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Drilling solder paste stencils

Discussion in 'CAD' started by oparr, Sep 29, 2006.

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  1. oparr

    oparr Guest

    These are usually laser cut ($$$) but what's wrong with CNC drilling
    your own using the coordinates for the SMD pad centers? Paste doesn't
    have to cover every nick and cranny of the pad. It will require a few
    tool changes for the various pad sizes but since we're dealing with say
    8 mils brass sheet, relatively cheap high speed steel bits can be used.
    Any other thoughts on this?
     
  2. Boris Mohar

    Boris Mohar Guest


    Interesting idea. For large pads multiple holes could be tried. Perhaps
    slightly thicker deposition could make up for the corners.



    Regards,

    Boris Mohar

    Got Knock? - see:
    Viatrack Printed Circuit Designs (among other things) http://www.viatrack.ca

    void _-void-_ in the obvious place
     
  3. oparr

    oparr Guest

    For large pads multiple holes could be tried.

    Even overlapping to some extent.
    Instead of an 8 mils sheet one could use 10 - 16 mils depending on the
    size of the smallest pads (smallest holes). I suspect at some point the
    paste would rather stick to the walls of the hole rather than the pad
    as stencil thickness increases.
     
  4. Even with really sharp drill bits, I suspect the holes would not have clean
    enough edges to release the paste consistently. Perhaps if you sandwiched
    the sheet between two boards while drilling it would help with that.

    I just measured one of my thicker stencils and it is 6 mils thick. 10 - 16
    mils would lay down way too much paste making the self alignment (during
    reflow) unpredictable. It does depend on the pitch of the parts you're using
    too. The smallest parts on most of my boards are 50 mil pitch IC's and 0805
    passives, and 4-5 mil stencils work fine.

    Success would also depend on the flux in your solderpaste. Water washable
    flux is somewhat tricky as it burns off quickly preventing the dots from
    flowing over the pad in time. RMA would flow better with round dots and give
    a better chance of flowing to the whole pad. While it is preferable to cover
    the whole pad with paste, dot's do work. I have a paste dispenser on my pick
    n' place which places dots. Reliability is not as good as stencils, but for
    low volume runs, it works out alright. The most common problem I see with
    dots is misaligned parts during reflow.

    I use Stencils Unlimited for SS laser cut prototype stencils. At $145 each,
    they're not all that expensive. Also, if you're going to try making your
    own, I would suggest using stainless steel sheets instead of brass. Most
    types of soldering flux will activate the brass.

    Good luck, I'd be interested in knowing how it works out.
     
  5. The two previous posters have hit the nail on the head with the solder
    paste release issue.

    On a laser cut or etched stencil the openings are usually just slightly
    cone-shaped to allow for easy release of the solder paste when the stencil
    is lifted. It may not be much but every little assist makes the job that
    much easier.

    That said, it is possible to do and my employer used to do their own as
    well. I wasn't around then and have no idea how successful they were.
    However, how much are you saving? I have heard some comments in this NG that
    people are buying prototype screens for slightly less than $100USD. So what
    is your time and the material worth? Brass isn't cheap, nor is it easy to
    handle in very thin sheets and every slight kink/bend is going to adversely
    effect performance.
     
  6. Guest

    Its not about drill bits - have you tried drilling full hard stainless
    with a jobbing drill bit?
    Most SS for stencils has a hardness of 370 Vickers and top end stuff
    runs at over 400 Vickers (divide by 10 to get Rc).
    You wont get positional accuracy and the steel will distort (due to the
    rotation of the bit or cutter) and the stencil wont be flat. It would
    not be possible to do fine pitch.
    The mechanics of it are not possible and I doubt that i would be any
    cheaper as a CNC system would be no cheaper than a laser - new laser
    systems can be bought for around 150,000 USD now.
    Some one did try to sell a stencil "cutter" that was based on stamping
    technology. No idea if it worked and I dont know of any stencil
    operations that have one.
    Ben
     
  7. oparr

    oparr Guest

    I would suggest using stainless steel sheets instead of brass.

    Either can be used with the paste I'm using.
    The alloy I'm using is very cheap....12" X 8" sheet is less than $5.00.
    I'm using brass. Also, have carbide bits of each size but see no reason
    to use them.

    Drilled a piece of .016" brass with the five hole sizes needed using
    jobbers bits(.021", .032", .036", .052",.06"), here's a picture;

    http://upload.pbase.com/eldata/image/67779851

    Squeegeed the paste at 72F on a solder tinned piece of copper and got
    this;

    http://upload.pbase.com/eldata/image/67779854

    Only the .021" hole didn't release properly. I'll try a pieces of .010"
    and .007" brass next week. So far so good, didn't expect it to work so
    well with the worst case .016" brass. This has got to be better than
    using a syringe.
     
  8. Those dots look pretty good, and so do the drilled holes. Much better than I
    would have expected. Since your sheet is so thick, you'll probably want to
    try using the smaller dots first and see how they reflow as not enough paste
    is usually better than too much. What type of flux is in your paste? RA,
    RMA, NC & WS flux will usually react with brass. Not a big deal though if
    you wash the stencils with the appropriate cleaner (depending on the flux
    type) after you're done.

    Yes, it's definitely better than using a hand held syringe.

    I'd be interested to see photos of your experiments with thinner sheets
    too...

    Chris
     
  9. Borat

    Borat Guest

    Would polyimide work? It may be gentler on the drill bits and easier to
    handle as well.
    I had a rep visit me showing off laser cut polyimide masks for lower volume
    production. From memory they were 1/2 or 2/3 the cost of s/steel.

    rob
     
  10. I've never used polyimide stencils. My exposure to Kapton has been limited
    to flex circuits and tape. I might be nervous about drilled Kapton tearing
    easily. I also remember it being absurdly expensive. The laser needed for
    Kapton would be much cheaper than that needed for SS so some cost could be
    made up there.

    Honestly the brass sheets looked quite good after being drilled, and brass
    is soft enough to be gentle on the bits. If he has a CNC mill already
    available, it seems like it would work out alright.

    Chris
     
  11. One of the assembly houses we use has tried polyimide stencils and says they
    can be torn easily by the squeegee.
     
  12. oparr

    oparr Guest

    If he has a CNC mill already available, it seems like it would work out alright.

    Actually, it's just a CNC drill. Haven't gotten around to the mill yet.
    And yes, the drilled stencil worked well, goodbye to hand soldering and
    syringes. Some after "pasting" and after "baking" shots were added at
    the link below. Click on slideshow in the upper right hand corner.

    http://www.pbase.com/eldata/stencils&page=all
     
  13. The soldered parts look just dandy!

    An aggressive flux (RA or RMA) might get the solder to flow more evenly on
    the QFP-32, but that's just being nitpicky. I'd ship any of those parts
    without concern.

    Chris
     
  14. Fred Bartoli

    Fred Bartoli Guest

    oparr a écrit :
    That's really impressive of a hobby outcome.

    Just one remark: your IC2/MC33886 has a thermal pad, which is intended
    for heat spreading. As such the PCB pad underneath must *not* have
    thermal relief and depending on your board stack up, the better way is
    to also have a ground plane and stitch the thermal pad to it with a
    bunch of via.

    This is all explained in the IC data sheet.
     
  15. oparr

    oparr Guest

    the better way is
    Freescale goes to great lengths in their datasheet in order to avoid
    using a conventional heatsink on the device IMO. The best way is to use
    an external heatsink in addition to lots of copper which my design
    calls for. Makes the board easier to maintain if the part should fail
    for whatever reason. Here are three homemade prototypes that survived
    all the abuse I threw at them;

    http://www.pbase.com/eldata/image/66582088
     
  16. Fred Bartoli

    Fred Bartoli Guest

    oparr a écrit :
    Aha, OK. I missed that and just quickly skimmed through the data sheet.
    Nice job.
     
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