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ESD discharge while handling

  1. Mar 13, 2015 #1
    What are the conditions when a ESD discharge occurs, in particular while handling components and assembled units. Can someone explain ESD discharge in the following cases -

    Let's say you are handling an IC, moving it around for soldering etc.
    What is the chance of the handler zapping the IC and under what conditions. There should be a potential difference between you and the IC. If you are just carrying the IC in your hand, where does the potential difference come from?

    Let's say you are handling an unpowered assembled unit (PCBA or a small product like mouse). Where does the potential difference come from for an ESD discharge to occur from your body to the product.

    Is a Metal object good enough for ESD discharge. There is discharge all the time when you touch a door knob or when you try to open the car door. Neither of these are connected to ground.
     
  2. jcsd
  3. Mar 13, 2015 #2
    The potential difference comes from static charge built up by various materials rubbing together.

    The static built up on your body can do damage to electrical components even whey you can't feel the discharge. And the damage is often unnoticeable, but it's cumulative.

    The only safe way to handle electronic components is with a grounded wriststrap (or other grounding mechanism) always connected. It also helps to avoid low humidity and to wear more cotton and less synthetic clothing.
     
  4. Mar 15, 2015 #3

    jim hardy

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    Well, really they are even if through tens of millions of ohms.
    Tens of kilovolts of static electricity will push a milliamp through tens of millions of ohms, and you can feel a milliamp.

    As KenJackson said -- something to discharge that static electricity before you touch the part is the "ounce of prevention".
    I never had any trouble is South Florida where humidity is always high.
    But in Canada or Idaho in winter one needs a conductive mat on his workbench, and a wrist-strap connecting him to it so no static potential difference can be generated.

    Lots of folks dont know this but those wrist straps have a series 1 megohm resistor so that accidental contact with a power source won't pass lethal current to ground through you and the wrist strap. If you're home-brewing one for your workbench keep that in mind .

    Anti static carpet has a few just barely conductive fibers woven into it to do the same function as a wrist strap.
    Anti static workbench mats are not very conductive so that accidental connection to power won't shoot sparks or shock you.

    Static electricity can be discharged by meg-ohms, you don't need or want milli-ohms for reasons above.

    hope above helps you with the basic concepts.

    old jim
     
  5. Mar 15, 2015 #4

    vk6kro

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    You can regard a car as a big capacitor.
    It has a big lump of metal above ground, which is a conductor.
    The tyres and air under the car are dielectric material.

    So if this capacitor charges up, it can discharge through a conductor, even if the tyres are nonconducting.

    Likewise, a person with very well insulated shoes can become another capacitor.
    Placing two capacitors in parallel still produces a current if the capacitors are charged to different voltages.

    This seems to be less of a problem than it used to be. Maybe modern tyres are made slightly conductive to avoid charge buildup.
     
  6. Mar 15, 2015 #5

    jim hardy

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    The carbon lampblack used to make tires black also makes them very slightly conductive. I figure that's why i get shocked exiting the car just after driving it to the store, but not when i come back a half hour later. The substantial capacitance of the automobile has had time to discharge through the meager conductivity of the tires.

    That's my theory.

    My 1949 Buick shop manual suggested placing a little bit of loose graphite powder inside the tires to reduce radio interference from static electricity.
    Whether there was any scientific basis for that i don't know, but in those days car radios had excellent sensitivity. So it's plausible.

    old jim
     
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