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PCBs correct first time?

Discussion in 'Electronic Design' started by Leon Heller, Nov 25, 2003.

  1. Leon Heller

    Leon Heller Guest

    No one in my experience (especially me) ever seems to get PCBs of any
    complexity designed correctly straight off, there's always at least one
    error on the prototype. They are usually OK the second time round, of
    course. How general is this observation?

    I've got a feeling that thorough checking of the prototype PCB design
    probably costs more ultimately than just getting it made and fixing any
    problems subsequently.

    Leon
     
  2. Pretty true.
    However the reasons nowadays, are not usually through lack of checking the
    design. Typical problems would be unexpected noise (even the best simulators
    still can't model things exactly like the real world), or a component that
    displays tolerances in production quantities that were not displayed on the
    original samples.
    A couple of recent boards I did (reasonable high density, four layer double
    eurocard size), had one problem on the first, and two on the second. One
    problem in each case was from the same chip (which was a thermal problem
    when built in the final casing), and the other was was an induced noise
    problem from a motor mounted adjacent to the board. Finding these, is what
    prototypes are all about.
    What has basically 'vanished', in the last few years, are the
    'transcription' errors between circuits and the final board, and 'silly'
    faults like tracks crossing one another. Rats nets checkers, and basic
    design rules, fix these. :)

    Best Wishes
     
  3. I think you meant "Designs correct first time" - As a PCB usually is
    checked against the netlist of the schematic, and mechanical
    design-rules.
    So if it isn't correct it may be a "design error" or some "mechanical
    error" or some "physics" (RF, etc) design error not covered by netlist
    verification. But this also could be described with and checked against
    design rules.

    From the projects I do I would say that nearly no prototype is correct
    the first time. And even if there is a prototype doing all things
    required by the customer the first time I will take some time to play
    around with it for "optimization". This helps keeping the "Models" in my
    Brain in sync with the prototypes.
    And often the confrontation with the Prototype brings some new
    ideas/requirements to the customers mind.

    But it also depends on how "new" each Project is. If it's like building
    a car already built, but painted in a new color, then chances for a
    first time correct design are very high.

    But if your "social succes" is measured by your "first time success
    rate" I would suggest learning strategies like.

    - justify errors so that they are no errors anymore.
    - avoiding doing something new, but concentrating at suggesting others
    that it is new
    - rhetorically blame others for your blatant errors.

    Raymund Hofmann
     
  4. nospam

    nospam Guest

    If you can't get it right first time when you try, just how bad is it going
    to be when you don't bother trying?
     
  5. Only a *very* small PCB has a chance of being 100% correct
    or at least useable. There are always little things that
    go wrong. New footprints that are wrong, little mistakes
    in the schematic.

    I check with automated design rule checking, that doesn't
    cost much.

    Even the second proto often has a few minor things that
    can be improved. Like the silkscreen layout, for instance.
     
  6. What do you mean by 'one error'? If this is the layout not matching
    the schematic, than the person simply doesn't know how to use their
    CAD package - all modern CAD can check layout against the netlist
    better than any human, and the post-processing can easily be verified
    by numerous viewers (like GerbTool in OrCAD for example). Of course, a
    responsible designer should never trust somebody else's footprints
    (including the ones bundled with the CAD package, they are often the
    worst).
    If, on the other hand, it is the schematic that needs correction,
    that's (partly) what prototyping is for.
    Regards,
    Eugene
     
  7. budgie

    budgie Guest

    Many replies posted here rely on DRC and netlists to ensure correct
    translation from schematic to PCB layout. As one who uses Protel
    Schematic and AutoTrax under DOS (remember DOS?), I rely on the Mk1
    eyeball for all checking - along with checklist procedures I developed
    yeras back.

    The last commercial design I did went straight from PCB file to
    production. There was ONE error, due to wrong pinout information for
    a concentric DC input receptacle. Fortunately, it only required a
    bridge between two adjacent lands to fix. The client was so
    unconcerned that he ran several more batches through production before
    even bothering to have the artwork revised. Needless to say, the
    Rev:1 board had 0 errors.

    IMOE the source of many problems is wrong pinouts or footprints in
    libraries, or in published data sheets.
     
  8. EEng

    EEng Guest

    There are many options today that didn't exist even 10 years ago. SMT
    protoboards are great for testing some designs, and breadboarding
    isn't expected to be actual finished size. As far as BGAs and other
    parts of like ilk, most companies have adapters to allow them to be
    breadboarded, you have but to ask. Remember that breadboarding
    doesn't prove PCB layout, it only proves schematic design. After
    that, PCB layout is only as good as the person doing the layout, and
    anyone with experience knows about EMI, thermal dynamics, etc.....
    Good layout technique cannot really be taught; it is learned by
    experience.

    Anyone that designs by breadboarding is not worthy of the position.
    Breadboarding is for proof of concept, NOT as a design method.
     
  9. In budgie typed:
    Spell checkers do have their uses though. ;-)
     
  10. budgie

    budgie Guest

    Fortunately, that was the result of sloppy typing, not dyslexia ;-)
     
  11. Leon Heller

    Leon Heller Guest

    Yes, that's what I meant.

    Leon
     
  12. Ian Buckner

    Ian Buckner Guest

    Good call! - one that nearly got us was an assumption about which
    way was "up" between a motherboard power connector and the
    connector on our board at the other end of a wiring harness, with
    the two engineers sitting 6k miles apart.

    Regards
    Ian
     
  13. My PCB's are always correct the first time. Its always the environment that
    is wrong...Oh, a don't forget those pesky little quirks that the IC
    designers forget to mention in the data sheets
     
  14. In your dreams ;)
    It would be nice if that's the only problem.
     
  15. Boris Mohar

    Boris Mohar Guest

    Mounting holes? What mounting holes?
    Test points? What test points?
     
  16. I have yet to (knock on wood) have even a prototype come back that was
    not shippable with a bit of rework. I've even ordered 1,000+ fairly
    expensive boards without a prototype. This may change as the cost of
    prototypes has dropped so much that it may be cheaper to spend less
    time checking, but I don't really think so. The issues like mechanical
    dimensions, 3-D fit, and so on are easier to check with laser prints
    and spray adhesive to laminate test bits. Footprints on parts that I
    don't have samples in hand are a nasty issue, but I try to always have
    samples around to lay on top of the laser print (crude, but effective
    check). Then there are possible schematic issues, but that's nothing
    to do with the layout. It's sometimes good to highlight the power nets
    on the screen, both to check they are going to all the places you
    expect, and to make the way the current will flow easier to visualize.

    I expect the prototype to be about as good as the first production
    units used to be, and then the actual production parts ought to be
    close to perfect. Sometimes I don't bother fixing stuff like silk
    screen markings over vias until production, but that risks
    accidentally getting them over SMT or other pads (but I run two
    independent DRC checks to catch most of that stuff). Of course if you
    made the footprint wrong, the padstacks wrong, or selected the wrong
    one...

    Best regards,
    Spehro Pefhany
     
  17. But you are a brilliant guy, Spehro! I always **** up something. My
    latest screw-up was not having a footprint for a PLCC44 PTH socket.
    Couldn't find one with google. I had a PLCC32 PTH socket, and modified
    it, turning it into a PLCC44 socket. Of course, with pin 1&2 starting
    in the wrong position. At the moment of creating that thing, I was
    sure I got it right. I could use the board for all other prototype
    testing, with a socket dangling of the board, with 44 wires. You
    don't want 1000 boards of that ;)

    The trouble is, it all looks so bloody nice on the screen. Zero
    complaints from the DRC. And you upload the files, happy as a
    monkey with 7 dicks.
     
  18. No I just put lots of time into checking and fixing what gets
    forgotten. It works better if you can set it aside and look at it
    later, or get someone else to check it. It's almost always something
    really stupid and obvious, like using the wrong width for a SOIC. But
    it's probably only a matter of time (knock on wood).
    I'd probably go back and check that about 4 times. Those thru-hole
    PLCC sockets have a really irritating pin pattern (I remember making
    an 84 or 68 footprint). Then there are relays and oddball connectors
    with metric dimensions, asymmetrical layout, pinout numbering
    including various phantom pins, and the drawing given from the
    *bottom*. Yuk.
    That's when you get the boards right out in the trash so you don't
    have to be reminded of the monumental screw up.
    Hahah. I haven't heard that expression in years. Then there was the
    guy with only five- but his underwear fit like a glove.

    Best regards,
    Spehro Pefhany
     

  19. I just did a board with only a couple of minor errors:

    o A 176 pin QFP with 0.5mm pin pitch. Except it should have been
    0.4mm.

    o A flash chip with all the address lines off-by-one.

    All in all, not my best work...
     
  20. Hahaha, I once used the PLCC schematic symbol for a dualported
    ram, 7C136, and used the PQFP footprint. Pin1 starts on the corner,
    not in the middle. I ditched it right away, about 5 minutes after
    that first and exciting 'power-on' event.

    You're not alone ;)
     
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