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Tiny PCBs - score, route, or ?

Discussion in 'Electronic Design' started by Richard H., Dec 28, 2005.

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  1. Richard H.

    Richard H. Guest

    I need to make some small PCBs (about 0.5" x 0.75" ea). I've compiled
    some options, but I need some insight on the best way to cut these apart
    (ideally, pre-scored / pre-cut and snap apart after assembly).

    V-scoring seems good because it might require less PCB space than
    routing, the parts are large enough to grip & snap apart, and the edge
    would be straight, although rough (which is OK).

    Tab routing would leave a clean edge except for the tabs, and it might
    put less stress on the board to break apart. Drawback is the "jaggies"
    left by the tabs.


    Do-it-yourself alternatives...
    The "string of holes" method using drill hits, which might prove
    cheapest, but also the roughest edge. Any tips on how to space the
    holes so they don't fall apart in assembly, but aren't too hard to break
    later?

    Cutting ourselves with a band saw or a shop router on a table stand w/
    guide. This seems a bit hazardous with small parts, and may cost more
    in the long run because the cuts may be wider (more waste). (Band saw
    isn't a bad idea...)

    Using a shear, which I've seen suggested, but not for an assembled
    board. Pop! Crunch! :)

    Any thoughts on these other other techniques?


    (I've found a few board shops that will do tab routing and/or scoring
    for protos, but they're pretty pricey so any references would be welcome
    too. This is short-run production, so cost per board is a concern.)

    Thanks,
    Richard
     
  2. Fibreglass board will wreck a bandsaw blade very quickly...
     
  3. I recently had some small boards made that used a combination of V
    scoring (done with the router, with a pointed bit) for straight edges
    between the boards and side rails (to hold the boards together for
    parts placement and soldering), and routing for the corners and curves
    and between boards in the row. This left rough edges only in the
    middle of 2 straight sides per board, that I clean up by sliding the
    broken board along a sheet of silicon carbide paper. Where the routed
    slot meets a V groove, the V groove aligns with the board side of the
    routed slot. So I sand till I can't see where the broken edge meets
    the routed edge and I have good dimensions.

    On another board that was designed by someone else, for us, they used
    the routed with tabs, but curved the routed line inward about a half
    slot width on each side of the tab so the tab broke just below flush
    with the board edge. Of course, this means that it broke sticking way
    out on the other side, but this was not another copy of the board, but
    a temporary support rail that was discarded.
    I think you need about a diameter of board between the holes.
    It takes a carbide tipped blade and makes a lot of dangerous dust.
    For small runs, you can use a carbide tipped hook knife to pull a V
    groove out of the board along a straight edge guide.
    If you break boards apart, either along V scoring or drilled holes, it
    is a good idea to keep surface mount parts a few board thicknesses
    away from the break line, to reduce the chance of pulling the
    metalization off the parts or cracking them in half. It is also a
    good idea to have the resistors and capacitors along the edge run with
    both pads equally far from the edge, instead of with one pad along the
    edge and one away from it, for the same reason.
     
  4. Richard H.

    Richard H. Guest

    All good tips, thanks.

    It sounds like tabbed routing might be the best approach for the
    smallest scale (which is important here), allowing us to have parts
    closer to the edge without the stress. Maybe with a throwaway strip
    between the parts for the indent technique you described.

    Or, I'll tryout your technique of v-scoring with a router before
    assembly, if we can't find a shop that'll do it for cheap enough.

    Thanks,
    Richard
     
  5. V-groove would be the way to go. Very little waste (you do have to
    keep traces away from the very edge of the board) and you can assemble
    and perhaps even test the little b*ggers before you have to break them
    apart. No tooling, and only a tiny cost per board (perhaps less than
    the cost of the PCB area otherwise lost to kerf).

    Shear is theoretically possible on an assembled SMT-only board but
    requires a lot of unused space around the cut on the component side
    for the clamp to come down.

    The only way I'd consider a saw is if I could find an appropriate
    'table saw' type configuration with a $$ solid carbide slitting saw
    blade. Something like this:

    http://www.discount-tools.com/ful-2450-4.cfm

    I think there are purpose-made depanelizing saws, but I've never had
    occasion to use one.


    Best regards,
    Spehro Pefhany
     
  6. Olimex (http://www.olimex.com) will panelize and separate boards at no extra
    cost. They start with standard 160mm x 100mm boards (single- or
    double-sided) and make as many of your design as will fit, for a single
    fixed price. For instance, I recently had them build a 1" x 1.5" design;
    that works out to 25 boards, for a total of about $43 including shipping.
    (The price varies slightly depending on the presence of nonstandard drill
    sizes.) I'm pretty sure I've had them do boards as small as yours, in the
    past.

    The shipping is a bit slow because they come from Bulgaria, and the quality
    is not quite as nice as what I've had done locally, but it's well within
    spec, and the price is very hard to beat.

    I have not yet found anyone else who will panelize for free.

    On the original topic, though: you can use tin snips to cut fiberglas board.
    It helps if you warm the board up with a heat gun first.
     
  7. Richard H.

    Richard H. Guest

    Walter, thanks.

    Actually Olimex is the most promising so far. I'd mailed them for
    details, but they are closed for the holiday season. (PCB Pool also
    offers this, but their prices are outrageous.)

    Olimex will depanelize, but I really don't want a bag of tiny PCBs in
    the mail - they'll be very hard to assemble. I'm hoping that if I
    provide the routing template as a layer file, they'll do tabbed routing.
    How did they deliver your tiny boards?

    The mailing is no problem; I can be patient. :)

    Thanks for the pointer on tin snips; I'd read about that one too, but it
    seems very unlikely with a populated board (lots of flex, it'd seem).

    For the tolerances we need, having them pre-tabbed/scored by machine is
    looking to be the only practical option, with in-house pre-scoring with
    a v-bit running a distant second place.

    Thanks,
    Richard
     
  8. Guest

    Sacrificial connectionstrips are a good way to work with small boards.
    You can have the board house route most of the board outlines, with a
    web of disposible strips connected just enough places to hold the
    little borads well enough for assembly.
    Our board house does that as part of the outline routing option. It is
    just an alternate router bit.
     
  9. Rich Grise

    Rich Grise Guest

    Something's impelling me to say that, if you go the V-score route,
    don't break them apart with only your fingers and thumbs - find
    some kind of fulcrum or blade or something to set them on and just
    push down.

    Probably pretty obvious, now that I think about it. Oh, well. :)

    Thanks,
    Rich
     
  10. qrk

    qrk Guest

    V-scoring is nice, but SMT parts near the score lines can be damaged,
    especially if the board thickness is 0.031" or less.

    I like using tabs with three 20 mil drill holes in each tab along the
    board edge. I can post a sketch of the tab w/ drill holes if you want.
    Easy to break off and the jaggies aren't too bad. Easily touched up
    with a bit of sandpaper. Hmm, perhaps v-score the tabs.
     
  11. stefanv

    stefanv Guest

    Don't forget the old-fashion way. In case of single side boards, I us
    Press n' peel (the blue stuff) buy a paper phenolic copper clad 12x12 fo
    about $12.00. Iron on the Press n' Peel. Put in in the old Ferri
    Chloride, and etch the boards myself. You can score the paper phenoli
    boards with a stanley knife and break clean. Gives you 144 sqare inces (
    lot of small boards).
    I just did 28 small controller boards last night .75" x 1.5" in an hr tim
    + $8.00 board. For low power boards, I still like the phenolic copper cla
    better, you can cut it drill it, file it, and your toold stay shar
    forever. Never lost a drill bit on these! se
    http://www.web-tronics.com/pp1212.html

    Stefan V
     
  12. Ben Bradley

    Ben Bradley Guest

    Just add that to the cost of cutting the boards apart. Don't use a
    big (12" or 14") bandsaw, get one of the $89 8" tabletop models, so
    you can use a thinner blade that will cut a smaller kerf. Also,
    consider a scroll saw, also a very small kerf, but perhaps not as
    fast.
    Also, in any of these cutting methods, use a dust collector, and
    don't breathe anything put into the air that the dust collector might
    not get.
     
  13. Richard H.

    Richard H. Guest


    Good info, thanks. These guys are local too. I'll keep that in mind in
    case this can be one-sided (i.e., larger). As-is, it'll need 2-sided
    with PTH.

    Thanks,
    Richard
     
  14. budgie

    budgie Guest

    On Wed, 28 Dec 2005 10:42:50 -0800, "Walter Harley"

    (snip O/P)
    Futurlec (www.futurlec.com) depanelise free. I regularly order batches
    comprising 30 double-sided boards each 1.3" * 0.8" and the shipped cost is
    $US41.

    Re the O/P, I appreciate he is concerned about stuffing tiny boards and appears
    to favour population before separation. I certainly prefer stuffing tiny boards
    in a small board holder rather than separating assembled boards from a panel.
     
  15. Richard H.

    Richard H. Guest

    Yeah, I hadn't thought about the issue of the dust generated during
    cutting too. By the time I rig this, it'll be cheaper / safer / easier
    to just pay the board house.

    We're definitely planning to assemble and test on the larger board, then
    snap off for packaging. What drives your suggestion for V-score over
    tabbed routing? I don't have enough data to weigh cost yet, but is
    there much difference in the waste? (Routers seem to be in the 0.12"
    range.)

    As for unused space around the perimeter, even reserving a 0.1" border
    would take a huge bite out of the yield. So anything that minimizes the
    perimeter space will be welcomed.

    Do you know board houses that'll do v-score or tab routing for
    proto-volume, and a reasonable price? I'm working through my list
    slowly (it's hard to find this detail info), and Olimex is the closest
    thing to reasonable so far. Volume is a problem, because at this size
    2-3 boards makes a huge batch of devices, and the one-time fees make the
    per-piece costs high.

    Thanks,
    Richard
     
  16. Guest

    In production we stuff the boards while still panelized. They are
    scored. We then use a "pizza cutter", which is a rotary diamond saw on
    a fixture that can slide back and forth along rails, to depanelize
    along the score lines.
     
  17. Richard H.

    Richard H. Guest

    I'm planning to use standard thickness, which IIRC is 0.062". Good food
    for thought.
    Yes, I'd very much like a sketch, thanks.

    After all these comments and some noodling, it looks like tabbed routing
    is a strong option, maybe with the indent technique mentioned by John
    Popelish. Instead of a whole strip of throwaway, see if I can make the
    tab to snap on both sides, recessed from the edge to hide the jaggies.
    Separate the first break by hand, and remove the second stub with
    pliers. Like this:

    | -ooo- #1 |
    |-----/ ||| \-----|
    tab
    |-----\ ||| /-----|
    | -ooo- #2 |

    The question then becomes how small to make the tab and how close to
    space the holes in the tab (balancing ease of snap vs. sag during
    assembly), which I expect will be a matter of experimenting. I imagine
    with parts this small you can't go less than one tab per side, unless
    I'm missing something.

    And of course, where to get these made in small proto quantities...

    Thanks!
    Richard
     
  18. As a bag of tiny PCBs :)

    Olimex's edge tolerances are not that great. In fact that's actually my
    main complaint with them - the edges are pretty rough.

    Budgie's comment about Futurlec was pretty interesting. I've used them as a
    parts supplier (though their online catalog is awful, at least last I
    checked) but I had no idea they did PCB manufacturing. Have to check that
    out - might be a bit quicker than Olimex.
     
  19. There is basically *no* waste with V-groove, provided you don't intend
    on putting parts and traces right out to the edge. 60 mils is usually
    enough. I have not tried it with >2 layers, you might need a bit more
    for that.
    Most moderate/small volume PCB suppliers will do it for a nominal
    charge. Not so many prototype companies.


    Best regards,
    Spehro Pefhany
     
  20. Richard H.

    Richard H. Guest

    Good pointer - they have very good pricing ($99 for 200 pcs)! I wasn't
    aware they did PCBs. Do you find their routing tolerances are good?
    Yes, this is SMT on both sides, and I expect it'll be much easier to
    paste and pre-heat / solder as a larger panel. One-offs could be
    managed, but panelizing seemed cleaner.

    Have you run into problems de-panelizing, or just not interested since
    you have a good one-off technique? How is your holder constructed?

    Thanks,
    Richard
     
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