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Check for PCB flatness with Newton's rings?

Discussion in 'Electronic Design' started by [email protected], Mar 12, 2007.

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  1. Guest

    We have a PCB that needs to remain as flat as when it left the fab,
    but it is bolted down to a metal base. We think the base is not flat
    enough. Before we call in a mechanical consultant, do you guys and
    gals think it makes sense to put a pattern of concentric rings on the
    PCB and then check for fringing with a lens?
    I just got the Edmund optics catalog in the mail, so I got to
    wondering.
     
  2. Have you checked it with blueing?
     
  3. Tim Wescott

    Tim Wescott Guest

    What makes you think it was flat enough when it left the fab?

    What makes you think that its thickness is consistent enough so that
    when it's bolted to said metal base it'll be flat enough?

    If you think it's the base, why not measure the base?

    I suspect that the board surface will be rough enough that fringing
    won't be a big help -- but it won't hurt to try. Getting a metrology
    expert to do a consult may be money well spent, particularly if you
    budget for at least one fool before you find one who can get the job
    done right.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  4. The newton rings are a check for optical flatness,
    which I doubt you will achieve with a pcb. You'll
    need an optical surface as well. A milled surface
    is hardly optical, meaning a point source reflex
    is still a point source. It shouldn't be too
    difficult to check a milled surface for flatness ,
    just giving a quantitative answer is not that trivial.

    Rene
     
  5. Guest

    dial indicators with stands cost about 20-30$ from Harbour freight and
    other low cost maching supply companies.

    Steve Roberts
     
  6. Seriously, the level of flatness from a PCB, is going to be so 'bad' in
    optical terms, that Newton's rings would be pointless. However for normal
    mechanical 'use', a well designed board should be pretty flat. The key
    though is in the design. If (for instance), you have a large ground plane
    area on one side, and not the other, the board _will_ be bent when it
    'leaves the fab'....

    Best Wishes
     
  7. Guest


    Thanks to all, much to consider. Upper management is resistant to
    spending money on a mechanical guy...
     
  8. Others have already answered your question pretty well (unambiguously with
    "No", I suppose). Let me just add that Newton's rings aren't a pattern that
    you put on things; rather they are generated by interference when a
    spherical glass surface touches a gainst a flat one (very simply stated).

    robert
     
  9. Ryan Weihl

    Ryan Weihl Guest

    instead of the PCB get a steel plate with the same dimensions
    and mount it.
    rw

    --
     
  10. Just how flat does it have to be?

    You can get a class B surface plate and DTI pretty inexpensively,
    which will allow you to check to within a fraction of a thou.


    Best regards,
    Spehro Pefhany
     
  11. James Arthur

    James Arthur Guest

    Homer's blueing idea with a machinist's "surface plate" is pretty
    good, but even a simple straight edge will reveal deviations from
    flatness on the order of 2-3um.

    http://msl.irl.cri.nz/training_&_resources/Measurement_articles/How_flat_is_flat.pdf

    Best,
    James Arthur
     
  12. James Arthur

    James Arthur Guest

    Homer's blueing idea with a machinist's "surface plate" is pretty
    good, but even a simple straight edge will reveal deviations from
    flatness on the order of 2-3um.

    http://msl.irl.cri.nz/training_&_resources/Measurement_articles/How_flat_is_flat.pdf

    Best,
    James Arthur
     
  13. James Arthur

    James Arthur Guest

    Homer's blueing idea with a machinist's "surface plate" is a pretty
    good gauge of flatness, but even a simple straight edge will reveal
    deviations from flatness on the order of 2-3um.

    http://msl.irl.cri.nz/training_&_resources/Measurement_articles/How_flat_is_flat.pdf

    Best,
    James Arthur
     
  14. James Arthur

    James Arthur Guest

    Homer's blueing idea with a machinist's "surface plate" is a pretty
    good gauge of flatness, but even a simple straight edge will reveal
    deviations from flatness on the order of 2-3um.

    http://msl.irl.cri.nz/training_&_resources/Measurement_articles/How_flat_is_flat.pdf

    Best,
    James Arthur
     
  15. James Arthur

    James Arthur Guest

    Homer's blueing idea with a machinist's "surface plate" is a pretty
    good gauge of flatness, but even a simple straight edge will reveal
    deviations from flatness on the order of 2-3um.

    http://msl.irl.cri.nz/training_&_resources/Measurement_articles/How_flat_is_flat.pdf

    Best,
    James Arthur
     
  16. James Arthur

    James Arthur Guest

    Homer's blueing idea with a machinist's "surface plate" is a pretty
    good gauge of flatness, but even a simple straight edge will reveal
    deviations from flatness on the order of 2-3um.

    http://msl.irl.cri.nz/training_&_resources/Measurement_articles/How_flat_is_flat.pdf

    Best,
    James Arthur
     
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