Wiring USB3 internally with not enough space for a real connector - Page 1
Forum section: electronics / projects

author: rx8pilot
author_rank: Super Contributor
timestamp: July 10, 2017, 07:26:48 pm
content: I am working on a project that is integrating an Intel NUC type micro-sized PC with some custom hardware into a very small enclosure.All seems to be going well until I got to the part where the only USB3 (a positively required interface) needs to connect to a high-speed IO interface internally. The only USB3 is a Type A connector on the outside edge of the PC. The connector is right up against the wall of the enclosure.The question is how likely is it that I could get way with manually soldering a cable to the pads of the existing USB connector? The existing connector would be a stub since it is impractical to remove it without damage. The idea is to use a high-quality cable from a certified USB3 cable and strip it, carefully solder it on both ends. The total wire length would be about 3 inches or so. Full speed data is important, power is not. The fiddly efforts needed for manually soldering is not really an issue (although I expect it to be a pain in the ass)If there is a VERY low profile USB-A right angle connector, I may be able to adjust the enclosure to accommodate. So far, the right angle connectors I have found are not even in the ball park. I considered buying a raw USB3 plug and attaching a cable to it manually, but that is not ideal in that it is also a pain and still requires modification of the enclosure.I do have the ability to look at the resulting eye, but only up to 6Ghz. I cannot check for impedance issues although it appears USB3 can tolerate a surprising variation in impedance (from what I read anyway).Is this:1. Silly and impossible?2. Possible but unlikely?3. No big deal?4. Other.....I will try to gather some pics if it helps.

author: Berni
author_rank: Super Contributor
timestamp: July 10, 2017, 07:46:11 pm
content: With how tiny the pads tend to be on these connectors it will be anoying to solder wires to for sure but if done right it should work perfectly fine.Desktop PCs use regular troughhole pin headers for internal USB 3.0 ports that head off to the front of the PC case. There is no need to tear apart an actual USB 3.0 cable for the wires. The wires in there might actualy be a bit thick to solder on to your pads. Any sort of wire can be used as long as the correct pairs are twisted together, however it is a good idea to look in to the impedance being somewhere close to 90 Ohm.It might be useful for you to use a SATA cable for it as SATA is electrically very similar to USB 3.0 (Even runs at the same sort of speeds) but the conductors in the SATA cables are layed flat next to each other. The way they are held in position between ground wires makes it a good transmission line similar to the twisted pairs used in USB, but it could possibly be convenient for soldering on to the pads as they come out nicely next to each other and usually are solid core wire.

author: rx8pilot
author_rank: Super Contributor
timestamp: July 10, 2017, 08:50:36 pm
content: Quote from: evb149 on July 10, 2017, 08:07:15 pmMy job is to take a project and put it in a specialized enclosure and design power, control, and various IO electronics. The Intel NUC is outside my sphere of influence on this project. I am the late arrival contractor tasked with finishing a bench project that has been in development for 2 years and now they are pressed to deliver (VERY quickly) to keep investor money flowing. I was the only one available that can do fancy custom CNC machined enclosures, electronics, and do it on a totally unrealistic schedule. The good news, is that the pressure is to build 5-10 of these, so some fiddly labor is not a deal breaker.The PCB of the NUC has been completely removed from the stock enclosure. The space restraints are very tight. The custom electronics have been built to fill in space around the CPU fan and any other spec of free air space. Some NUC models had USB3 headers on the mother board, but this generation only has USB2 on the PCB.Quote from: evb149 on July 10, 2017, 08:07:15 pmYes the USB-SS 5gbps. So, the two 90 ohm pairs for SS are the primary concern and of course the most delicate in terms of signal integrity risk. I did a test to de-solder the USB3 connector by pre-heating in our convection reflow oven followed by a hot air station. The surrounding components partially heat sheilded with Kapton. the connector never seemed to come close while the surrounding plastic parts were already starting to show damage. Even less practical than trying to solder wire directly to the back side of the through hole connector pins.The SATA cable is an interesting suggestion, but it is 100ohm. Not deadly, but going in the wrong direction right off the bat. I feet like I will already be battling the impact of the connector being a stub. Looking for a 90Ohm flat cable that is reasonable to solder and not some special thing that I have wait for.I may be able to make a little PCB interface to facilitate this....use the pads on the connector footprint to carefully route it to match a cable and provide some mechanical restraint.

author: Monkeh
author_rank: Super Contributor
timestamp: July 10, 2017, 09:22:25 pm
content: Does this particular NUC board have a mini-PCIe slot and the room to populate it?

author: rx8pilot
author_rank: Super Contributor
timestamp: July 10, 2017, 09:36:08 pm
content: This model is the NUC7i5BNK - the PCIe is not present, seems to have been replaced by M.2PCB IO connections are M.2, USB2 (2), SATA. I wish the high-speed interface could happen over SATA, but that is not going to happen.I just cut apart a USB-SS cable. The SS pairs are individually shielded and do not look ridiculously difficult to solder. Mechanical strain relief may be a challenge though......

author: Monkeh
author_rank: Super Contributor
timestamp: July 10, 2017, 09:43:18 pm
content: Are you using the M.2? If there's room, you can get a converter board and drop a mini-PCIe USB controller in.E: Very likely to obscure the SATA port though..

author: rx8pilot
author_rank: Super Contributor
timestamp: July 10, 2017, 10:36:02 pm
content: The M.2 is being used for SSD, unfortunately.

author: Berni
author_rank: Super Contributor
timestamp: July 11, 2017, 05:08:33 am
content: Ah its for an actual product not just a one time hack.In that case i recommend you create a flat flex PCB that fits on the connectors footprint.http://www.flex-ability.co.uk/images/flexconstructions_singlesided.gifThe side with the holes design so that the holes line up on the pins and the other side you plug in to a FPC connector on your custom board. That way you just line up the flex PCB on to the connectors pads and just solder each pad without any time consuming fiddling. Suddenly you have a motherboard that has USB3.0 on a flat flex cable.

author: rx8pilot
author_rank: Super Contributor
timestamp: July 11, 2017, 06:43:09 am
content: Quote from: Berni on July 11, 2017, 05:08:33 amIf I can try to negotiate more time for the project - this sounds like an interesting option. I believe I need to sort out a way to safely remove the USB connector though.Quote from: blueskull on July 11, 2017, 05:22:05 amI recently finished my first true high-speed project that included a design for 6gbps signals with edge rates just below 100ps on 100 ohm differential and 75ohm single ended. I learned that even some really small stubs create issues. Really, if the wind outside changes direction things would change (exaggeration a bit). These very high-speed signals are very unforgiving. I am going to have another try getting the USB off the PCB and then making an adapter for a FFC that can be soldered on. I can sacrifice a NUC if needed. Almost all of my projects have been leaded solder too, on this production PCB, the temps will be higher of course. Fingers crossed that I can not only get it removed - but sort out a procedure that a bench technician can repeat without many PCB's being damaged. They are about $350/ea.Another option that could meet in the middle is this weird idea. I managed to cut the top of the USB off. This exposes the high-speed pads that are easy to attach a tiny PCB without the stubs being as much of a penalty. What a pain in the butt this is.....EDIT: Who makes an off the shelf, controlled impedance 90 ohm flat flex cable or FPC with good connectors?

author: Berni
author_rank: Super Contributor
timestamp: July 11, 2017, 07:06:45 am
content: If you use a off the shelf 0.5mm FPC cable and create a pin out that has each pair sorounded by ground pins should get you reasonably close to your transmission line.But if you really want to be pedantic about it there are connectors like the Hirose FH41 series that are designed for special shielded FPC cable that has a ground plane running along the whole cable and creates a controlled impedance.

author: RGB255_0_0
author_rank: Frequent Contributor
timestamp: July 11, 2017, 07:31:38 am
content: Would something likehttp://m.ebay.co.uk/itm/NEW-Genuine-Dell-XPS-17-L701-x-3-0-USB-Board-DAGM7BTBAE0-CN-0861CJ-0861CJ-861CJ-/152038109145?epid=1923645325&hash=item23662d6fd9%3Ag%3AaXQAAOSw2s1UwdTc&_trkparms=pageci%253A998dea1c-660a-11e7-8a78-74dbd1804a7d%257Cparentrq%253A308d0e8815d0ab45429d15affff98d8d%257Ciid%253A17Be what you're after?

author: marshallh
author_rank: Unknown
timestamp: July 11, 2017, 01:48:05 pm
content: USB3 is very resilient. If you're only runnin the channel a few inches, you have a lot more loss budget to blow. If you just wire the pairs, twist TX-/+ together with its own ground return, do the same thing with RX-/+ and D it should be rock solid (The only thing you'd be missing from a real cable would be the shield mesh)

author: rx8pilot
author_rank: Super Contributor
timestamp: July 11, 2017, 07:51:35 pm
content: I just met with the project manager and convinced him to make a concession for the first 10 units.....which BTW needs to be delivered in 21 days (including weekends). Still doing mechanical design and have not yet started on the PCB layouts5 axis CNC programming & machining (myself), anodizing, PCB assembly & test (myself). I appreciate the comments and help here for sure.We are able to make the prototypes big enough to squeeze in an off the shelf right angle USB3 connector into the enclosure. This buys the time needed to work out a solution that preserves the desired long-term size and shape of the enclosure. Excellent.So the next step is to:Figure out if the USB3-Type A connector is practical to remove.Work out a PCB interface to a FPC or FFC solutionSince this is a low-volume effort, it would be nice to work with some off the shelf design that already has been worked out for 90 Ohm pairs suitable for USB3. Not sure where to look first, but at least I got myself more time to sort it out.All the feedback is appreciated. I am on the edge of my comfort zone here.

author: Berni
author_rank: Super Contributor
timestamp: July 12, 2017, 05:12:16 am
content: Yikes that's a tight deadline considering the PCBs are not even designed.I am sure you won't find off the shelf FPC cables specificaly made for USB 3.0. But even if they are 100 Ohm impedance that's perfectly fine. For short runs under about one meter its more important to keep the pairs close for as long as possible so that the whole differential signaling thing works. Its common practice in consumer products to run high speed LVDS-ish signals over FPC cable.If you do design your own flat flex circuit to fit on the footprint, then you could use online calculators to find out what distance apart you need to run your wires in a pair to get 90 Ohm. Ground traces sorounding the pairs is also reasonably effective shielding. But do consider that if you press such a cable up against a large metal surface that will change its impedance, so its also important how you run your cable inside the product if you really want to maintain proper 90 Ohm.

author: alm
author_rank: Super Contributor
timestamp: July 12, 2017, 06:14:53 am
content: Some excellent suggestions.Something else that might help is (mostly) grinding through the connector vertically in the direction of the plug. There should be relatively little material to grind to, because of the hole for the connector, and it might help against the clamps that evb149 describes that can be a pain to remove. Obviously you want to stop a millimeter or so above the PCB, as not to damage the PCB. Depending on the construction of the connector, this might be faster and less messy than abusing a diagonal cutter.

author: rx8pilot
author_rank: Super Contributor
timestamp: July 13, 2017, 04:01:59 am
content: Quote from: Berni on July 12, 2017, 05:12:16 amYes - nuts is an understatement. I will setup the CNC machines and while the parts are cycling, I will be laying out the PCB. noise canceling Bluetooth headphones with some music and it is the multitask wonder ride.The PCB is largely stuff I have done before just being integrated on a single board that fits the project. That means there is not much need to create new parts, fiddle with schematic design of a brand new circuit, and the layout will allow some cut-n-paste. If I had to start from complete scratch, it simply would not be practical. The high-speed 6gbps serial section alone has a steep learning curve, but I have that part dialed in. That was last months effort that has another thread here on the EEVblog. Lots of amazing insight from the members.At the moment, I am pushing forward getting proto units completed for some pre-sales events. I will then loop back to sort out a final solution. The technicians that will assemble these are deathly afraid I will continue down the path of removing the USB connector - which I can understand. I am leaning toward taking a raw USB3 connector and using that to launch into a flex cable of some sort. That way, I can make an assembly that any tech can assemble and service.What a party.....On a side note for those that remember....I was the winner of the first Keysight Test to Impress contest. This project is my first commercial release of a design that was enabled by the new MSOX-6004A 6Ghz scope. Literally, would have not been able to take this job without it. Super excited as this is the career step-up that I was after. Hopefully, my customer will let me take photos when it is done.

author: Monkeh
author_rank: Super Contributor
timestamp: July 13, 2017, 04:07:08 am
content: Making a flat flex to fit onto, say, one oftheseis probably the best bet, if there's enough room. Shouldn't stick out all that far from the socket. A dab of epoxy and careful handling should be all the strain relief needed.

author: rx8pilot
author_rank: Super Contributor
timestamp: July 13, 2017, 04:22:22 am
content: Quote from: Monkeh on July 13, 2017, 04:07:08 amExactly! Pretty sure that will allow the tight mechanical requirements to stay happy. Wondering how difficult it will be to test it. The plug it in and see if it works method is a bit scary. I would like to know that there is some headroom in the signal integrity. I have struggled probing 100 ohm signals without damaging them - at least it appears that the probing is closing the eye and/or creating some bumps.

author: WastelandTek
author_rank: Frequent Contributor
timestamp: July 13, 2017, 05:19:20 am
content: I hope you're getting paid enough for this BS

author: Berni
author_rank: Super Contributor
timestamp: July 13, 2017, 05:24:35 am
content: Well you are gonna need a even better scope than your shiny MSOX-6004A to see if your USB3.0 solution is inside the spec. You could try getting a eye diagram if you have any fast active differential probes for it. Will probably look fuzzier and rounder than it actually is with that scope but if the middle of the eye is open you should be good.The more low tech way of seeing if something high speed is on the edge of working is to simply grab as much of the surface area of the flat cable as possible in your hand or poke your finger at any exposed pins on the connectors. This introduces extra capacitance and loss and other funky effects that make the signal worse so if its on the edge it will likely drive it over it. Also a good idea to repeat this with the entire product chilled to like -20°C or with it hot at about 70°C

author: rx8pilot
author_rank: Super Contributor
timestamp: July 13, 2017, 05:51:47 am
content: Quote from: Berni on July 13, 2017, 05:24:35 amWithout a doubt.....I think I would need nearly triple the bandwidth to see the real signal. I have one 6Ghz probe, but that can only see the fundamental. Better than nothing I guess.As I have been learning about high-speed design, one of the most painful lessons is the stunning cost of test and measurement gear. A 20Ghz sampling scope and a 20Ghz VNA seem to be pretty standard for others going after the high-speed TV signals that I am after. This one really only sees input signals 3Gbps which is within the usable range of the scope. The 6Gbps signals can only see a rather rounded eye and no harmonics. After a few training wheels projects, I will be digging through ebay for the 20Ghz gear.....At the moment, I am revenue generating at least and doing better than when I was only offering power management products.

author: Berni
author_rank: Super Contributor
timestamp: July 13, 2017, 08:03:35 am
content: You might be able to find a 20GHz sampling scope on ebay for cheap if its one of those old CRT mainframe digital scopes, might have to fix a thing or two on it or hunt for a missing plugin module.But when it comes to a 20GHz VNA...well you better have deep pockets even on ebay. Id expect to have to dish out 10 grand for one of those. There are some old 20GHz spectrum analyzers that can be had for cheap, but you still need lots of extra stuff to turn it in to a network analyzer(Scalar one tho). Even things as simple as a coax cable get expensive at those frequencies. I once tried putting 20GHz trough a regular cheap SMA cable to see what would happen. The meter or so of cable turned in to a 30dB attenuator. It likely also had lots of reflections but i don't have anything that can measure that.

author: alm
author_rank: Super Contributor
timestamp: July 13, 2017, 09:00:46 am
content: Quote from: Berni on July 13, 2017, 08:03:35 amThat sounds like the way to go for 20 GHz on a budget. If you have a 6 GHz 'main scope', then you may be able to tolerate less features on the sampling scope that you only use when you need the bandwidth. But if you need a differential probe at that bandwidth, then I am not sure if there is a remotely affordable option.Quote from: Berni on July 13, 2017, 08:03:35 amWell, 20 GHz is pushing it even for decent SMA connectors (18 GHz bandwidth is common). You will need either the special high frequency SMA connectors or even 3.5 mm.

author: rx8pilot
author_rank: Super Contributor
timestamp: July 13, 2017, 05:38:32 pm
content: I am not sure if I can pull it off yet, but I am considering a round of financing. That would allow me to look at modern test gear. Specifically, I am looking at the 20Ghz Keysight E5071C with an eCal and TDR option. Obviously, this is big money but it offers me access to the markets I am after. In TV, the most common high-speed signal is 3Gbps. 6Gbps is fairly quickly replacing that aging specification. 12Gbps is next, and not too far off. Internally, these signals are de-serialized, distributed, and generally moved all over the place through numerous connections, PCB layers, etc. I have seen small businesses crash and burn perfectly good concepts because they underestimated the challenges of dealing with these types of signals. The customers are generally unforgiving.BNC connectors are now being designed for 12Ghz and generally tested to 18Ghz - really nuts. When you put a whole system together, signals can fall apart quickly if every detail is not handled correctly. The TV environment requires serious reliability so you won't last long if your gear glitches a picture. Testing whole signal chain, as well as individual elements and connectors, is important - especially before you ship stuff that will be on-air for a high-profile event. I have done the industrial and mechanical design for many broadcast systems. I was able to watch the EE's from a safe distance to see how they handled validation of the various parts of the system. It was good to see that from that perspective and learn how to best stay out of trouble. This current project is not an on-air system, which gives me some room to learn more safely.The older gear on eBay could work, but I would be more nervous buying $15k worth of old gear and fiddling with issues and limitations than just making payments for 2 years on a current and supported model that simply gets the job done out of the box. Since high-speed design is a new thing for me, a huge learning curve lies ahead. If I am able to avoid the learning curve of a half-working 15 year old VNA, that is what I will shoot for.

author: rx8pilot
author_rank: Super Contributor
timestamp: July 13, 2017, 05:42:02 pm
content: Quote from: evb149 on July 13, 2017, 05:26:36 pmI will agree with that. Also, I doubt USB3 analysis will be needed much for what I am hoping to do. This is a unique situation (I hope). I will, however, need the same or very similar gear for all the other signals I will have to deal with. A conventional scope is only one arrow in the quiver for sure.

author: alm
author_rank: Super Contributor
timestamp: July 13, 2017, 08:57:32 pm
content: Quote from: rx8pilot on July 13, 2017, 05:38:32 pmWho in their right mind would, when faced with a 12 GHz signal, pick a BNC connector and start tweaking it? As opposed to picking a connector with a more stable and reproducible geometry like N or SMA, or even TNC.

author: WastelandTek
author_rank: Frequent Contributor
timestamp: July 13, 2017, 09:04:02 pm
content: Quote from: alm on July 13, 2017, 08:57:32 pmA German perhaps?

author: rx8pilot
author_rank: Super Contributor
timestamp: July 13, 2017, 09:20:33 pm
content: Quote from: alm on July 13, 2017, 08:57:32 pmIn broadcast, it is all about backward compatibility and habit. The BNC has been pushed and pushed and it appears that 12Ghz is indeed the final practical number. The nicer connectors are Swiss machined and far more precise than yesterday's connectors. Samtec really nails it, but there are a few others that can take on 12Gbps over BNCFor fixed installations, the HD-BNC which is quite a bit smaller, higher performance, but more fragile. For field operations - BNC is the de-facto connector and 12G will carry the industry for a considerable amount of time.

author: Berni
author_rank: Super Contributor
timestamp: July 14, 2017, 05:23:23 am
content: That is quite the investment to take. One of those 20GHz VNAs brand new costs as much as 5 brand new cars. But as long as you have good paying business with it to cover its cost in a reasonable amount of time.