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PCB Mounting Holes | PCB Design Techniques
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- Опубликовано: 17 авг 2024
- Mounting Holes are a huge component in PCB Design. But where should they be placed? Should you ground them? Should they be plated? Today, Tech Consultant Zach Peterson dives deep into PCB Mounting Holes to answer these questions and more.
0:00 Intro
1:15 Where Should You Place PCB Mounting Holes?
2:22 Should You Plate Mounting Holes?
6:20 Return Current Considerations
8:40 Capacitive Coupling and Common-Mode Noise
9:10 Common-Mode Noise Closer Look
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Great video! Thanks
I love this guy
Thanks! Hit that subscribe button to see more!
Another consideration with NPTH is how much pull back you call out for your copper planes. Doing RF boards I try to give a generous amount of pull back as to not have new parasitics when mounting a PCB to some sort of chuck. Also using nylon or polymer fasteners rather than metal fasteners can help with grounding or coupling issues though mounting holes.
That's a great point, parasitics always change things, including with mounting holes and coupling around the board. I bring up something like a metal screw since the stuff I work on always needs strong mechanical mounting, but something like a plastic standoff is a good solution. I've done that in the past with prototype wifi modules, but since we didn't have an enclosure on those prototypes I never worried about it.
Somewhat off topic but in multi layer boards remember to set up rules so that a screw never will be able to "chew" in to the board material and cause a short from ground to the internal power planes With the result of overheating and even a fire hazzard. An example of this is the NZXT H1 PC case debacle. NZXT had to do a recall of 32000 PC cases sold in the US (data from the Verge)
Ultimately one has to mount the pcb with the metal enclosure but regardless the mounting hole is grounded or not, either the multiple ground return paths or common mode noise propagation through the capacitive coupling occurs. Then how to get rid of the problem?
Sometimes capacitive coupling from ground to chassis can be a good thing. In instrument amplifiers, it is common practice to place a 100nF capacitor from the instrument input's ground pin or lug to the chassis, which supposedly shunts RF picked up by the instrument cable shield to earth; there are quite a few stories floating around about amps without such a capacitor acting as wide-band receivers picking up ATC or CB chatter. Supposedly the capacitor provides a lower impedance return path than the amplifier's ground plane or bus. It occurs to me that a non-grounded plated through mounting hole is a convenient way to connect the RF bypass capacitor to chassis earth.
This video feels not complete. How to get rid of the common mode? I have an AC/DC adapter powered board, grounded all mounting holes to metal casing. Users experience tiny shocks sometimes. I think because of the y-caps inside the SMPS?
Awesome vids!
I'm also curious about the best practices when the power source is an AC mains cord. There's probably a lot of authoritative-sounding conflicting advice out there on what to do with neutral, what to do with ground, and whether to put anything between the holes and the enclosure, etc.
Yeah there is a decent amount of information out there, and I think the difficulty is in when to apply various recommendations to different systems. Recommendations about using the GND and neutral wire are sometimes conflicting, but different recommendations can both be right in different situations.
If you think about AC mains the point of the GND wire is top provide a single reference point for other signals, so it can be used as ESD/RFI shielding, a protective GND point, and/or as a sink for high frequency noise depending on how it is connected on the PWR input of the system. We did a video previously that looked at some of this stuff, you can find it here: ruclips.net/video/hMyYYERfPhQ/видео.html
Hi, this video reminds me a lot the desktop computer's power supply where is grounded on enclosure (I think with one only screw) and it have common mode choke for the rejection of the common mode noise that is created from the parasitic coupling. If you please could you make a video about the structure of a computers power supply. I beleive it includes many design challenges and rules that you have said on many from your videos.
Hello, that's a great idea, I'll try to put together a teardown and walkthrough video.
Did Altium change circuit maker from paid to free? Last I heard it was a $500.00 program. Love these videos.
CircuitMaker is a open-source, community driven free tool. Circuit Studio is an affordable, professional level tool.
circuitmaker.com/
www.altium.com/circuitstudio/
Do you only need to connect one mounting hole to chassis to prevent safety issues? Am I right?
Not necessarily... It depends where you expect some kind of safety problem to occur and what exactly that safety problem is.
If your main worry is an ESD event inducing a high voltage pulse in the board, then you want to have low-impedance connections to some safety ground near where you expect the event to occur, like the enclosure. This is really common in industrial Ethernet: the port is usually where this occurs because, even in an insulated enclosure, some metal on the port will be exposed. In these systems, the port is connected directly to a section of the board that is connected back to the chassis with a low-impedance connection (large GND plane plus multiple screws), so any ESD would follow this low impedance path back to GND rather than follow a path upstream into nearby components. Other systems that have shrouded connectors can do the same type of thing for the same reason. The risk here is that poor grounding in the system creates a ground loop that carries noise, or it's possible for capacitive coupling into a transformer/ferrite (people who do Ethernet have debated this forever) if you don't know how to predict return paths throughout the system. You can read more about this here as it's related to your question: www.signalintegrityjournal.com/articles/1808-ethernet-connectors-and-routing-above-ground-planes
What ticks me off is the number of off the shelf AC/DC brick switchers that think they are doing you a favor when they short DC gnd to AC earth ! And they have a plastic case.
You've just given me an idea for another article/video
How are we supposed to square away the seeming conflict between what were taught in physics, that EM radiation can't escape a faraday cage, and what is being described at 5m, that a (effectively) faraday cage can aid EM radiation?
Hi Matt, at 5m I'm showing a conductor that isn't grounded yet. And it's not necessarily that adding the enclosure makes radiation worse, but it can still permit radiation. In a physics class, we assume (but we don't state clearly, I am guilty of this from when I was teaching) that an ideal Faraday cage is a perfect conductor, so it will have enough free charge to counter any electromagnetic wave or electric field that is incident on the metal, thus the E-field inside the metal is zero. Real conductors are not perfect and they do not provide infinite shielding effectiveness when used in a Faraday cage. When left floating, they can couple to the PCB or external sources at high frequency and radiate or conduct noise. Grounding is known to increase shielding effectiveness. You have a similar problem from heat sinks, which can act like antennas when left floating.
Zachariah Peterson You might also mention the skin effect and skin depth. The current in a conductor doesn't flow uniformly throughout but instead mainly along the surface. How deep into the surface is called skin depth and has a relationship with frequency.
If the frequency is such that the skin depth exceeds the thickness of the shield, the shield may radiate.
Don't forget to call your fabricator!
Wich software did you use @ 9:15 ?
I'm not very artistic so I just did the drawing with the shape tools in powerpoint.
zooming in/out on the whiteboard is pretty annoying. Please do not do that again.