ESD question

Discussion in 'A+ Certification' started by Tuna Sunrise, Jan 5, 2004.

  1. Tuna Sunrise

    Tuna Sunrise Guest

    Hello

    This is a question I was asked, and haven't been able to come up with
    an answer for - hope someone can help. Maybe I'm wrong in my
    understanding of something. If so, please correct me.


    If you set up an ESD workstation, you basically equalise the charge of
    the electrons on you and on your equipment.

    An anti-static bag acts as a Faraday cage, keeping the charged
    electrons on the outside of the bag.

    If you, connected to your workstation, take a component out of it's
    anti-static bag, is the level of charge on the component different to
    the level of charge on you and the bag? If so, whats to prevent you
    zapping the component, despite the ESD workstation?


    Thanks for your help,

    Tuna Sunrise
     
    Tuna Sunrise, Jan 5, 2004
    #1
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  2. Tuna Sunrise

    David Hough Guest

    Lets look at this a little differently. If you are grounded with the
    connected wrist strap and holding the bag, then you are both at ground
    level. The only possibility of a differential would be if there is a
    charge built up on the chip inside. Presuming the person who packaged
    the chip was also grounded, the only possibily of a differential would
    be a difference in the shippers ground and your ground. Any possible
    difference would have to be nil, and the components are manufactured
    with a piv of 50 volts. I would say it is safe as long as you are on the
    same planet(same ground).
     
    David Hough, Jan 5, 2004
    #2
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  3. Tuna Sunrise

    Tuna Sunrise Guest

    That makes sense, I never thought of it like that - thanks a lot.
     
    Tuna Sunrise, Jan 5, 2004
    #3
  4. Tuna Sunrise

    Tuna Sunrise Guest

    On Tue, 06 Jan 2004 01:40:22 GMT, Barry Watzman
    <> wrote:

    >The anti-static bag acts more like a bag made out of aluminum foil than
    >like your description, which seems to suggest inside and outside
    >surfaces insulated from each other.


    No, I understand this - my description wasn't very clear I think.

    >

    <snip>

    >
    >A semicinductor device is only damaged by static current passing THROUGH
    >the device, where the device is simultaneously in contact with two
    >surfaces (one of them presumably being you) that are at different
    >potentials, with the device in question being the "connection" between
    >them. This can happen when you (at potential "X") insert the device
    >into a circuit board (at potential "Y") where you are holding one end of
    >the chip and the other end touches the circuit board (but if you touched
    >the circuit board first, then no problem)


    Are you saying that it would be completely OK to handle the component
    with complete disregard for ESD while it is not in connection with the
    board, and it is not until I insert the device into the board that the
    static discharges, frying the component in it's path?

    .. There is almost no way to
    >damage a device in an ESD bag, but it's not absolutely impossible. Take
    >this example: Bag lying on grounded surface, you walk across the carpet
    >and then, having raised yourself to 2500 volts:
    >
    >1. Get the device out of the bag. No problem, you have to touch the
    >bag first.
    >
    >2. Pick up the bag with your "insulated glove" left hand, and dump the
    >device into your right hand. Still no problem. True, the device is at
    >0 volts when it contacts your hand at 2500 volts, but it's in mid-air
    >and not touching anything else.


    Is this on the same principle as a bird landing on electric cabling?
    Because there is no connection to ground, there is no discharge?

    >
    >3. The device is sticking out of the bag, part inside, and part
    >outside. You touch a part of the device sticking out. Now THAT can
    >cause a "ZAP" and you can ruin the device, because the current passes
    >from you, THROUGH the device, to the grounded bag.
    >


    As opposed to here, where the component itself provides the link to
    ground?

    I'm pretty sure I understand , but I just want to be clear before I go
    passing on spurious information to anyone else.

    Thanks very much for your help.

    Tuna
     
    Tuna Sunrise, Jan 6, 2004
    #4
  5. Tuna Sunrise

    Tuna Sunrise Guest

    On Tue, 06 Jan 2004 03:18:09 GMT, Barry Watzman
    <> wrote:

    >Re: "Are you saying that it would be completely OK to handle the
    >component with complete disregard for ESD while it is not in connection
    >with the board, and it is not until I insert the device into the board
    >that the static discharges, frying the component in it's path?"
    >
    >Yes, sort of. Tell me, just HOW would you "handle the component with
    >complete disregard for ESD while it is not in connection with {ANYTHING
    >else (doesn't necessarily have to be "the board")}"



    :eek:) I've always been very careful when handling components, and made
    sure I was all strapped up - I think maybe I was a little
    over-cautious.

    >

    <snip>

    >
    >Also, your bird analogy is a good one. For the device to be damaged,
    >there has to be a "circuit" from a point at one potential to a point at
    >some other potential.



    I always understood the importance of taking anti-static precautions
    and the amount of damage ESD can do, but I never really grasped the
    plot completely - Thank you for your explanation.

    While we're on the subject, how effective are the wrist band and wire
    things with no mat? They always semed a bit lame to me.
     
    Tuna Sunrise, Jan 7, 2004
    #5
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