Fixing botched prototypes

in hacks by DP | 28 comments

We go through a lot of prototypes, and many are not perfect the first time around. Here is a pic of one of our prototypes that had 2 pins cross-connected. The simplest solution was to cut the offending traces and reroute the signals with wire and tape.

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Comments

  1. Where’s the best source for that magnet wire you use – I’m assuming it’s the kind where you burn off the insulation with your soldering iron, right?

    *Brian

  2. Jim says:

    You switched MISO and MOSI, ouch!

  3. DanyO says:

    what gauge/type if wire is that? seems fairly small…

  4. JesseJ says:

    I recommend using plenty of Xilinx CPLDs and just reprogramming to swizzle your pinouts as needed.
    Of course, you gotta make sure their JTAG pins are accessible!

    • brentbxr says:

      thats a bad reccomendation. CPLD’s are expensive. there trying to create prototypes to later sell to the public. The best method is to create the PCB and schematic as perfect as you can the first time, if it doesnt work perfect; do what he did above. alot more common, alot easier then working in a CPLD on every design (let alone programming it, powering it, and so on..).

      this is quite common; even for the highest of manufactures they end up having to cut traces and re-route… (even post production/publicly sold items.. you crack them open and often see some after the fact swaps! its great)

      • JesseJ says:

        I’ll challenge this one. There’s a reason that CPLDs are called ‘glue’. I don’t know where you buy your
        parts, but small CPLDs not only collect other logic, which is expensive, but offer indefinite ‘swizzling’ of
        signals, as the design evolves. Seldom see wires ‘glued’ onto boards with CPLDs, but then I only have
        about twenty years experience with them. Maybe I shouldn’t comment, as the number of ‘free proto’ boards from Dangerous Prototypes might diminish . . . You ever check out the price for simple
        level shifter chips?

      • piotr1600 says:

        I get where you’re coming from JesseJ but at the cost of a single board spin one time – versus having to buy the CPLD every time – it doesn’t make economic sense – UNLESS you intend to make a lot of changes or perhaps see a way to add functionality via SW upgrade.

        But for simple stuff? I’ll take the wire-hacks and maybe even a deadbug mod or two and save the cost of the CPLD…

      • brentbxr says:

        Yeah sorry JesseJ for prototyping one off projects that is a horrible idea. If you design yourself somthing and you forsure want it to work; by all means throw a CPLD in it and ensure everything works without needing revisions…

        These guys are trying to design a PCB with the smallest part count possible to make profit. So revisions happen; cutting traces happen; all that happens. They would never use a CPLD in every design they create.

        I would never do that; cutting the line and some solder works just as well; fix the .brd file; send them to seed your done…

        Otherwise you need to create a BRD with the CPLD design it all perfect; figure out all the CPLD ‘swiggles’; then design a new BRD file without the CPLD for mass production… and then sometimes you will still end up having to cut traces after the fact :P. If your trying to make money avoid this CPLD idea all the way..

  5. AJ says:

    @Brian Schmalz: The wire is available under the Roadrunner brand name – they also make the wiring pencils to run it with. The gauge is 0.15mm or 0.19mm and yes, it is the type that you can burn the insulation off with a soldering iron, although experience says that you will want a temperature controlled iron and you will want to run it HOT (400C) to do this easily.

    Newark stock number 20J5555
    Element14 stock number 5017233

  6. AJ says:

    Also note – although tape is OK for temporary repairs or testing purposes, I prefer Tak-Pak for holding down permanent mods, particularly if they are going into the field.

    • brentbxr says:

      hot glue too; as its a common hobbiest tool available.

      • Sleepwalker says:

        Hot glue won’t stick well to the copper unless the copper is preheated, the lack of heat in the metal causes the bond to be really crappy I’ve found. I use the hot air rework unit to preheat it if I use hot glue for anything like that.

      • brentbxr says:

        I disagree. but thats just me. to hold down temp wires like in the image above, i find tapping just a tiny tiny bit of glue on a few key spots you will be fine; and it will hold forever (without extreme conditions and handling of course)…

        Heating the board and everything is just a waste of effort imho; the glue will hold just fine to the PCB’s soldermask, ICs (with a couple legs to hold to, even better), plastic, whatever.. It holds great!

      • brentbxr says:

        Are you talking about the wire? what copper are you talking about? Just a tiny dab of glue will engulph the whole wire and stick to the PCB; it giving the wire a nice tight hug; not trying to stick to it.

      • Sleepwalker says:

        Brent, I’m not staying it won’t stick, I’m staying it won’t stick *well* – perhaps I should rephrase that to ‘Not nearly as well’. Also I should clarify I didn’t mean directly to the copper, but I meant it will not stick as well to the areas were the copper is underneath(underneath the soldermask), as distinct from fibreglass. I can guarantee you 100% the bond will not be nearly as strong without preheating – I’ve been doing it on boards professionally for more years than I care to remember. Maybe it’s OK for general stuff and if it’s OK for you fine, but you *will* get a much better bond with preheating. Try it on a piece of steel and see how strong the bond it without per-heating compared to with it – I think you’ll be surprised.

  7. IPenguin says:

    Magnet Wire is the correct name (Kupferlackdraht in German). Actually it’s used for transformers, motors, inductors, speakers.

    http://en.wikipedia.org/wiki/Magnet_wirehttp://de.wikipedia.org/wiki/Kupferlackdraht

    In Germany we buy it from reichelt (CUL XXX/x,xx – 0,10mm – 2,00mm), Franell, Distrelec.

    • arhi says:

      Actually no, it is not. Magnet-wire uses completely different type of insulation from prototyping wire (wire-wrap / roadrunner / pro-wire /…). The prototyping wire uses polyurethane that you can cleanly remove with iron heated to 400C, while magnet wire if not stated explicitly uses polyvinyl that’s capable of taking bit over 100C only. The bigger problem with polyvinyl is that between 100C and 260C it “flows” (it’s thermally unstable). For magnets, transformers etc polyvinyl (aka formvar) is common as they are all dimensioned not to go over 90C and as you know your transformers, inductors, magnets will “short” if they go over 100C and that’s exactly because they are using standard magnet wire insulated with polyvinyl. The polyurethane insulation is for order of magnitude more expensive and is used only in special applications (like prototyping wire) or if higher heat resistance is requred. But that’s not “regular magnet wire”, it is “very special case”. The polyamide / kapton insulation is even more expensive (4-5 times compared to PU) and is very rare…

      • IPenguin says:

        @arhi, yes and no :)

        You can use both – equally well and simple as long as the operating temperature will not exceed ~80°C ! :)

        For magnet wire you use a mechanical stripper – or simply a nail clipper you prepare with little nodges according to the diameter of the wire – and for wire wrap wire you yan use the soldering iron or the stripper/clipper as well.

        However, I prefer the magnet wire over the wire wrap/prototyping wire as it can be bend and formed and will stay in position while the wire wrap wire tends to stretch back and this is far harder to keep in position.

      • arhi says:

        @IPenguin, I was just pointing out what wire was on picture and what is prototyping wire.

        As for what wire you can use, I seen ptfe insulated wire, pvc insulated wire, all the way up to 1.5mm in dia used for this.. so yes, you can use magnet wire or basically any type of wire to fix error on pcb, I sometimes use just a piece of soldering wire (to make some short bridges you just touch the wire to the pin that’s already cooling and solder wire will melt to it but stay there so you do same with other side and …) .. prototyping wire is imo easiest one to use (0.10mm magnet wire is very hard to strip for e.g.), especially if you use prototyping pencil and good soldering iron

  8. Arup Basak says:

    You could have used superglue along the wire instead of masking tape.

  9. JBeale says:

    I use hot-melt glue for holding fine wires in place, but it’s annoying having to wait a few minutes for the glue gun to heat up, and although I have a palm-sized one (“surebonder mini glue gun”), I think it’s still larger than you really want for this sort of job (the kind of thing you do under a lens or microscope). Is there a “micro” size glue gun for this type of very small work? The techs at work usually use superglue for this.

    • Sleepwalker says:

      I have a ‘normal’ size hot glue gun, but also a little baby one that I don’t use much. I think it cost $5 at a local fabric/crafts store and it works OK, though takes quite a bit longer to heat than the bigger one. See my post above also about preheating the board.

  10. JayPal says:

    Ah ah!!! Quite old school!
    My father used to do the same on the Sperry/Univac 1100/70!
    But I have nothing against it!
    Quite by the contrary, it’s a great way to correct something!

    • IPenguin says:

      Hah, that’s where I learned about wire wrapping … on a Sperry Rand/Univac 90/70 assembly line and in their lab about using wire wrap wire for quick patches on prototypes and engineering samples. :)

  11. ericwertz says:

    I like Weldbond glue for this purpose. It’s got the consistency of hot glue after it sets (you can tug it off), but it goes on like an all-purpose glue. Dries fairly clear, too.

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