capacitance at the corner.
This increase in capacitance is simply due to the added metal.
In fact, if you simply look at the picture below, W2 is > W1.
Figure 2 - 90-degree bend trace width difference
How does this relate to capacitance?
The capacitance for a 50 Ohm line can be calculated as follows:
Where:
Ccorner = the excess capacitance in a corner we want to calculate.
w = the line width of the 50 Ohm transmission line.
Clen = the capacitance per length, approximately 3.4 fF/mil is the capacitance per length in a 50-ohm line with a Dk = 4.
Based on this finding, we can also observe the following:
The influence is not as significant when using narrow traces compared to thicker traces.
Meaning:
Narrow traces lead to less added metal at the corner, resulting in less added capacitance, meaning fewer reflections.
Wider traces lead to more added metal at the corner, resulting in more added capacitance, meaning more reflections.
Now that we can put this into numbers, we can decide if this added capacitance is important for us or not. So the question we should ask is, do I have enough capacitance budget on this trace for my signals to tolerate these disturbances? Are my signals so sensitive to these variations?
Well, this will depend on which signals I need to transmit on these traces, so we should determine how much change in capacitance we can tolerate.
What if my budget for capacitance variation is low?
How can I improve the quality of my signal transmission and reduce reflections?
Usually, I use rounded traces (or at least 45-degree bends), even when I can tolerate a good range of disturbances. This allows me to:
First, reduce reflections.
Second, give my PCBs my personal aesthetic touch.
So, it's a win-win.
I hope this helps,
Dario
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