Not a very long back in history, the PCB design job was merely about the “connecting-the-dots” and it was like solving a puzzle or playing like a game. Well not quite so, I am exaggerating a bit. But for sure, it was considered a relatively easy job until the technology trends changed and electronics industry entered in the era of technologies such as high speed Ethernet, RF, USB, WiMAX and WiFi, LTE, DDR, PCIe and few others that I missed here.
With these new technologies, the PCB design was no longer a simple job. It became a mixture of art, knowledge, wisdom, analytical judgement and more importantly an ability to see something which can’t be seen. Many times, people say, when you work with high speed PCB design, you are actually dealing with “BLACK MAGIC”
So,what are the issues in high speed PCB design (Printed Circuit Board Design)? Let tell you a few.
When you see a PCB in a product in working condition, let’s say, one of the electronic items that you have at your home, be it a modem, TV, some gadget, a laptop or be it anything. So, when you that PCB in working condition in that item, you would be tempted to believe that all the electronics and everything related to it is enclosed in that item and there is nothing outside that box. However, it’s not the case. That PCB is radiating some energy in the air. It’s called the electromagnetic energy and the issue about this radiation is called EMI – Electromagnetic Interference. And this radiated energy can be fatal. It can couple with or can be absorbed by the other electronic product (victim) and it can start working unexpectedly.
If you are traveling in a plane, you would be told to put of all the electronic items with you or put them in flight mode. The reason for the same is – EMI. The equipment’s in the pilot’s cabin for the communication with the station can start operating weirdly and it would be a nightmare.
Another issue with high PCB is the signal integrity. The signal on the board can lose its integrity, i.e. it’s intended value or amplitude by the source can be received erroneously by the receiver and all the calculation can go wrong.
We will see all these issue and few more in this article and see all the High Speed PCB design considerations that a PCB designer needs to make when he is dealing with high speed circuits.
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What is High Speed?
The speed of the circuit is its frequency element. What is the frequency of the signals in the board? So, we can easily categorize the frequencies in mostly three segments. In practical terms for PCB, Upto around 25 MHz (Mega Hetz), call it a low speed. From around 25 to 100 MHz, call it a moderate speed. From 100 to 1000 MHz, it’s the high speed PCB. Above this range, it’s the ultra-high, that is, somewhere around radio frequency (RF). So, we are in 100 to 1000 MHz band when we are talking about the High Speed PCB design considerations.
The maximum speed a signal on the board can have is equal to the speed of the light i.e. 3 x 10 raised to 8 m/s. i.e. 300000000 m/s. This speed is its speed in the free air. But when the medium is different, the speed is not the same. The speed with different medium is relative to the speed of light in the air, and it’s always less than the speed in the free air. In the PCB, this medium is dielectric. So, the dielectric constant of the material used as the dielectric, is what decides the speed of the signal.
When the current passes through a signal track, there is always an electromagnetic field generated by that current around the track. The direction and the amplitude of the field are dependent on the original signal. It is best described with right-hand-rule. Now this electromagnetic field travels with the signal wherever it goes.
Some part of this field is absorbed in the system itself, however, some part is radiated away from the system and that what the radiation is. When we deal with the High Speed PCB design, our aim is to reduce, if not completely eliminate, this radiation which is emitted away from the system.
The issues related to EMI and other high speed behavior
- Signal Reflection
- Ground Bounce
All of the above listed, any many more, issues are primarily witnessed due to one thing, the inadequate return path for the signal. There have been many books and other materials available on this topic, but to cover in depth anatomy of the matter is impossible in one article. But, the important thing is, it all depends on how the signal returns back to the source. Ideally, the signal and its return have to be exactly equal and opposite in nature. Which also implies that the path of the two has to be same as well. This path is a loop. As the loop increases, that means, as the return current path increases, all these issues arise.
High Speed PCB design Considerations
So, to sum up, we have to do some considerations and apply the solutions to the above mentioned problems while laying out the PCB.
Adequate return path– all the high speed signals have to be routed such a way that the return current flows very close to it. It implies that the return current has to flow on the adjacent layer exactly below the original signal. The return current will flow from the place where it finds the least impedance. The impedance is least on the continuous copper planes. That means, every high speed signal has to routed such a way that its entire path of run is a above related and continuous, unbroken copper plane layer.
Board Partitioning– Partition the different sections of the circuit and place them isolated from each other. Such as, the digital section has to be away from the analog section of the circuit. The I/O section has to be isolated from the digital and analog sections.
Avoid unwanted coupling– The two adjacent traces will couple into each other thereby bringing the unwanted effects. The impedance seen by each of these traces will differ due to this coupling. The coupling is inversely proportional to the separation between each other. As you move them away from each other, the coupling effect will reduce. This is also called as crosstalk.
Board stack up– The board stack has to be defined such a way that
- Every signal layer is referenced to a continuous plane layer
- There is at least one power-ground pair adjacent to each other
Impedance matching– Every transmission line has its own characteristic impedance. If this characteristic impedance is not same as that of the output impedance of the source or the input impedance of the receiver, the reflections will occur. The traces will have to be designed such a way that the characteristic impedance seen by the signal on the trace is matched to the source output impedance. Sometimes, terminating resistors will have to be added to match the impedance.
Reduce inductance– At higher frequencies, the behavior of the electronic components changes. A resistor is no longer a resistor alone. It also acts as an inductor. Capacitor starts acting as an inductor as well. This inductance can come from solder pad joints, inner parasitic elements, component leads etc. The aim should be reduce the inductance added to the circuit due to such things. Such as using a wider trace to connect the power and ground for bypass capacitor.
Bypassing and decoupling– Bypassing and decoupling play very important role in high speed design considerations. The placement of the capacitors and the connections to the power rails and the ground is key to the effective bypassing.
3-W rule– 3-W rule is used basically to minimize coupling between transmission lines or PCB traces. The rules states that the distance separation between traces must be three times the width of a single trace measured from center to center. This rule for trace separation will reduce the cross-talk ﬂux by approximately 70%. (For a 98% reduction, change the 3 to 10.)
20-h rule– There is inter-plane coupling between the power and the ground plane. This creates a magnetic flux linkage. Off the edges of the PCB, RF currents will radiate due to this flux linkage. This coupling is called fringing. It is more commonly observed on very high-speed PCB. If we make the power plane on the layers that are adjacent to the ground plane layer smaller than the ground plane, this flux will be absorbed by the ground plane and will not radiate to the outside world. The general rule is to make the power plane smaller than the ground plane by 20 times H where H is the dielectric thickness between the adjacent power and ground layers.
Critical Frequencies– A portion of RF current waveform subjects a product to RF corruption and it is called as the Critical frequency. Any wavelength less than λ/20 of its respective frequency is the critical wavelength. Stitching vias are often used to tie ground planes and ground metal pours to planes at regular distance and the distance is preferably λ/ 20.
As listed, these are few considerations we have to make when we work on the layout of a PCB that is high speed. There are many more books and reference available for high speed PCB design considerations and some of the considerations become stale as the new technology evolves.
As popularly said, it’s a thing to deal with the black magic. There are no strict rules that apply to all the design scenarios and an experienced PCB design has to make rational judgments based on the design requirements.