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Reverse polarity, Scotty.

Discussion in 'Electronic Design' started by Phil Hobbs, Sep 9, 2013.

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  1. Phil Hobbs

    Phil Hobbs Guest

    Hi all,

    Because of a miscommunication with a client's layout person, I find
    myself in need of a series-connected dual Schottky diode in SC70 whose
    pinout is opposite to the BAV54SW, i.e. I need something like this:


    *------*
    K ==| |
    | |== COM
    A ==| |
    *------*

    It'll be used at 20 mA, 12V, no inductive kick. I can use the BAV99RW,
    but that'll cost me a volt's worth of drop, which I'd rather avoid. Any
    suggestions?

    Thanks

    Phil Hobbs
    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC
    Optics, Electro-optics, Photonics, Analog Electronics

    160 North State Road #203
    Briarcliff Manor NY 10510

    hobbs at electrooptical dot net
    http://electrooptical.net
     
  2. RobertMacy

    RobertMacy Guest


    If you can stand the overhead, put the BAV54SW on a little 'sub' PCB and
    stick that down.
     
  3. Joerg

    Joerg Guest

    Before time runs out too far I'd consider a re-layout. The only series
    Schottky I ever saw in an "oops" package was SOT23, and that was a while
    ago and an RF part that IIRC didn't take 12V.

    To get product tested and maybe even into the field (via deviation) you
    could have them hand-soldered reversed for cathode/anode, sort of
    standing at a 45 degree angle, and then have COM handwired.
     
  4. an SC70 is 2x2mm if there's room for a daughter board there's room for two discrete diodes

    simple fix is like Jan suggested; mount it upside down,
    might not even need to bend the legs, just be little generous with the solder

    -Lasse
     
  5. Joerg

    Joerg Guest

    If you have to make a board anyhow, why not fix the real McCoy instead?
    It's less effort.

    But before that, tear off a page from the "anger pad", crumple, and
    chuck it into a corner of the room, with gusto :)

    http://www.flickr.com/photos/[email protected]/2714686795/
    http://www.werswissenwill.de/images/wutzettel_360.gif

    For some reason I've never seen those in the US. I remember when we told
    our mechanical engineer at my first company that we had a "slight"
    change in the mold for a plastic part and at that point he really needed
    this pad.
     
  6. Phil Hobbs

    Phil Hobbs Guest

    Thanks, I didn't see any either. However, happily it looks like the
    BAV99RW will work okay. I may need to make the 47 uF cap a little
    bigger to compensate for the extra voltage drop, but oh well.

    If there needs to be a spin for other reasons we'll go back to the
    BAT54Ses. Right now we have to figure out how to get the board clean
    enough. I really miss PbSn!

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC
    Optics, Electro-optics, Photonics, Analog Electronics

    160 North State Road #203
    Briarcliff Manor NY 10510

    hobbs at electrooptical dot net
    http://electrooptical.net
     
  7. Joerg

    Joerg Guest

    A RoHS board? My condolences. If it's any comfort I'll have one of those
    coming up in fall 2014. The bureaucrats in Brussels took away the
    exemption for medical. Hurumph. Grumble.

    Make sure there is a liability waiver in your agreement for when the
    lead-free stuff fails in the field.
     
  8. Joerg

    Joerg Guest

    ANGER SHEET .. Crumple in case of anger attack and chuck into a corner.

    (in German a corner in this context means the corner of a room)

    I just had one with a 2-pad SMT diode reversed because the layouter
    scooted the "C" mark to far. And here I thought they'd always go by the
    CAD file. Oh well, happens.
     
  9. Guest

    At a PPoE the layout guy was always getting diodes backwards. He
    ASSUMEd that pin AO1 was the cathode. "It's IPC standards."
    Unfortunately, not all diode manufacturers got the memo. We
    eventually solved the problem by using 'C' and 'A' for the library pin
    names (for arrays that becomes "C01" and "A01").
     
  10. Guest

    I've been using socialist solder processes for five years. After they
    got the paste/stencil and temperature profiles right, I haven't seen
    any problems. Nothing from the field.
     
  11. Joerg

    Joerg Guest

    Five years in the field is not a long time. It also depends on how it is
    used, harsh environments, vibrations and so on. It's not just issues
    that can develop with IC packages, also with passives:

    http://www.calce.umd.edu/articles/openart/Page_2_Flex_Cracking.pdf
     
  12. Guest

  13. Nico Coesel

    Nico Coesel Guest

    Make sure they solder it in a nitrogen atmosphere. The joints will
    look just as good as with leaded solder.
     
  14. Can you put pads both top and bottom? Maybe only 10% compression on each side? ([email protected] the expense and full steam ahead.)

    George H.
     
  15. I've never had any luck bending the leads on those little SMT
    packages- they tend to break right off. I'd stick it on upside down
    with blobs of solder re-inforced with little single strands from
    stranded wire, if necessary. Okay for up to a few dozen pieces, with
    some small loss in dignity, reliability etc.

    Best regards,
    Spehro Pefhany
     
  16. Guest

    That does nothing. The schematic already has the anode and cathode
    clearly marked. The real problem is that the pick-n-place machines
    can't read the schematics, only the footprint.
     
  17. Guest

    I've seen leads pulled off components but no failed solder joints. The
    gloom and doom from five years ago was that commie solder wouldn't
    make a year without tin whiskers. None so far.
     
  18. Joerg

    Joerg Guest


    They should be reading the CAD file. Older ones needed a XYRS file for that.
     
  19. Oh my.. tell me about it! (no thermal instincts)
    I keep trying to 'wash my hands' of the current project.
    (I worry a bit that it'll get me fired.)
    At the heart is a LN2 cryostat. My boss thinks more mass is better, 'causethen the temperature will be stable... As it stands now the probe must have several pounds of copper. Byzantine layers are added to try and correct a 'flawed' design. I guesstimated a time constant of ~1000 seconds with 100 Watts of heater power for one internal structure. When I try and talk about low mass, fast designs, it's like I'm speaking a foreign language.

    George H.
     
  20. Joerg

    Joerg Guest

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