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ESD protection

Discussion in 'Electronic Design' started by [email protected], Nov 23, 2008.

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  1. Guest

    I'm having trouble understanding ESD protection. I hope somebody can
    help.
    Should an anti-static mat on a lab bench be connected directly to
    ground or connected to ground through a 1Mohm resistor?
    What is an anti-static mat made out of? Is it conductive? If so then
    why don't I measure any resistance if I poke ohmmeter probes into two
    spots on the mat?
    Do I need to have an anti-static mat on the floor around my lab bench?
    If I have a charge on me do I have to touch a circuit board to damage
    it or can the charge jump through the air?
    How do I safely move a circuit board from one lab bench to another lab
    bench? (I see people carrying circuit boards around my lab by the
    edges of the board with their bare hands and they think by not
    touching any IC's with their hands that the board is safe.)
    I'm often out in the field working on electronics and I have no ground
    strap. Should I touch a ground point on a metal object first before
    handling a circuit board, to remove any charge on myself?
    Thanks for any help.
     
  2. The mat direct to ground is fine as the mat has an inherently higher
    resistance. It is your wrist strap cord that usually has one integrated
    into it.

    Many times, a carbon impregnated material. These days there are
    several types, and most are multi-laminar in nature.
    Yes... well... it has a high resistance, so it conducts "slowly".
    Because the resistance is so high, you need a larger surface area of
    contact than the probe tips provide. A couple of 3 inch foil patches
    should allow you to see some motion (get a reading).

    It is not required. If you are doing High Voltage work, it is not
    recommended at all.

    When you are in an elevated condition, you also carry a field. That is
    why we wear smocks. They contain any field you would otherwise radiate.
    I ALWAYS touch grounded structures before I touch a PCB or approach an
    ESD work station with open assemblies on it.

    In an ESD shielded container, such as a metallized or carbon black
    dissipative bag WITH the top folded over. This can also be toted around
    inside an ESD tote box with a lid on it, especially for longer hauls.
    If you were talking about power supplies, I'd say the parts are pretty
    hardy.

    Since you are likely referring to delicate digital circuit assemblies, I
    would be sure that everyone involved takes every precaution possible.
    Such failures do not immediately show themselves. Sometimes they take
    time to propagate a failure mode.

    With a PC as an example, I would:

    Keep the chassis plugged in, as this keeps things grounded, and gives
    you a place to be grounded as well.

    Touch the chassis to equalize you body charge to the chassis. This
    should also be ground, but equality is more important. NOW you are able
    to touch circuit assemblies, etc.
    Yes. If you use the chassis for the device in question, you bring
    yourself to its level, regardless of where ground is, though the two are
    typically equal.

    You're an idiot.

    Conductor: WILL dissipate a electrostatic charge.

    Insulator: Will NOT dissipate an electrostatic charge.

    You were saying?
    Only if one intends to energize circuit card assemblies at their ESD
    safe workstation. It also depends greatly on the voltage in question.
    Low voltage DC circuits have no problem with ESD mats.
    That is not the reason for it. It is there so that a charge does not
    get dissipated to fast, which can cause damage to a circuit element as
    well.
    It doesn't mean a damned thing if he does not have on a heel strap.
    Anyone around and ESD workstation should ALSO have an ESD smock on as
    well.
    Three minimums for an ESD workstation.

    Bench mat.

    ESD Smock,

    and to be grounded.

    The best smocks have the cord integrated into them, and you ground that,
    and there is no wrist strap.

    Recommended:

    Ionizer.
    If you are causing zaps, you shouldn't be anywhere near an electronics
    lab, and said lab should look into the humidity conditions.
    I am done teaching... You're a goddamned retard!
    No shit.
    Pink are useless completely in low humidity.
    Nice logic, but flawed... kind of like the old "duck and cover"
    instruction for what to do if you see a nuclear blast.
     
  3. Guest

    So the 1Mohm resistor is only for protection against me getting
    shocked? If there is no dangerous voltage around me or I'm just
    working with 5V electronics then I can be connected directly to
    ground?

    When I'm carrying a board in an antistatic bag does it matter if the
    bag is open, or does the bag need to be closed?

    If I step on a grounded mat, any charge on me will flow through the
    rubber soles of my shoes? Isn't rubber very resistive?
     
  4. Guest

    Never mind that last message... the questions are answered in post
    #3. Thanks for all of that valuable information mook.
     
  5. The mats, straps, smocks, etc and the instructions governing their
    installation and use are designed to protect the electronics equipment
    and personnel from damage, even if the people don't why they are doing
    it. For people that do know why, they eliminate the need to think
    about it.
     
  6. Eeyore

    Eeyore Guest

    Yes it does. Dependes what you define as 'conductive'. ;~)

    But the volume resisivity is usually so enormous, I don't recall any such
    problem. Rememeber it's not zero ohms, it's in the megohms or tens or 100s of
    same. Enought to discharge a few nanocoulombs but high enough not to interfere
    with operation. Do check the data sheet though. They do come in various grades.

    Graham
     
  7. Eeyore

    Eeyore Guest

    Depends what's on the floor, what clothes you wear (try ALL pure cotton),
    your shoes etc. You will acquire some charge simply by walking.You can get
    ESD safe carpet, it has conductors woven into it, or spray the carpet with
    ESD safe spray every so often. Some say dilute detergent does the job as
    well but I stick with the Electrolube et all product in a ready to spray
    dispenser.

    Graham
     
  8. Eeyore

    Eeyore Guest

    As long as the equipment is Class II !

    The board needs to be totally enclosed in the bag but the zipper is
    probably not that important.

    That's why you have wall mounting grounding plates you touch with your
    hands.

    Graham
     
  9. Rich Grise

    Rich Grise Guest

    Howcome none of you googlies seem to be able^H^H^H^Hwilling to use the
    other side of google?
    http://www.google.com/search?hl=en&q=Anti-static+++mats&btnG=Google+Search&aq=f&oq=

    Hope This Helps!
    Rich
     
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