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PCB footprint insertion origin

Discussion in 'Electronic Design' started by Michael, Feb 25, 2008.

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  1. Michael

    Michael Guest

    There are two reference points when a footprint is created: insertion
    origin and "datum" (0,0).
    There are two "special" points in a footprint: pin one center and
    component center (not always clearly defined).
    Is component center preferred insertion origin and datum location?
    What are the pros and cons of alternatives?
    Any relevant comment will be appreciated.
    Thank you!
    P.S.: we are using ORCAD layout and will move to ORCAD PCB editor
  2. Guest

    Are you referring to how the tool will generate a pick and place file?
    It doesn't matter, eventually your assembler will translate to
    whatever he wants for his particular tool.
    Anyways, most PCB tools let you choose to what point the cursor sticks
    to when placing on-screen, you can usually choose from centroid, 0,0,
    pin 1 or user select (which can be any pin# or a x,y).
  3. Probably depends on the component. I might want to place the center of
    a coaxial jack on a precise center line, but not care much where (and
    whether they were off-grid) the hard metric pin locations turn up on
    my inch standard board. OTOH, for a part that does not have some
    mechanical relationship with the outside world other than through the
    padstacks, probably you'd rather use pin 1 or something like that.
    Best regards,
    Spehro Pefhany
  4. donald

    donald Guest

    You don't solder a part down from the center of the silkscreen.

    All parts should reference a real copper "foot print", not painted art work.

    My .02

  5. Michael

    Michael Guest

    Th reason I posted this question: some time ago we were asked by
    assembly house to place origin and insertion origin in the middle (if
    my memory serves me right). Now we are trying to clean up the
    libraries and I started having doubts.....
  6. Look at the IPC requirements. Most assembly shops use this standard.
  7. donald

    donald Guest

    I have goggled for "IPC requirements", but I still do not understand
    what IPC means.

    Please elaborate a little more about IPC requirements.

    Thank you

  8. donald

    donald Guest

    Found it:

    Institute of Interconnecting and Packaging Electronic Circuitry

    I'll have to check this out.


  9. Have a look here...

    They have a std IPC-7531 land pattern calculator, SMT only but it should give you an idea....

  10. qrk

    qrk Guest

    In Layout, you want to place the datum at the center of the part. This
    is contrary to what Orcad wants you to do with the datum. Layout was
    developed in the days of parts on 50 mil pitches. This will prevent
    the component from being placed on a weird grid after you rotate it
    and/or place it on the opposite side if the pads aren't on a multiple
    of your placement grid. When the part gets placed on a weird grid, you
    need to pick up the part and place it down again to get the part to
    snap to the proper grid (you'll see the part jump to the new
    location). This is a very annoying bug in Layout. Placing the datum on
    the center of the part solves this issue. Layout, which was the
    Massteck(sp??) router of yore, had a lot of issues with coordinate
    translation and rotation. Back in the mid 90s, I got sick of moving
    Layout projects back to PCAD to give proper pick & place coordinates
    for bottom mounted parts. I figured out the translation/rotation math
    and gave it to the Layout manager so they could post a work around
    using Excel. They actually fixed the next release of the program. Boy,
    folks think Layout is a bear to use these days, you should have seen
    how horrible it was back then!

    Always put the pick & place point at the center of the part for a
    symmetrical part (like smt resistors). For odd parts where the body
    isn't centered on the pads, I'll put the pick & place point on the
    center of the body.
  11. Jim Thompson

    Jim Thompson Guest

    But you're still the dummy one.

    ...Jim Thompson
    | James E.Thompson, P.E. | mens |
    | Analog Innovations, Inc. | et |
    | Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
    | Phoenix, Arizona Voice:(480)460-2350 | |
    | E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
    | | 1962 |

    America: Land of the Tree Abusers, Because of the Bastards
  12. Joerg

    Joerg Guest

    Maybe that's why everyone was using Autotrax back then :)
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