Connecting a PCB to a slotted enclosure - Page 1
Forum section: electronics / projects

author: Dave
author_rank: Super Contributor
timestamp: December 04, 2018, 07:28:31 am
content: I've been struggling to get a good solution for this problem: Connecting a PCB to the slotted enclosure it sits in. Sounds simple enough, right?This is the profile of the type of enclosure I'm talking about:The PCB slots in horizontally.Since the enclosure is going to be anodized, I think I can not rely on putting copper on the edges and pulling back the solder mask, as this should be a connection reliable enough to provide a return path for the occasional ESD current spikes.Here is what I imagine would be a reasonable solution for this:Kitagawa OGSC seriesA contact that sits on the PCB that has a prong sticking outwards. It gets compressed when the PCB is slid into the groove and maintains pressure on the enclosure. The part of the aluminium that comes in contact with the prong gets sanded a little bit before assembly, to remove the oxide insulating layer (yes, I know aluminium passivates itself in air, but that oxide thickness is minuscule compared to the one made in the anodizing process - voltage required to penetrate it is very low).The problem with these particular connectors is that I have trouble locating them for sale in Europe. I couldn't find them at any of the major distributors (Digikey, Mouser, Farnell, Arrow,...), nor was I able to find a suitable alternative. I need a source that can provide a proper invoice, as this is being purchased by my company.Have you faced a similar design challenge yourself in the past? How did you tackle it?

author: bugrobotics
author_rank: Regular Contributor
timestamp: December 04, 2018, 07:43:51 am
content: If you are sending the enclosure out to be anodized you can request that the appropriate slots be masked. This will leave the raw aluminum exposed. Masking is a pretty normal in the anodizing world.

author: Dave
author_rank: Super Contributor
timestamp: December 04, 2018, 10:42:39 am
content: Well that's a simpler and cheaper solution than anticipated. Thanks!

author: Ian.M
author_rank: Super Contributor
timestamp: December 04, 2018, 12:11:23 pm
content: If the PCB's a sliding fit, you cant rely on the edges forming a reliable contact to the raw aluminium groove surface as there will be negligible contact pressure. It will be fine at first, as enough oxide will be scraped off by PCB insertion but as the oxide film reforms, and contamination builds up from outgassing of board and components, odds are the connection will become intermittent.

author: cdev
author_rank: Super Contributor
timestamp: December 04, 2018, 02:13:19 pm
content: Quote from: Dave on December 04, 2018, 07:28:31 amI have run into a similar problem several times with low noise amplifier cases for RF and had a long discussion about a related problem with some enclosures here the other day. The theme was avoiding problems caused by improper grounding and ground loops in RF enclosures. Maybe your issue is similar enough that this might help you.Two ideas that came out of it were,LNAs and bias tees already often use a feed through capacitor or more recently combo feed throughs that incorporate an inductor as well, to route the DC power line out of the enclosure. Typically they will have a tinned solder post on their exterior to connect the DC ground to. But because of the skin effect, this may not always be enough to provide the lowest impedance ground return forRFgrounding. Because the RF travels along the skins (outside surface) of conductors and DC goes through those same conductors. This may seem counter intuitive and breaking the rules of RF construction but it makes sense to me because Ive observed this happening in low noise (less than 0.7 db) amplifiers ive used for VHF/UHF reception. (which are a very good way to learn about RF enclosure issues because they amplify mistakes!)1.) An additional ground, going more directly to the active device ground may help make the LNA quieter.This involves, counter intuitively making an additional (to the connection to the case outside) separate connection via a thick soldered wire to the most important spot on the PCB ground (backplane, sides of board) and route that ground out of the box in the shortest possible manner via a feedthrough capacitor of adequate enough value (Adding this: one that does not have a built in choke-some do incorporate an inductor and one doesn't want to use them in this specific use.) This provides a good both case-and PCB connected ground with both low DC and RF resistance. One should take care to make it short and direct and I would also suggest running it to the back of the PCB, right underneath the multiple vias that connect the ground pads - the ground pins of the RF-active device to the back of the PCB which is a solid continuous ground. The edges of that PCB should also be sonnected via soldering to the case -2.) a surface which can be provided in the case of an aluminum case- which is un-solderable- is makable by lining the interior of the case with (if possible) a single large piece of the thickest practicable conductive copper tape* that also makes contact with the exit holes for the RF connectors or screws that make direct contact to the aluminum as well, ideally using lock washers and additional pressure. This way you have multiple redundant grounding. Solder can improve on this.*line theinside(of a non-slot containing aluminum enclosure) with copper tape and solder the edges of the PCB to that.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: December 04, 2018, 02:51:12 pm
content: Quote from: bugrobotics on December 04, 2018, 07:43:51 amBare aluminum will oxidize. It may not make a good long term contact. Can you run a wire to the case?Also you can chem film the aluminum. I like yellow chem film. It makes a good contact and prevents corrosion.https://en.wikipedia.org/wiki/MIL-DTL-5541

author: bugrobotics
author_rank: Regular Contributor
timestamp: December 04, 2018, 03:29:28 pm
content: QuoteQuoteAbsolutely correct. I shouldn't have left my response as the end all solution. If I was stuck using the raw aluminum to PCB contact I would use an electrically conductive paste such as No-Ox-Id or similar products that inhibit the oxide formations. Thanks for keeping me honest.EDIT:Or if the PCB connection isn't great in the mounting channels you could you use the end plates of the extrusion as your main connection. Adding some screw terminals to the end of the PCB and using a screw you could control your connection to the box better. Of course, as mentioned you would need to control the masking of the end plate and coat the surfaces with a corrosion inhibitor to make the connection long lasting.

author: max_torque
author_rank: Super Contributor
timestamp: December 04, 2018, 04:50:46 pm
content: Put a large round pad on one side of the pcb, solder a spring to that pad. When you slide the pcb into the box, compress the spring, and then let it extend and push against the box. You'll have to mask or remove the anodising to ensure a good contact, but this works nicely.There are also spring loaded type solder on contacts that can have quite sharp points that "may" cut through the anodising. Note the use of the word "may" ;-)ie

author: Dave
author_rank: Super Contributor
timestamp: December 04, 2018, 08:04:55 pm
content: Thank you all for your responses.Perhaps I need to add more context to this. I'm currently not at work, so I can't show you a 3D model right now, but I'll paste it in here tomorrow.Soldering a piece of copper tape into the enclosure would be a decent solution for a couple of boards, but I'm looking at a small batch of them here. It needs to be simple-ish and repeatable to assemble.The board will not butt up to the end plates, so screwing it down to those is also not an option. It's really sitting right in the middle of the enclosure, only really contacting the grooves of the enclosure. The rest of the circuit will be connected through an FPC cable, which isn't an option for sinking the current spikes, as it has considerable inductance.I've already looked at those springy contacts, but they all seem to be intended to make contact normal to the mounting surface, not horizontally. The only ones that did the latter (that I managed to find) are the Kitagawa contacts from my first post, but I have trouble sourcing those.I have just foundthese. I'd have them soldered near the edge of the board and compress them slightly before slotting the board in place. What do you guys think?Some of you mentioned treating the aluminium surface with something to prevent oxidation over time. Do you have any particular product in mind that you could recommend?

author: Dave
author_rank: Super Contributor
timestamp: December 05, 2018, 07:30:54 am
content: As promised, here's the 3D model of my project.I need to couple the rear part of the PCB (the part with the flanges that slot into the case), the narrow prongs towards the connectors will be electrically isolated channels, which I can not use to conduct away my fault currents. The rear part of the PCB will also be connected to another PCB (located on the rear panel of the enclosure) through an FPC.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 05, 2018, 02:14:47 pm
content: If it's isolated then I fail to see what you're looking for -- nowhere for ESD/surge currents to flow, nothing wrong!If it's not truly isolated (in the wideband sense, merely galvanically), then do what Ethernet does, add a ~1nF capacitor to ground (which would be front panel ground, the nearest accessible point -- and also the location the offending ESD is coming in through!).Tim

author: cdev
author_rank: Super Contributor
timestamp: December 05, 2018, 02:28:22 pm
content: What are the parts on the top and where is the FPC on the back?You probably have the best likelihood of successful RFI suppression if you use gold plated RFI contacts and have the amount they need to deform be exactly what is specified by their manufacturer for maximal longevity. Take advantage of whatever support they have. Don't have something that somebody who is closing up the box needs to muck around with in any unusual way because people never re-read instructions a few months or years down the line. They will invariably not do anything unusual like press the fingers down to position them properly for contact. That would be almost guaranteed to not be done by users.The most effective RF shielding for aluminum boxes Ive encountered has been having lots of flat surfaces and lots of screws through them spaced closely together. (bringing us back to the beginning)A good example of a problematic situation is a computer. Shielding most computer cases adequately is very difficult.On YouTube Leif Asbrink, SM5BSZ has four videos entitled "Probes" or something like that. He shows how to make your own e-field and H-field probes and using them to locate where RFI is being created. He is very knowledgeable and I always learn useful things from his videos. Tips that work. Thats a really good practical set of tips on figuring out RFI problems from nearby devices unsing basic probes and thats just worth knowing.You can make really small probes and use them with a variable attenuator(s) and broadband SDR to get a very fine grained level of RFI origin sensing. Maybe you could first get or make the probes (if you dont already have them) You can make probes that can sense EMI and determine the 'sense" of the magnetic H-field. They are sold commercially too, but fairly expensive for something you can make yourself fairly well.(but worth it if you do this a lot) around a year ago one of the fine people here, Frazier posted a deal on a set of probes, for very little. See if you can find his post, and even if the deal is gone you can see from the post the construction details and make one.One you have suitable probes and some tools to sense them (you probably already do) Try the brushes out and see. If your device will see a lot of card insertion and 'desertion' (?) the brushes may fatigue and no longer work well, but if not and you use enough of them they are probably okay. See if you can pick up the signal from them with various shielding methods including the one with the copper tape so you have a point of comarison. You may have RFI no matter how well its shieldedtherebecause the RFI is actually escaping someotherway.-----Edit: If your signals can handle it they also sell flat ferrite cables in many different sizes as well as clamp on flat ferrite blocks to suppress EMI. I have a bunch of them also for ribbon cable. If you still have a problem from the back you can slap an appropriately sized block around the cable on the back. If your signals are differential, as I understand it (others here please correct me if this is wrong)the reactance (might only be a few hundred ohms and you need to pick the material by the frequency of EMI you want to suppress) should cancel out some well definable amount of the unbalanced EMIand leave nothing but your significantly cleaned up (balanced so unaffected) signal(s).My best luck with probes has been when I made them very small and connected them up using multiple ferrite beads on the feed line to prevent the feedline picking up the signals. Look at the wildly expensive EMC sniffing gear and try to see if you can replicate its functionality. They often try to make it look esoteric in order to discourage reverse engineering it. But its not rocket science.A simple clip with a ferrite split bead between its jaws and a few turns of wire connected to a coax can sniff RF riding on cables. Again decouple the feedline with ferrites.

author: max_torque
author_rank: Super Contributor
timestamp: December 05, 2018, 06:23:23 pm
content: Ok, looking at those connectors tells me your product is isolated thermocouple amplifier! If you are doing the isolation for HV reasons, rather than noise, and the device is in a metal box, then you WANT to have a large, bolted ground stud or connector on the unit, and the instructions need to make it very clear indeed that this MUST be earthed at all times. That earth point can then be simply connected to the pcb with a suitable earth wire or ground strap (which needs to carry any potentially large fault currents, ie be big!)If you are isolated just for common mode noise rejection, then it would be better to just try to arrange your design to have the lowest possible capacitive coupling path out to the real world and between channels. Here, every pF counts ime........

author: Dave
author_rank: Super Contributor
timestamp: December 06, 2018, 08:26:38 am
content: Quote from: max_torque on December 05, 2018, 06:23:23 pmCorrectamundo!Excuse the awfulness of the drawing, this is basically what I'm doing here.The thermocouple contacts are going to be exposed and prone to getting zapped here and there when connecting thermocouples to them. Each of them is connected to an isolated channel, which is what the long fingers on the PCB are for. The fault currents are going to be directed to the isolated ground through a pair of TVS diodes.As the channel capacitance towards the system ground is going to be quite small, it wouldn't take much charge to move the potential of the isolated ground sky high and punch through the isolation barriers of my power and signal isolators, which are used between the isolated channel and the main system circuit (rear part of the PCB). Therefore, a GDT is placed between both grounds, to ensure that it sparks over before the isolators could get damaged. The fault current now needs to go from the system ground to the earth grounded enclosure, to complete the circuit. A Y-rated capacitor is going to be placed between the two for this purpose.TL;DR I need to get the fault current from the rear of the PCB to the enclosure somehow.The solder-on horizontal contacts (linked above) seem like the most viable option at the moment, but I'm always open to new ideas.Quote from: cdev on December 05, 2018, 02:28:22 pmAs max_torque already figured out, they are thermocouple connectors. The FPC connector is not visible, because I haven't put it in yet.It's not really an RF issue, but I do appreciate you taking the time to share your RFI shielding techniques.I have attached another image of the current state of the PCB design. A fair bit of the circuit is still missing (including FPC connectors), but I'm slowly getting there.

author: Ian.M
author_rank: Super Contributor
timestamp: December 06, 2018, 08:52:36 am
content: Its hard to beat a machine screw with a zinc plated serrated washer under its head for getting a reliable ground to an aluminum case or chassis.Take a look athttps://www.pemnet.com/fastening-products/fasteners-for-mounting-into-printed-circuit-boards-2/Right Angle Fastener - Type SMTRAif you can tolerate an extra hole and screw in the case side, orRight Angle Fastener - Type SMTRAif you'd rather have it more central on the PCB with a scrw through the top or bottom of the case.

author: Wendy_Preston
author_rank: Contributor
timestamp: January 10, 2019, 11:12:21 am
content: Hi Dave, I spotted the link you posted to our Horizontal Spring Contacts (S1951-46R) - we also have an option without the slot in the mating contact, which gives it a slightly higher force (S1941-46R).It's difficult to say if it's going to achieve exactly what you are aiming for, but it's certainly an option that I think is worth considering. It will be important to ensure it's making good electrical contact with the area on the case where you choose to locate the interface. You can request samples from our website - go to the product page, load up the sample basket, then checkout. You can also see stock at all our authorised distributors there.Do let me know if I can be of any assistance.

author: IconicPCB
author_rank: Super Contributor
timestamp: January 10, 2019, 12:00:20 pm
content: Consider alodining instead of anodising

author: Dave
author_rank: Super Contributor
timestamp: January 24, 2019, 02:24:41 pm
content: Quote from: Wendy_Preston on January 10, 2019, 11:12:21 amAttempted to order some samples from your website on the day that you wrote this message, no response has been received from your team in two weeks.

author: Wendy_Preston
author_rank: Contributor
timestamp: February 05, 2019, 09:38:00 am
content: Quote from: Dave on January 24, 2019, 02:24:41 pmDave, I'm so sorry, I only just managed to get back on the forum!Totally not an excuse, but our website is having the occasional glitch that we are having real problems pinning down. If you still have had no response, please do email the Samples team on samples@harwin.co.uk with all the info you put in the form (if you got an automated response from the website, you can send that), and they will get onto that for you. Once again, I can only apologise.We're at the start of redesigning the website, brand new rebuild, as we have a few glitchy things like this that just refuse to get pinned down. Plus it's due for a refresh anyway.