High-Speed Routing on a Two-Layer Board
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- Опубліковано 20 тра 2024
- Two-layer boards are generally seen as the hobbyist's friend, but can they reliably be used to route digital or high-speed signals? In this video, Tech Consultant Zach Peterson dives deep into both if and how PCB Designers can route high-speed signals on two-layer boards.
0:00 Intro
1:11 Using 2 Layer for Digital & High-Speed Boards
2:18 Impedance
6:12 Input Impedance
7:21 Two-Layer Board Interfaces
9:06 Trace Length Considerations
12:54 Aren't Two-Layer Boards Differential?
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Why is There a Transmission Line Critical Length?: resources.altium.com/p/why-th...
What is High-Speed Design?: resources.altium.com/p/what-h...
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Hello Zach Peterson, I want to congratulate you for sharing your knowledge with everyone without any profit motive, and I take this opportunity to tell you that I am a bus driver here in Venezuela but that all my life I have been passionate about electronics at a hobby level and I have I have become a crazy fan of all your videos mainly because UA-cam offers them translated into my native language, which is Spanish. I have also subscribed to your personal UA-cam channel. At this moment I am studying signal integrity and SI/PDN power integrity and your videos have made many things clear to me. Thank you and greetings from my beautiful and macho Venezuela.
We’re so glad you’re enjoying Zach’s content! We always try to have fully translated captions in Spanish and a bunch of other languages, so it’s great to hear that that’s valuable to you 😊
Comments like this are the reason I do what I do, it's great to hear that you are studying to work in this field. Best of luck and feel free to ask me questions any time.
Haven't stopped to do the trace width calculation but using a 0.8mm thick 2 layer board would make the trace widths more manageable? Surely this would be better than the usual 1.6mm board.
Please tell me how do you calculate the Power handling in the Power divider or combiner?
Thank you very much for your help.
Eric Bogatin did a good video about 2 layer routing, including how to route signals on other sides and maintain signal integrity (as much as can be done on 2 layers).
Can you provide then Link to the Video ?
@@paulhome2023 probably this one, in the second half of it: ua-cam.com/video/DIMIzKRmync/v-deo.html
Really the practical advice would be to just go for 4 layer PCB. They now cost just a little bit more than 2 layer one overseas, but advantages they bring are massive and are totally worth a little extra.
Totally agree with you! That's why I call a 4-layer stackup "entry level". However, some people still insist on doing some stuff on 2-layer, so it's worth bringing up. Realistically, you can only do two interfaces on 2-layer and do them well: USB 2.0 and 10/100 Ethernet. I'm interested to see if anyone else has done a 2-layer board with other interfaces (maybe pre-DDR SDRAM?) just to see it worked.
@@Zachariah-Peterson I've seen HDMI TX IC breakout boards done on two layers for up to 1080p (so 1450 Mbps per lane). This interface is surprisingly robust, so you can get away with a lot of sketchy stuff. Like you mentioned in the video, as long as traces are not very long, it should still work. I imagine that other widely used cable-based serial interfaces will also work as they are designed to work even over crappy $5 cables.
Very Good Super Tutorial .
Many many thanks
Wild. I managed to successfully make a single-layer USB device, years before I knew any of this. I expect it wasn't high-speed though. Roughly speaking, it only converts USB OUT packets to 250kbaud RS485, so even a 12Mbps connection is exceedingly fast for that.
Thank you for video!
My question - Can I use single coplanar in two-layer board? how much ground should be near signal trace?
I see that for 50 Ohm and clearance 0.15mm width of trace must be 0.65mm - it's thick trace but may be there's still some proplems?
Yes that sounds accurate but it depends on the board dielectric constant. You would need to have low clearance to the nearby ground to get to the right impedance. In Altium Designer, using the default Dk value of 4.8 on a 62 mil board, I found that the width was 15 mils and clearance was 4 mil to get a 50 Ohm trace.
@@Zachariah-Peterson thank you very much for answer!
Great
How do we decide if the PCB t requires high speed design or not required?
If the interface you are using on a component has an impedance specification then you need to design the traces to have the required impedance value. This is a basic requirement in any PCB that has high speed interfaces. We call a PCB "high speed" based on the rise time/fall time of the signals in the components, not on the clock rate or data rate. Generally, when signal rise/fall times are around 10 ns, then it is best to start with a 4 layer board. Some boards will use two layers with coplanar ground, and sometimes they pass EMC testing but they will fail if not laid out correctly on two layers. That's why the best case is 4 layers, and it is not much more expensive to design on 4 layers instead of 2 layers.
.6mm 2 layer board makes for small matched traces!
I've sent a question to that email, but received two notifications stating that address is not found. Maybe these addresses belong to folks who have left the company. Either way, you might want to check it out if it works properly.
I think I received it, hopefully IT fixes it soon. We'll be filming the Q&A session next week before we do our podcast with Eric Bogatin!
@@Zachariah-Peterson Sounds good, looking forward to it!
I asked about high speed tools for Altium. They don't do an equivalent of Hyperlynx I'm told, sadly.
Hi Robert, there is a new connector for AD + Hyperlynx that should be coming out soon, it will let you open your AD board inside Hyperlynx so that you can use all of their utilities.
@@Zachariah-Peterson Thanks. It was a reason we didn't use Altium back when I did car ADAS with Mentor Tools. Let's hope it's not just Boeing, NASA and Apple that can afford the tool though.... 🤣
USB signals uses differential pair so this was bad example. They do not use, virtually, ground as impedance references. (strictly only for imbalanced current due to non-ideal transceivers) Another problem is you have to provide current return path for high speed signals, no one tell that have to be done in different layer as long as it is easyest for many layer pcb.
I talked about differential pairs at the end of the video, and we in another video I showed an example specifically with USB as an example on a 2 layer board. You need to have ground somewhere otherwise those traces might get too fat, or you have to put them too close together, depending on board thickness and distance to the GND plane/pour. And yes, differential pairs can and do use ground as a reference, it is done to define the impedance and to reduce parasitic coupling to other interconnects, including other differential pairs, it affects differential crosstalk and differential-to-common-mode crosstalk. You can put a return path somewhere else, but if it's too large be prepared for a lot of noise and crosstalk. There are other reasons to use ground plane/pour as well, mainly power integrity once you have a system that is sourcing a lot of high speed with fast edge rates.
you do not need a ground plane just a plane
Hi Roger! Technically you need a low-impedance path to ground, which in a 2-layer board is most easily provided by a ground plane. If you can figure out how to do that with a power plane as the reference and a capacitive path to a ground trace/rail without creating highly inductive current paths then that's great, it's just harder with a 2-layer board than with a 4/6/8 etc. layer board.