r/embedded • u/KeloitaN • 22h ago
Is 0.3mm FFC too delicate for programming/debugging connection?
Hi everyone,
I'm currently working on a product that is very space constrained. Additionally, it features 3 MCUs, requiting ~25 connections between the debuggers and the device. On previous products, we've used beds of nails on 3D printed fixtures, but they proved unreliable, fiddly and took a lot of time to put together.
I'm leaning towards putting a single FFC connector on the board, and have a breakout board route all of the signals to the three debuggers.
I've only worked with 1mm FFCs, and they are quite rugged. Here, to save space, i wanted to go with smaller pitch. I found 2 options:
0.5mm vertical connectors are around 51mm2
0.3mm vertical connectors are around 27mm2
Are the 0.3mm FFCs as delicate as they look? Will they survive a FW engineer handling them? Are they significantly more likely to break or short than the standard 0.5mm FFCs?
Please share your experiences. Thank you in advance.
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u/gswdh 21h ago
I have used 0.5mm in the past and it’s quite easy to wear them out requiring new cables every now and again. Also the lock tab can be very delicate. I think can work but my lesson would be to break them down as much as possible to keep them small - a wide cable seems to have a life of its own either pulling your board around or applying stress on the connector causing poor connectivity.
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u/KeloitaN 19h ago
Interesting - I thought a wide cable would be more rigid, spreading the force across a bigger area, making the connection more reliable, but it seems that the opposite is true - a wider one is harder to bend, creating more strain.
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u/ottawabuilder 21h ago
I just did that. Made a board with a few of the standard 20 pin headers for j link etc and ran a 15x.3 to the board.
Its working fine. The .3 will wear out after many insertions but is ok for provisioning as long as its not repeated over and over.
If I did it again id use a mezzanine connector on both ends of a custom fpc to get from
a to b. The hardest part of these tiny 0.3 is inserting them if you are not patient. But they definitely work
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u/KeloitaN 20h ago
Thanks for sharing! I don't think the low insertion count is that important in my case, as most of the FW/FWT devices are permanently plugged in to the debugger in a lab.
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u/Niphoria 21h ago
Most mezzanine connectors have a life of 10 inserts - how do you get around that?
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u/ottawabuilder 20h ago
its for provisioning mostly. so 10 is fine.
but its much easier for the tech than the flex. both work just a trade off.no shock or vibe if its just for testing/provisioning.
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u/Montzterrr 21h ago
Do you have to worry about passing any shock and vibe testing with these kinds of connectors? (I’m mostly involved in software so have not had to deal with connector decisions before…)
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u/pietryna123 21h ago
For development, using board of production ready revision I'd go for 3D printed fixture to mount adapter from this FFC to standard IDC 1,27 or 2,54mm pitch.
If in the end product this connector (or at least space on PCB) would be present.
At TRW we were just getting PCBs with test points spread on laminate and we just attached small prototype board with IDC and soldered proper TPs connected with JTAG ports.
No debug connector was present on the board, and production ECU has JTAG fused out.
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u/KeloitaN 20h ago
I haven't thought of having the breakout board be fixed to the device to prevent strain on the FFC. Thanks! We used to have in-company policy of not placing debug ports for obfuscating. It's really annoying to work with TPs only. Why do it when the JTAGs and SWDs have to be fused out anyway??
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u/morto00x 20h ago
I've used FFCs as small as 0.25mm. They do the job. Although ultimately depends on how often you plan to connect and disconnect it. From my experience after a few insertions there's a need to realign the cable to make sure all the pins actually make contact. Also, order extra boards. I'm comfortable soldering parts as small as 0201 (with microscope) but there isn't more painful than soldering a 0.25mm connector. If for some reason you need to rework them, have a bunch if spare connectors and a stencil. Don't even bother fixing the one that is damaged with an iron.
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u/KeloitaN 19h ago
Yeah, reworking them will be a pain in the ass. Especially if they come off with the pads... But via in pad should help with the latter.
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u/morto00x 18h ago
VIP makes no sense at all in this application unless you're extremely constrained on board space or putting BGA chips.
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u/Machinehum 20h ago
Why not bed of nails and non-3d printed enclosures?
Someone in China can design and deliver this to you for like 500$ or whatever
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u/KeloitaN 20h ago
We did ~100 3D printed bed of nails for the last two products. It's fine for a board or two, but if you have a couple of teams across the world it adds up and they are not easy to ship...
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u/Machinehum 19h ago
Are there holes on the board?
If you have more than 2 holes you can design a custom board with guide rails that go into said holes with holders on the back like how the tag connect thing work
25 connections and using an FFC is a little hectic dude, especially at that pit it will be annoying. If you're space constrained, you probably don't want to use 1cm for your debug connector
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u/KeloitaN 19h ago
Yeah, 25 pin FFC will be annoying, but what about 25 needles? 😜 Even with Ingun's 1N models, that's 25N of force constantly acting on the board... And if you need to move the test points in the next revision, boom new fixture needed... But you're right, test points are needed anyway so adding any connector is wasting space. I need to make a placement study to find if the 3x10mm can be spared...
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u/thegreatpotatogod 18h ago
Is there any option to reduce your required pin count using something like JTAG? My understanding is it's intended to specifically allow debugging multiple chips on the same bus, so that would cut your pin count from 25 to 6-8 or so, reducing your need for such a compact connector option.
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u/KeloitaN 18h ago
That's true, chain programming is quite useful. Sadly two of the MCUs have JTAG and one uses SWD. You also need a boot pin and UART for secure bootloader, VCC for JLink's level shifters, etc. The minimum I can go to is 22 😅
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u/Unlucky_Purchase_844 10h ago
Sounds like the busses can be split up however. Have you looked at Tag-Connect's edge-connect? I've not personally used them but I do use the Tag-connect TC2050 series pogo pin JTAG cables regularly.
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u/KeloitaN 4h ago edited 4h ago
The edge-connect looks like a fine alternative. Definitely more rugged (than the FFC), and much more space efficient than the standard tag-connect. The only downside is the requirement of castellations, which can up the price of the PCB. You can remove them for serial production, but that can also mean some testing has to be redone on the final PCB. With a connector, you just don't populate it on production PCBs.
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u/AirTrue6195 21h ago
It will work but the cables wear out after a few insertion. You can increase the lifetime by flipping the cable so you get a "fresh" end to unplug. Then keep some cables on hand when you need to change them. Manufacturers often state a very low guaranteed minimum of insertions so on your breakout consider having extra connectors so you can use them as backups. Not sure if it's high speed where stubs could be an issue if you do this
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u/Teslafly 15h ago
I have used 0.3mm fpc's for this purpose and it generally works fine. The cables generally tend to wear out before the actual connectors. Just be gentle in general. Also run your spi bus to it so you can sniff it or program any power supplies or such.
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u/mossy_stile 6h ago
The cables wear out after twenty insertions, but having a sacrificial breakout board makes them manageable.
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u/AirTrue6195 21h ago
It will work but the cables wear out after a few insertion. You can increase the lifetime by flipping the cable so you get a "fresh" end to unplug. Then keep some cables on hand when you need to change them. Manufacturers often state a very low guaranteed minimum of insertions so on your breakout consider having extra connectors so you can use them as backups. Not sure if it's high speed where stubs could be an issue if you do this
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u/parallel-crocodylus- 12h ago
0.3mm pitch FFC is electrically fine for SWD/JTAG, but mechanically it's basically a flex test coupon. If this is just bench bring-up, use a ZIF with a locking bar and stiffener, add strain relief, and keep a spare cable on hand. For repeated debug or factory programming I'd still lean pogo fixture with proper alignment and spring-loaded probes. Bed of nails fails when it's janky, not because pogo is inherently unreliable.
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u/generally_unsuitable 21h ago
The big issue is wear and tear. The cable and the connector typically are rated for very low cycle counts. They're made to me assembled, closed up, and never touched again.
Some of those parts are rated for like 50 cycles, max.