Stacked (and not plugged vias) - Page 1
Forum section: electronics / general

author: stcoso
author_rank: Regular Contributor
timestamp: April 11, 2022, 07:42:35 pm
content: Hi, I have a question that may not have so much sense in PCB design (because it's not intended as a PCB).I'm trying to understand if it's possible and economically reasonable have manufactured a 8 layers pcb with 4stackedvias (one between layer 1 and 2, one between 3 and 4 etc...) but with the hole opened (like a through hole). I'm essentially trying to design a sensor (for water), i need 4 independent electrodes, and the idea was to use 4 stacked vias (gold plated) but i can't figure out if the manufacturer would be able to produce such a stack with the hole opened throughout all the layers.Thank you in advance.

author: twospoons
author_rank: Frequent Contributor
timestamp: April 11, 2022, 11:59:07 pm
content: Put pads on your 4 layers and specify a non-plated hole (NPTH). The PCB fab may come back and ask you about it, as its not usual, but just tell them that is definitely what you want and they will do it.Trying to do NPTH between buried vias might be a tough one, as I don't think the typical process flow would allow that.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 12, 2022, 06:54:18 am
content: So you get separate electrodes inside a single hole?I don't think that makes any sense, as, the holes would have to be drilled and plated beforehand -- which is fine for a layer-pairs stackup -- but then when they're glued together, how do you not have resin oozing up inside the formerly plated holes? And you can't just drill them back out, either some resin will be stuck on the sidewall, or the copper gets chewed up. No way they're drilling precisely enough to just perfectly skim-cut the copper.Can it not just be castellated holes (edge metallization, full height) or face pads or whatever? Even with AC bias, I'd be worried about corrosion on a segmented thru-hole. If this is something like a microfluidics thing and you were thinking shortcuts with PCBs, ehh, dunno about that?That said, perhaps you can do a custom build, for example 2-layer PCBs interleaved with thin FR-4 / G10 laminate, glued together with a very thin bead of whatever adhesive (epoxy, sealant, etc.). Done carefully, little enough glue could squidge up into the hole that it might be fine as-is, or with a light pass with a reamer (which might still cause tear-out, so, be careful).Tim

author: stcoso
author_rank: Regular Contributor
timestamp: April 15, 2022, 05:16:41 pm
content: Quote from: twospoons on April 11, 2022, 11:59:07 pmWith "Put pads on your 4 layers" you mean on the 4 non connected layers?Quote from: T3sl4co1l on April 12, 2022, 06:54:18 amIt's exactly what I thought. It's not my idea, I suggested that the production process would plug the holes but I'm not an expert, so i didn't want to give a bad advice. Anyway... even IF possible would be really pricey (600$ at least)... I think we've found a nice solution though, involving some nice stainless steel sheet laser cutted by (again) pcb manufacturers

author: BreakingOhmsLaw
author_rank: Frequent Contributor
timestamp: April 21, 2022, 07:53:53 am
content: I'd advise against it. I tried something similar years ago, with less electrodes. The PCB will pick up water through osmosis and delaminate eventually.It may work with a different carrier material though. Check the datasheet of your available base materials.