Electronic – ESD Mat circuit theory

circuit-designesd

I've tried researching this extensively, checking existing threads, but I suppose with any uncertainties in this subject, they are hard to convey properly and therefore hard to find answers to.

So I understand static electricity (I think) and don't need to be convinced about the merits of taking precautions to protect electronic components from getting fried. But I fear there are gaps in my knowledge and/or don't understand things correctly. So I'll ask questions as I try to convey my understanding.

My basic problem is: I need the right setup for working on small electronic components. Let's say a RAM chip.

For working on internal PC components, most places I've come across suggest use of an anti static wrist strap hooked to the metal chassis of the computer and that's it.

Question 1: The internal components are all eventually connected to the conductive metal, so won't they absorb the static discharge from your body?

Then there are places which suggest connecting yourself to an ESD mat using an anti static wrist strap.

Question 2: Wouldn't this just create an open circuit to ground?

Question 3: The mat is high resistance so electrons from you would flow very slowly through it. I know electricity still moves really fast but my mind doesn't and I'm seeing discrete intervals of time. So I'm thinking that if you touch the sensitive component straight after connecting to the mat, you might still discharge to the component.
So how quickly does your body take to achieve neutral charge?

Question 4: Can there be a state where you and the mat achieve equilibrium but your body still has a charge that can damage the component if touched?

Question 5: Can there be a charge in the mat that can transfer to the component once the component is rested on it?

Finally, some have suggested connecting yourself using a wrist strap to an ESD mat and then connecting the ESD mat to mains ground with the outlet switch off.

Question 6: If the switch comes on by accident or there's a fault somewhere on the circuit, will the high resistance of the mat stop the current from flowing through you freely and hence stop you from getting fried?

Question 7: Even after you've discharged, wouldn't you, the mat, and the strap be on a different (and possibly higher) potential than the component (which currently sits outside this circuit) you will now touch and therefore damage?

Now let's say with the previous setup, you are working away on the component that is resting on the mat and it's still fine. Then you rub your feet on the carpet subconsciously and gather static.

Question 8: Will there be a constant discharge through the mat while you gather charge? If so…

Question 9: Wouldn't the discharge cause current flow through the mat where the component is resting? and if so…

Question 10: Could that current damage the component?

Best Answer

Question 1: The internal components are all eventually connected to the conductive metal, so won't they absorb the static discharge from your body?

A wrist strap ensures that the potential between you and the computer is near 0V instead of thousands of volts that comes from clothing and walking across the floor. The damage comes from current flowing from you into the small gates that make up transistors. If the voltage between you and another object is 0V no current can flow.

Question 2: Wouldn't this just create an open circuit to ground?

No, when you connect the wrist strap to the mat, they really mean to ground, look at the diagram below and wrist strap two. Wrist straps should be bought with the 1MΩ resistor built in.

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Answer to Questions 4-10:

An ideal ESD setup will lower all potentials to near 0V (which means connecting it to earth ground). There will be no current transfer into parts because there is no voltage to start with, it has been lowered to ground by a wrist strap or mat. These have a 1MΩ resistor to prevent a short circuit to ground in the event the user or other voltage should come in contact with the mat or wrist strap.

If you have a wrist strap only to the computer that's probably fine in most cases. A better setup is one with a mat and a wrist strap to ground. A true ESD controlled environment has the floor, the table surface (and no paper, or plastics that are not ESD safe), tools and lab coats (to prevent electric fields from clothing) and the humidity of the air controlled.