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PCBs correct first time?

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

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  1. Greg Neff

    Greg Neff Guest

    These days our PCBs tend to be on the complex side. We do
    double-sided SMD, BGAs, fast signals, 16-layer construction, >800
    components, etc. We usually have to spin the board a second time.
    Occasionally a third time, but not too often (if we get bit at this
    point it's usually due to a signal integrity or ESD/EMI issue). We
    get close because we carefully control the design process:

    1) We have a library of verified footprints. If we need a new
    footprint then it is made and checked against a mechanical sample
    prior to it being used in a design.

    2) Our schematic libraries include every part number in our part
    database. Each part in a library includes the part value,
    manufacturer, manufacturer's part number, our part number, and
    footprint. Yes, this means that we have a library part for each value
    and size of resistor we use. The generation of the parts in the
    schematic libraries is where most of our mistakes are made.

    3) A schematic is entered that includes only mechanically critical
    components. A PCB is designed with these parts placed as per the
    specification. A copper-clad piece of FR4 is drilled and routed by
    our PCB house. The mechanically critical parts are soldered and/or
    glued to this board. This mechanical sample PCB allows verification
    of form and fit in the target equipment. It also allows the
    mechanical guys to work on the enclosure and wiring harness design.

    4) A complete schematic is entered and run through DRC. The schematic
    is peer-reviewed. A human-readable netlist (wirelist) is generated.
    You would be surprised how many errors can be detected by reviewing
    this wirelist.

    5) A PCB netlist is generated. The netlist is imported into the PCB
    tool.

    6) The PCB layout is done, with several reviews along the way by the
    design engineer. Any changes required at this stage are carefully
    forward or back annotated as required.

    7) A BOM is generated from the schematic, and a kit of parts is pulled
    to allow building of two or three prototypes.

    8) We build the prototype, and then the fun begins...

    Breadboarding is a thing of the past for digital designs. It simply
    won't work. The last wire-wrap prototype that we did was about 15
    years ago.


    ================================

    Greg Neff
    VP Engineering
    *Microsym* Computers Inc.
     
  2. Well this bit is cheating!
    Interesting; I think I will try this idea.
     
  3. John Larkin

    John Larkin Guest

    I'd guess that we've had two or three throw-away rev A boards in our
    20-year company history. We expect to be able to manufacture and sell
    the first board, and we formally release it to manufacturing as rev A.
    They build it, they inspect it, we test it, and we are embarassed if
    anything serious is wrong.

    We don't breadboard entire designs. Once in a while we breadboard one
    specific circuit, generally because the electromagnetics is too
    complex to simulate, or because a part isn't characterized well
    enough. If an engineer can't design the product architecture right
    without a breadboard, we just fire him.

    Correct-the-first-time is a matter of policy and culture, and I try to
    encourage it here. Most boards do get revved past 'A', to add
    features, replace obsolete parts, or improve manufacturability, but
    it's seldom under duress.

    John
     
  4. John Larkin

    John Larkin Guest



    See my post to a.b.s.e. Luckily, I can fix this with a diode or
    something.

    John
     
  5. Nah, you have to do better, to win this 'most horrible PCB blooper
    contest'.

    ;)

    Wish I had a digital camera some 20 years ago...
     
  6. Brian

    Brian Guest

    I breadboard everything.... on a prototype pcb:)

    I may test parts of a design, but the first "whole show" test is done on a
    prototype pcb. I hardly bother checking this first board for layout mistakes
    in PADS. I use this board as a tool to shake them out. Then I'lll run a
    second proto, hopefully w/o errors to check the circuit and layout. 3rd time
    I send artwork, its for real.

    With SUPER cheap protos available fast and the fact I am always working
    multiple projects, this is most time efficient for me. Too time consuming to
    check that first board for my own stupid mistakes, and many times a databook
    error sneaks in anyway.


    http://www.speff.com
     
  7. Brian

    Brian Guest

    Don't feel bad... the guys here claiming to go from concept to production
    with one layout are:

    A) Designing simple toasters
    B) Have ESP and can predict data book errors
    C) Spend the time doing everything we find on a proto on paper (holding
    parts up to monitor to verify footprints, heehehe), just spending 25 times
    the time we need
    D) Do only one design every 3 years
    E) Lying

    Brian
     
  8. You forgot F) And when there was a stupid mistake, it was made
    by a co-worker.

    ;-)
     
  9. John Larkin

    John Larkin Guest

    F) Disciplined.

    G) Smart.

    H) Profitable. Very profitable.

    John
     
  10. Mike Page

    Mike Page Guest

    "Thorough checking" is a culture thing. You have to have a certain
    expectation of correctness, because there are hardware engineers whose
    mantra is "this isn't the final board, so it doesn't matter".

    Missing a deadline or an expected performance level has systemic costs
    that aren't always obvious, and usually hurt the whole team.
     
  11. Mike

    Mike Guest

    or G) A client who has an attack of featureitis and decides to move
    the goalposts in the middle of the prototype phase.
     
  12. I screwed up my first ever production pcb by designing a VGA connector
    upside down (if that makes sense). Didn't read the data sheet properly.
    Since then I have never had a wrong footprint on the PCB. What I do now is
    print out the overlay with the holes on a decent laser printer and stick the
    paper to a piece of polystyrene foam. I then insert every through hole
    component into the overlay and lay every single SM part ontop.
     
  13. Brian

    Brian Guest

    Thats covered by E)
     
  14. Brian

    Brian Guest

    John,

    Is toaster control that profitable?

    I'm betting you fall under E)
     
  15. Brian

    Brian Guest

    I find protos to be cheaper than thorough checking many times. Unless I have
    breadboarded the entire design, I often don't find certain errors until the
    1st proto is made. These include data book errors, misinterpretation of
    data, and wrong assumptions. Often, this first proto also leads to new ideas
    for the design (better performance, lower cost) and change is needed on the
    pcb. If you aren't finding that you often want to change the way you did
    something on your initial design, chances are you have put out a product
    that isn't as good as it could be. Occassionally I even rip 2 protos with
    differences to similar circuits just to see whats better (Simulation still
    sucks). With protos being so EXTREMELY cheap, I can't see why not to use
    them this way. Spend 100 bucks on a proto to save $1 on every unit, heck ya!
     
  16. Well, we are all just kidding a bit I guess, but when
    I see the pictures of John's work, I rank myself as
    'toaster-business' category.

    I never did anything > 4 layers and with perhaps $75
    worth of components on it. And 95% has been 2 layers
    anyway. At those levels, a proto does not cost much.

    But the point was made, and there is some truth in
    it, for some of us.

    [snip]
     
  17. budgie

    budgie Guest

    I glue the laser printout to cardboard (cereal box) and "drill" all
    the required holes with a special pin. To avoid static risk, I use
    already-dead IC's of the various pinouts. It's a straightforward way
    to ensure mechanical fit, and often shakes out a problem or two before
    boards are fabricated.
     
  18. budgie

    budgie Guest

    you left out the most important part ....

    OR

    F) brought up in a culture of checking things thoroughly. Goes with
    the idea of accepting responsibility for one's own ****-ups, and the
    old-fashioned notion that it pays to do it right once rather than
    half-right a number of times.
     
  19. I guess you never have software problems either.
     
  20. Seems most of us use somewhat similar independently developed methods
    to catch as many errors as possible as early as possible.

    Does anyone else edit the NC drill G-code files manually to fit the
    proto house's standard rack?

    Best regards,
    Spehro Pefhany
     
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