Conformal coatings (silicone + copper-based) to shield PCIe Gen4 adapters? - Page 1
Forum section: electronics / rf-microwave

author: aBavarian Normie-Pleb
author_rank: Unknown
timestamp: April 23, 2021, 11:04:54 pm
content: Hi,I'm trying to get better signal quality when using PCIe Gen4 with adapters and cables, for referencecheck out my previous topic.As a maker noob I've never used these before but would it make sense to spraythe adaptersfirst with a few coats of insulating silicone-based conformal coating (obviously mask the contact pins) and then shield 'em with a conducting copper-based coat...?Hope this topic isn't stupid and that I'm not mistaken placing this question in the RF sub-forum.Thanks for any constructive input!Regards,aBavarian Normie-Pleb

author: miken
author_rank: Regular Contributor
timestamp: April 24, 2021, 07:32:48 am
content: The transmission lines on the adapter PCB are (presumably) designed to operate in free air, so I wouldn't use a conformal coating on that board. And definitely not a conductive shield over a conformal coating, that is close enough to the lines to change the characteristic impedance significantly and that would be bad.I don't think interference is likely to be a problem, that is what differential pairs are good for. If you do want to experiment with shielding use a box or foil at least a few cm away from the board.At Gen4 I think the losses at high frequency are more likely to be the problem. It appears that you are already using a short cable though. Doing transitions at these high frequencies is not trivial either. The Delock board claims "Data transfer rate up to 32 Gbps" but one lane of Gen4 is already 16 Gbps... you may want to contact them and see what they say about your application.

author: aBavarian Normie-Pleb
author_rank: Unknown
timestamp: April 24, 2021, 07:47:05 pm
content: Thank you for your repy!I would like to reduce the number of plugged together parts but this seems impossible since there isn't such a thing as an M.2-PCIe-to-SFF-8673 cableIf it helps you thinking about what would make sense to try, as far as I know PCIe Gen4 uses a frequency of 16 GHz (source).Would applying shielding locally to the area where the adapters/cables are plugged together make more sense than "spraying everything"?

author: miken
author_rank: Regular Contributor
timestamp: April 24, 2021, 11:23:53 pm
content: You can try shielding a few cm away but I doubt that is the source of your problem.At 16+ GHz I don't know how realistic it is to build your own adapter. At work we have PCIe extenders that can have problems at Gen2 depending on what platform they're plugged into. So I'd say there are a lot of variables and unless you have specialized equipment and software to do the analysis I wouldn't expect to do better than the off-the-shelf solution, and at what cost? Like, you could try building something with a bunch of U.FL or SMA but it would be a mess. The adapters at work are cut/soldered coax but I suspect nobody analyzed the board->coax transition...Talk to your vendor, that is why you paid them money, to do the work for you

author: aBavarian Normie-Pleb
author_rank: Unknown
timestamp: April 24, 2021, 11:50:39 pm
content: I've already exhausted the customer "support" of the companies involved:- Delock gave a generic response: "Try using less adapters, PCIe Gen4 is very sensitive to external influences" (no sh.., Sherlock)- Icy Dock (Backplane): "Go ask your motherboard's manufacturer about it."- Motherboard manufacturer: "We don't support PCIe adapters."That's why I started these topics.

author: miken
author_rank: Regular Contributor
timestamp: April 25, 2021, 09:23:54 pm
content: It's unfortunate that they won't stand by their productAt these frequencies though nothing is trivial. Your best bet is finding a really short cable, like 15 cm.

author: aBavarian Normie-Pleb
author_rank: Unknown
timestamp: April 26, 2021, 07:23:09 am
content: Do you have an idea if there is something like a DIY M.2 board to which one might directly solder the leads from a cut SFF-8673-SFF-8673 cable to to avoid the multiple transitions of the current adapter chain?And yes, I'm a bit desperate :-/

author: miken
author_rank: Regular Contributor
timestamp: April 26, 2021, 09:03:51 am
content: Well, if you are desperate enough to try something like that, maybe start by shortening the cable yourself? I would be wary of designing the cable/PCB transition, but there is an example inside each SFF cable end. Given the tolerances involved I'd be worried about making things worse.Does it need to be Gen4? And does it work at Gen3?

author: aBavarian Normie-Pleb
author_rank: Unknown
timestamp: April 26, 2021, 11:10:55 am
content: Yes, I'd like to use PCIe Gen4 since the SSDs are to move large files and Gen3 would halve the usable speed.Looked around and found different M.2-to-SFF-8654 adapters that have PCIe Gen4 redrivers, will give these a shot with different cables:https://www.microsatacables.com/m-2-m-key-notch-to-oculink-with-redriver-adapterAnother option seems to be OCuLink-2 for PCIe Gen4, but am a bit uncertain about its Gen4 compatibility.