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Rubber stamp PCB?

Discussion in 'Electronic Design' started by Winston, Aug 12, 2012.

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  1. Winston

    Winston Guest

    Much to my horror, I discovered one of my 'standard' PCB footprints
    is 40% larger in both dimensions than the part it was intended for
    on my latest board.
    (It's a 16 pin device with pads on 0.5 mm centers)!
    The board is about 70% assembled and is much later than I would prefer.
    If I had unlimited time and money, I'd just spin the
    artwork with the correct footprint. Lacking either, I want to
    patch the existing board.

    I've not located a prototype board that has the necessary footprint
    terminating in matching pads.

    Here's what I'm thinking. I can upload a big bitmap of the
    correct footprint with exactly the right terminating pads to
    these guys and stamp a piece of my 0.005" PCB stock using say
    Epson Durabrite yellow ala:

    Then I can stamp and etch as many adapter boards as I want.
    (I need one [1] adapter board.)
    Mount the device to the adapter then solder the terminating
    pads to the huge pads of the incorrect footprint.

    What do you think of this idea?

  2. mike

    mike Guest

    I've done considerable experimentation over the years.
    I liked a flatbed pen plotter best, but with .005" resolution,
    mine ran out of gas years ago.

    I switched to toner transfer.
    I use a laserjet 4MP at 600DPI with real HP toner.
    I use a gerber viewer and have it print with
    a scale factor of .987. The printout isn't exactly 1:1.

    The secret is in the paper. Hardest part is to get
    the paper off the copper without taking the traces.
    Shiny photo-paper works best. There's lots of discussion
    of this on the web.

    I augment the etch with an eyedropper. Getting uniform etch
    is difficult if you just stick it in the etchant. The
    plastic traces aren't as robust as one might like.
    There's not a whole lot of room for overetch with narrow traces.

    Takes some experimentation, but turnaround is quick and cheap.

    I'd worry about ink thickness with a rubber stamp.
    I don't know anything about the ink suggested, but the stuff
    that worked best in the pen plotter was so nasty that they
    couldn't sell it in the USA. I got one plot session out of a pen
    before the plastic dissolved.
  3. I've used two prototyping methods in the past. One is to use
    prelaminated PCB material that i illuminated with a simple uv tube. As
    mask i used normal transparencies that were printed on a
    thermosublimation printer, as they put down a pretty decently thick film
    of ink with little sidewallroughness.
    The other method is actually cooler, involving a milling system (doesn't
    need to be super fancy LPKF ones), where you would just do isolation
    milling. The cool thing about this one is that once you take your board
    out it's even already got holes. Obviously no conductive vias possible
    with both methods.
  4. Nico Coesel

    Nico Coesel Guest

    My idea exactly. When I have a small PCB made I order two panels. The
    PCB house I order from has a special prototyping tarif where you get
    two boards. Because of the start-up costs the price for a small PCB is
    the same as a larger one (say the size of a euro card). So for around
    €100 I get 10 to 30 PCBs. I estimate that a simple adapter board is so
    small you can easely fit 30 pieces in one panel.
  5. Ian Malcolm

    Ian Malcolm Guest

    For a one-off, if this is a leaded part (not J leaded) just glue it to
    the board and hand wire it. 0.5 mm is tricky to hand wire, but far
    from impossible IF you use fine enough solid tinned wire (e.g. a single
    strand taken from some stranded wire) and have an iron with a fine
    enough bit. No coffee that morning!
  6. Rich Webb

    Rich Webb Guest

    This is probably the best chance, especially since the wiring should be
    all straight out from the pin to the pad. At least, I'd try this first.

    For the other approaches:

    0.5 mm is probably beyond the capability of a rubber stamp to lay down
    with sufficient accuracy or to leave a uniform resist layer on bare

    Press-n-Peel Blue <> could probably do it but it
    requires some finesse with fine pitch artwork as well as a laser printer
    (not ink jet) that uses a compatible toner. Helps to have a hot-roller
    laminator, too; much better reproducibility than a clothes iron. I have
    done 10/10 rule boards with P-n-P but other than the glory of doing it,
    it's a P-in-the-A. Pro tip: bare copper isn't the sweetest thing for
    soldering, especially fine pitches. MG Chemicals has a "liquid tin"
    solution that can be used to lay down a thin tin layer which makes the
    soldering easier. More shelf-stable, too, as compared to the dry mix
    tinning powder.

    There are carrier boards that might be adopted, e.g., over at

    But I'd bite the bullet and get a "real" PCB fabricated. There's a time
    versus cost trade-off, of course. A double sided, silk-screened, solder
    masked prototype from Sunstone could be in your hands fairly quickly or
    go with SeeedStudio for a really low cost ($1/board for 5cm x 5cm) but a
    longer wait.

  7. Seems too risky. Best way is a professional double sided PCB with the actual
    components footprint on top and the main PCB footprint on the bottom. If you
    place the vias in the middle of the pads of the bottom and (let) mill the
    outer half of them, you can solder them pretty easely.

    BTW What's the component you're talking about or at least what's its

    petrus bitbyter
  8. Winston

    Winston Guest

    On Sun, 12 Aug 2012 00:22:42 -0700, mike wrote:

    (Toner Transfer)
    OK. Thanks for your thoughts, Mike.

  9. Winston

    Winston Guest

    On Sun, 12 Aug 2012 02:07:41 -0700, Adrian Nievergelt wrote:

    I tried that and actually made a usable PCB.
    Then I learned all about 'actinic keratosis'. :)
    I used my cnc mini-mill to make a couple double-sided boards that way.
    Very time consuming but *much* better quality than anything I have
    tried, before or since. (With the exception of PCB houses of course.)
    Properly centered, I might add. :)
    Axial component leads and wires placed through holes work well.

    Thanks, Adrian.

  10. Winston

    Winston Guest

    OK. Thanks!
    I had this board made 'way out of country as well.
    Fab was very speedy but the board still took 8 weeks to get to me.


    Thanks guys.

  11. Winston

    Winston Guest

    Your original post didn't get here in it's entirety and I don't
    know why that is. Thanks for your thoughts.

    Sounds like a futile exercise to me, given the thermal conductivity
    of the wire and its resulting tendency to part company with the
    component when soldering to the pad ~ 1 mm away (or vise versa).
    One stamp vendor claims resolution of better than 0.045 mm.
    (I agree that it would look very rough.)

    Yup. I've used that stuff and like it too. Say! That'd work!
    Sixty smackers! (Thud)

    OK. Thanks, Rich.

  12. Winston

    Winston Guest

    On Sun, 12 Aug 2012 16:03:42 +0200, petrus bitbyter wrote:

    OK, thanks.
    It's the LTC5564IUD in the 16 pin 3 mm QFN package with pads on
    0.5 mm centers:

    Thanks, Petrus.

  13. Winston

    Winston Guest

    On Sun, 12 Aug 2012 09:54:22 -0700, John Larkin wrote:

    I've bought from AP Circuits as well and have been delighted with
    their quality. I will have to check back with them now because I
    recall they didn't allow milled channels inside a board in the
    past, for their inexpensive prototypes.
    Kewl! Looks like Petrus and you think similarly.

    (Perhaps I will just spin the artwork and send it to AP Circuits.)

    Thanks, John.

  14. Rich Webb

    Rich Webb Guest

    What I've done in the past when I've needed similar small "jumpers" is
    to strip a length of 30 AWG wire-wrap wire and then, with some shaping
    of the curve, a piece or two of Kapton tape (high melting point), and
    holding my tongue just right, lay the wire so that it bridges the gap
    between the pin and pad and stays in place *before* applying solder.
    Flux it up and then sort of drag-solder both the pin and pad. Tada!
    Doing this to 0.5mm pitch pins is challenging but quite do-able.
  15. Ian Malcolm

    Ian Malcolm Guest

    Not futile at all. Start with a reasonably long piece of wire, solder
    wire to pin, bend it till it lies freely along and in contact with the
    pad, tape down across the wire beyond the pad to hold it there and
    solder it to the pad. Yes the joint to the pin may reflow, but there
    isn't enogh stress in the wire to spring it clear and the tape stops it
    going anywhere. After the wire is soldered down, cut off the excess
    with a very small scalpel or a small flat-blade jeweller's screwdriver
    sharpened to a polished knife edge.
  16. Winston

    Winston Guest

    On Sun, 12 Aug 2012 19:05:25 +0000, Ian Malcolm wrote:

    Thanks, Rich and Ian.

    You guys have much more faith in my solder-fu than I do. :)

    It's the LTC5564IUD in the 16 pin 3 mm QFN package with pads on 0.5 mm

    Side-glued 30 AWG kynar would get me my .5 mm spacing
    with little error but I'd still be worried about capacitive
    coupling to the central ground plane, particularly on the
    RF input (pin 1). Were this an analog, or even low-freqency
    digital issue, I'd consider trying that.

    As it is, I think I'll just spin the PCB.

    Thanks for your thoughts on this, guys.

  17. Ian Malcolm

    Ian Malcolm Guest

    Ah, QFN yechh! No I wouldn't care to do that myself. I DID say any leaded
    package (except J lead)! ;)

    I see that it has a RF input of between 600 MHz and 15 GHz. Even with a
    high precision double sided PTH daugerterboard that's far from being a sure
    thing. A respin is probably cheaper and is certainly the best way
  18. Winston

    Winston Guest

    On Mon, 13 Aug 2012 10:13:55 +0000, Ian Malcolm wrote:

    So you did.

    I should have played fair and revealed the package type on
    my first post. Sorry about that. I wish I'd chosen the
    'small', accurate 'QFN' library footprint instead of the
    'incredibly enormous', accurate 'QFN' library footprint. :)
    While the respin is being fabbed, I can do firmware development on the
    present board.

    Thanks again. :)

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