Vias and traces under shielded inductors - Page 1
Forum section: electronics / projects

author: foxtrot
author_rank: Contributor
timestamp: April 01, 2024, 10:52:19 pm
content: I've been working on a particularly space-constrained PCB layout, and there are several places in which I've routed non-sensitive signals on the outer layer beneath fully-shielded/encapsulated inductors (i.e. theBourns SRP series), including vias. I'm now second-guessing myself as to whether this is reasonable.Googling around provides very limited advice on the matter – and there doesn't seem to be a lot of good explanations as to why this may or may not be done. Can anyone provide some well-reasoned guidance on the matter, and maybe some quick heuristics as to when this is permissible (if ever)?Thanks folks 🙏

author: thm_w
author_rank: Super Contributor
timestamp: April 01, 2024, 11:03:36 pm
content: Its probably fine, but screenshots of the layout and some explanation as what the "non-sensitive" signal you are referring to would help.https://www.monolithicpower.com/en/when-is-it-beneficial-to-place-a-copper-layer-beneath-dc-dc-power-supplies

author: foxtrot
author_rank: Contributor
timestamp: April 01, 2024, 11:27:54 pm
content: Quote from: thm_w on April 01, 2024, 11:03:36 pmThank you for the link! That's a good resource.I can't post screenshots, but when I say "non-sensitive signal," in this context, I'm referring to low-frequency, logic-level (3.3V) digital signals being received by devices with input hysteresis sufficient to prevent small, induced voltages from introducing meaningful noise.Obviously, best way for me to answer this question precisely would be to use something like Elmer FEM to simulate the magnetic fields around the inductor and the induced voltages in the underlying traces, and then see if they exceed the protection offered by input hysteresis – but that's a massive rabbit-hole to go down, so I was hoping someone would give me the answer you did, which is "it's probably fine"

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 01, 2024, 11:38:19 pm
content: Well, again hard to tell without seeing the exact layout. Yes, maybe, "it's probably fine". I always try to avoid routing digital signals too close to, or right under an inductor myself. But how many layers is this PCB? If you have at least a ground plane between top & bottom, then it's not nearly as bad, although if you put a via right there, it kind of defeats that.Just look at the datasheet of said inductor too: "shielded" can mean various things and is rarely a full shield.

author: foxtrot
author_rank: Contributor
timestamp: April 02, 2024, 01:43:31 am
content: Quote from: SiliconWizard on April 01, 2024, 11:38:19 pmYeah; it's a six-layer stack up (S/G/P/S/G/S).Quote from: SiliconWizard on April 01, 2024, 11:38:19 pmMolded inductor – fully shielded (with some margin of error) 👍

author: kimballa
author_rank: Contributor
timestamp: April 03, 2024, 04:36:44 pm
content: For what it's worth I put together a board very recently with Bourns SRP inductors on it. They're enclosed in a very chunky solid metal case. I don't have the equipment to measure leakage from them but it's probably as good as you can hope for...The actual interference will depend on the inductor function though - what frequency content runs through it in practice, and at what power.Likewise the digital line - is it mostly static or does it transition a lot?I also had to route some signals under the inductors but I didn't worry too much because there was an internal ground plane between the inductor and the signal, and the digital signals were basically unchanging, they were on/off control signals that only changed when a user hit a button, so essentially DC. If you can nudge the vias a couple of mm away from the inductor that would probably help but I can't quantify by how much

author: Neilm
author_rank: Super Contributor
timestamp: April 03, 2024, 06:27:56 pm
content: I'm not sure what you mean by a molded inductor, but the only issue I have ever had was with uncovered vias shorting to the metal shield of inductors. If that shield is connected to the inductor you get short circuits between the via and the signal in the inductor.

author: coppercone2
author_rank: Super Contributor
timestamp: April 03, 2024, 07:05:10 pm
content: IMO they should have a little plastic pad like a capacitor does, it seems bootleg