My understanding of ESD is that there has to exist a path to ground for static charges to discharge. Take a MOSFET as an example that is vulnerable to ESD and packaged in a way to protect against it, as long as the component is free standing or none of the leads is connected to ground or a lower potential, I should be safe to hold it with my hand without taking any precautious. Essentially my hand and the transistor form an open circuit both sharing the same electric potential with no current. But it is a fact that touching sensitive components even when they are not powered or just being held by itself can cause damage through static discharge - for that I just can't understand. As if a plane can safely fly through a densely charged cloud (me holding a sensitive component), only until a lightning strikes through the fuselage on its way to reach ground (the component has a path to ground).


A drawing should help clarify my question. (A) A hand accumulates some static charges (5000V of static charges to ground). Assume the body is not properly grounded and the static charges on the hand have no path to go. A transistor has 0V or no static charges presence. (B) A finger comes in contact with the drain of a transistor. The drain is now 5000V above the source and goes beyond the maximum voltage rating. Thus the transistor is damaged. (C) All three terminals of the transistor come in contact with the hand at the same time. The drain, source and gate go from 0V to 5000V in parallel. There is no potential difference between the terminals and transistor is not damage static discharge. enter image description here

  • \$\begingroup\$ Please re-write this as a clear question. \$\endgroup\$
    – Drew
    May 21 '20 at 6:25
  • \$\begingroup\$ No, there does not need to be a ground anywhere. If something has a different charge than some other thing has, they have a potential difference, and when they touch, the charges move around to redistribute themselves to equalize the potential difference. \$\endgroup\$
    – Justme
    May 21 '20 at 9:18

The damaging happens when you touch the component. Just at that moment a spark can hit between your hand and component's leg. If that hasn't still happened and the component is inside your hand it's in safe. The next dangerous moment is when you let the component touch something. For ex. you keep it between your fingers and one of the legs touches something that you do not touch.

If it happens that you haven't an ESD safe workplace nor an ESD safe package and you must handle a component there, have at least a metallic platform for the component. Touch the platform at first when you take the component from it and touch it also before you leave the component on the platform. It's not fully safe because it's far too easy to have one second time gap between the touches. That's enough for you to generate a new potential difference for ex. by moving your feet on a plastic floor.

Components are often in ESD.safe bags which are said to be conductive. That can be true for the interior of the bags, but the outer surfaces can be of normal plastic. Thus placing a component in an ESD safe bag is effective if the component is placed inside and you touch the interior when you place or take a component.

BTW all semiconductor parts and circuit boards can be damaged by ESD. A component can stil work after ESD hits but it's operation can be less reliable. It's operating voltage range and temperature range can be reduced substantially. Handling a computer circuit board with poor ESD protection is a waterproof way to get a computer which either does not work at all or it suddenly stops and must be reset.

  • \$\begingroup\$ No I don't understand. For a damage to occur to a MOSFET for example, there has to be a sudden increase of charges (or voltage) controllably in one terminal vs. the other that leads to a current above its maximum rating. Imagine if my hand touches all three terminals of a MOSFET at the same time, there cannot be any potential difference between the terminals to induce any damaging current. Or if I first touch the black plastic of a MOSFET, the charges will evenly distribute to the entire transistor body including the terminals and neutralize it - that again ESD damage would not have occur. \$\endgroup\$
    – KMC
    May 23 '20 at 15:31
  • \$\begingroup\$ That's the same as I wrote. Let one leg touch something that you does not touch. You are in contact with yourself all the time, so let just one of the component legs be free and let it alone touch something charged and get your component damaged. \$\endgroup\$
    – user287001
    May 23 '20 at 15:37
  • \$\begingroup\$ So theoretically if I touch the plastic package or body of a semiconductor component before touching the terminals, then the charges will get neutralize and static damage won't happen? \$\endgroup\$
    – KMC
    May 23 '20 at 15:42
  • \$\begingroup\$ Nope. You must touch something conductive that is much bigger than the semiconductor parts of the component, say 100x larger dimensions AND has contact to the legs of the component. \$\endgroup\$
    – user287001
    May 23 '20 at 15:49
  • \$\begingroup\$ Large dimension (or surface area) only means you have fewer quantity of charges per unit of area and has no implication on the potential difference between component legs. Even if I touch a small conductive object in contact with a semiconductor component only means each legs takes in fewer charges. Regardless of how many static charges each leg "consumes" or attaches, there is no voltage difference between them. With proper handling, I don't need any grounding wrist attachment or protective packaging. \$\endgroup\$
    – KMC
    May 23 '20 at 17:45

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