PCB Grounding/Shielding Question - Page 1
Forum section: electronics / projects

author: homebrew
author_rank: Frequent Contributor
timestamp: December 19, 2020, 07:52:58 am
content: Hi everyoneI probably screwed up big time in my strive to to produce good shielding at the level of the PCB.When designing the board, I followed an unrelated application note from nxp (https://www.nxp.com/docs/en/application-note/AN4215.pdf,section 2.3 page 3), wehere an unbroken chassis ground ring along the perimeter of the entire board is suggested. At the time of designing, I found this a good idea to follow.However, I now read some posts (e.g.https://electronics.stackexchange.com/questions/37710/ground-ring-good-or-bad/37714) that state that unbroken ground rings could resemble a loop antenna and actually make matters worse. Actually googling the topic I find much more examples of ground shield rings that do have a slot at some point.This is a 6 layer design with two internal, unbroken ground layers connected to the shield ring via parallel R/C right next to the connector shields.The boards are already manufactured (just a few prototypes, however still expensive!) and I don't want to throw them away.Now what to do?1) Leave the boards as is?2) Use a saw to manually cut a slot into the ground ring?

author: jonpaul
author_rank: Super Contributor
timestamp: December 19, 2020, 10:14:55 am
content: Hello: Board is used in what application eg audio, video, broadcast, etc?Freq min/max of signals and clock?I would assemble one and test 99% its fine.Jon

author: Siwastaja
author_rank: Super Contributor
timestamp: December 19, 2020, 10:41:45 am
content: Actual EMC compliance requirements? Or just a prototype/research/hobby?EMC is complex. Single ground plane rule-of-thumb is good because it's the simplest, and produces typically good results. It doesn't mean there sometimes isn't a real reason to split planes or do AC coupling of ground regions, but doing them "by default" by inexperienced designer as a "rules-of-thumb" is not sensible.Whatever you do: if EMI is important, test & measure. If you have an actual problem, then you can try joining your planes together, scratch away some solder mask and solder jumpers every inch or so, then test again. It may get better, or worse.Next time, when in doubt, start from the simplest iteration and add tricks like split plane or separate chassis ground only if you A) know what you are doing really well, or B) have measured actual problem and havemeasuredthat doing the trick improves the situation.

author: homebrew
author_rank: Frequent Contributor
timestamp: December 19, 2020, 11:47:29 am
content: Hi everyonethanks for the comments and suggestions.Calling me an inexperienced designer? Yes, indeed! This project is at the very edge of what I feel capable of doing. Maybe it was a bit more than I could chew. But classic Dunning-Kruger-Effect will trick you quickly into the "Come on, how hard can it be regime ..."No, there aren't (at least not yet) any kind of EMC compliance requirements, it is just a "research" prototype sitting on my bench.The board is part of a proof-of-concept for a remote (later, without Covid-19, hands-on) networking lab to support teaching of computer network basics. Based around a Raspberry PI compute module (CM3+/Lite), it offers multiple network ports and USB/SATA connections to explore firewalling and high availability concepts.So yes, unavoidably there are fast signals on the board (100M ethernet, USB2.0, HDMI, SATA), stuff like that ...All these fast signals are routed on the outer layers with an uncut GND plane directly below them to have the impedances controlled as good as possible in that low cost setup. Planes are stitched heavily together using viasStackup:-- Signal / Ground-Pour-- GND-- Signal (Low-Speed / GPIO)-- Power-- GND-- Signal / Ground-PourI've one assembled prototype that is functioning very well.The desire to have a separate chassis ground started with the question on how to connect the shields of ethernet or USB to the board. Many designs do apparently not directly connect them to system GND but rather do only AC coupling through a cap and add an R to remove any static charges if I understand that correctly. Finding the aforementioned application note it seemed like a good idea to keep the shield separated from the ground planes and tie all the shields together. The driver behind that was rather ESD and not so much EMI.After reading more on that subject I became concerned that I introduced an unnecessary EMI issue and that's why I did start this thread.

author: jonpaul
author_rank: Super Contributor
timestamp: December 19, 2020, 12:33:08 pm
content: For that app, just use chassis ground and connect to PCB ground.Do not worry about the ring!100 MHz is not high nowadays, as many circuits are far higher.For EMI the bypass caps and power decoupling will be more useful.Finally most modern PCB layouts do not use a ring for ground, but an entre inner ground layer.EG for 6 layer PCB, top/bottom: circuit tracesInner 2 = GRDinner 3 = via tiesInner 4 = VccA full and uniform ground plane is the ideal.IF you want to learn more read some of the excellent books on EMC compliance and PCB layout for compliance.Kind Regards,Jon

author: homebrew
author_rank: Frequent Contributor
timestamp: December 19, 2020, 01:53:35 pm
content: Quote from: jonpaul on December 19, 2020, 12:33:08 pmThank's Jon, that's good news to start :-)Quote from: jonpaul on December 19, 2020, 12:33:08 pmLuckily, I didn't use a ground ring either. Layers 2 and 5 are solid and uncut GND-Planes. Everything references to them. However, I do have a separate 'shield', 'chassis ground', 'connector ground' (whatever it is correctly called) ring around the entire perimeter of the board. That ring is connected to the connector shields and to the system GND planes via Rs and Cs at each connector.Quote from: jonpaul on December 19, 2020, 12:33:08 pmCould you suggest a particular one? Especially ones that teach 'practice' and not just pure theory.

author: jonpaul
author_rank: Super Contributor
timestamp: December 19, 2020, 02:19:22 pm
content: Hello again HomeBrew: Did EMI, filters and compliance in 1980s..1990s as EE, consultant.I have a library shelf of EMI and compliance references.Many are very old but Maxwells Equations have not changed!Here are the best:Schlicke: Electromagnetic Compossibility (very practical) pub DekkerOtt Noise Reduction Techniques in electronic systems (classic) pub WileyElectromagnetic Compatibility Handbook Violette, White, Violette (comprehensive, practical) Pub Van Nostrand ReinholdFinally recommandée Dr Howard JOHNSON excellent writer, EE and consultant in this field of high speed layout and EMIHigh Speed Digital Design: A Handbook of Black MagicWebsite:http://www.sigcon.com/Hope you can learn a lot and enjoy this interesting and complex field!Jon

author: jkostb
author_rank: Regular Contributor
timestamp: December 19, 2020, 04:59:46 pm
content: The main reason why people break the chassis ring is that if currents circulate through this ring it can cause EM fields which then can be injected directly into your circuit. It is good practice to break the ring. However I don't think that these high frequency currents will circulate through your chassis ring if you have enough chassis connections distributed over the ring. I have seen a USB 3.0 hub PCB design in a plastic housing and using a continuous chassis ring passing the medical device directive! The product passed all EMC/EMI tests with a continuous chassis ring, but there were a lot of chassis connections distributed over the ring. Normally if you have a metal housing/chassisI would never break the ground plane in a chassis ring. For EMC/EMI it is sufficient to have chassis connections close to connectors. This shorts common mode currents to chasis . You must also ensure that shielded cables are terminated in 360 degrees at chassis. Problems start when you don't have a metal housing/chassis and are completely in plastic enclosure. The idea of using a ground ringi in that case is to prevent ESD current to propagate through your system ground plane. But usually this does not work, the RF fields will capacitively couple into your ground. The best strategey is to use a metal plate and mount your PCB on this plate and use a continuous ground plane with enough chassis connections to this plate (especially close to the IO connectors). Lastly you need to be carefull with application notes. Especially on this topic there are a lot of application notes, which sometimes give contradictonary recommendations. If you want to read more on this topic: Henry Ott Electromagnetic compatibility engineering is a good resource.

author: Neilm
author_rank: Super Contributor
timestamp: December 19, 2020, 05:55:21 pm
content: If you want a book that explains EMC in enough detail that you can understand it without burying you in Maths then I would look at "EMC design Techniques" by Keith Armstrong. I read a series of articles taken from that book and published in the EMC journal (before its demise) and found them a very useful guide to what to do and why it works without delving too far into the mathmatics of modeling that some books jump into.

author: homebrew
author_rank: Frequent Contributor
timestamp: December 19, 2020, 09:17:14 pm
content: Quote from: jkostb on December 19, 2020, 04:59:46 pmOk, I think I can live with that easily. It is demo only and for now the prototype keeps working flawlessly. I've never seen any misbehaviour (e.g. spurious resets or other glitches). However, whether or not it radiates too much can't be measured using the equipment I have. As there is nothing connected to the ring (apart from the ground plane at several points) and the shields of the connectors of course I don't see large potential for current flowing in the loop caused by the prototype by itself.Quote from: jkostb on December 19, 2020, 04:59:46 pmYes, I learned my lessen here. Next design will feature a single unbroken groundplane.Quote from: jkostb on December 19, 2020, 04:59:46 pmAnd here we go ... Although designed to mechanically fit into a 3U sub rack enclosure (100x160mm), I intend to use it primarily in the way eval boards (or Raspberry PIs) are normally used - just the bare PCB without any enclosure. Exactly because ESD I went into this ring thing in the first place. But yes, I totally see your point why that might not even work at all as intended

author: S. Petrukhin
author_rank: Super Contributor
timestamp: December 20, 2020, 02:12:09 pm
content: If you are worried about the ring as a receiving antenna, then you need to remember about the signal level that the antennas give - whether this level will leak into the circuit if the antenna is grounded, at least, by hand, not to mention screwing it to the ground. Personally, i do not see any danger in this regard in the presence of grounding. This ring can also not be a resonator (re-emit the received signal) because it does not have free departures. if we consider the ring as radiating antenna, then the question of flowing currents through this ring is important. If external circuits that can receive crosstalk that will drain to ground are connected close to the point that connects the ground.Simply put: this ring will not create any problems for you.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: December 20, 2020, 02:15:36 pm
content: By the way, connector screens can have high currents if the wire screens are connected on both sides and the connected devices have separate power supplies..