Questions on basic ESD safely

Question

Let me first apologize as I know this topic appears to have been discussed extensively yet something isn't clicking for me after researching over many days in stack, google, and youtube! I want to clarify a few thoughts in my head on the applicability of anti-static devices (such as wrist straps and mats) and how they work. I'll list my current thoughts/questions as follows...

1.) The purpose of the resistor in the strap/mat grounding cable: As I understand it the purpose of the resistor is that when there is a potential difference between you and the device you are working on the resistor will limit the current fed into the device to reduce damage. Is this true?

2.) How ESD occurs when working on a device: I understand that ESD occurs when there is a voltage difference between you and the device. I also understand that voltage is a relative measurement between two points. Additionally I've been taught that "ground" and "earth ground" are two different things. My big source of confusion is how you prevent an initial static discharge. As I understand when we are connected to "ground" it is not necessarily earth ground. This "ground" potential can vary depending on the electricity provider. As such, wherever your device is developed will have achieved equilibration in charge relative to whatever "ground" it was prepared relative to (assuming manufacturers develop devices by grounding themselves and all components). Where I am getting mixed up is that say when I take out a computer motherboard (which will be at a certain potential) and put it on an anti-static mat and connect the mat to my local ground source, isn't there still a possibility that very act may lead to a voltage difference (due to different relative "grounds" and cause an initial discharge? Or is the variability in what is considered "ground" standard throughout the U.S?

3.) When working with devices is it best to connect everything to ground? Or do you connect everything to the device? As I understand it you either set yourself and your device to ground potential or you set your-self to the same potential as the device. In both cases wouldn't there still be an initial discharge? A scenario in my head is that I touch the bare metal of a grounded outlet to disperse any static on me initially. When I go to handle the device how can I be sure that the device is at the same grounded potential within its anti-static packaging and that I won't discharge current through it?

4.) Assuming that there is a natural inevitable discharge and that my concern about that is correct, is it just a matter of which is safer, the "natural" discharge or the ESD? As I understand ESD can lead to a potential difference of hundreds to thousands of volts. If I use a CPU as an example device I believe anything over 2-volts has the potential to damage/fry the CPU. In this case ESD is definitely bad news. However, due to just transportation across the U.S and the potential of difference of relative grounds is it likely that there is always a few volt difference between me and anything else that I interact with? In this case, wouldn't I potentially always damage the CPU when removing it from its packaging?

5.) Lastly, as I see it now with all my concerns, ESD devices such as wrist straps and mats do not protect you and your device against any initial discharge (since there may always be a difference in your resting potential), they only prevent the future build-up of static as you are working on the device. Is this true?

Thank you very much for your patience in helping me figure this out!

Answer 1

1. The resistor is there mainly to protect you. If you accidentally touch a live wire when standing on an insulating floor, you will get a very unpleasant jolt, which will probably not kill you (but don't try it!). If you touch a live wire while one wrist is connected firmly to Earth, then a much larger current will flow, which is a lot more likely to kill you. The resistor reduces the current to something much safer. That said, the resistor also limits the current that flows when you first put the strap on - avoiding the static electricity jolt you sometimes get when touching metal on a cold dry day.

2. "Ground" can mean several things. It can be the Earth, and the voltage it's at. But in small DC circuits, it's often the 0V terminal of the power supply, or the negative terminal of the battery. In the case of battery-operated gadgets, there may be no connection between "ground" and the Earth. Here in the UK, the term "earthed" is often used instead of "grounded", which tends to be clearer. Anti-static mats are normally "earthed", but will actually work, even if they are not. The important thing is that you, the mat, and the circuit you are working on are all connected together.

3. You're trying to get rid of the tens/hundreds of thousands of volts caused by static electricity. The odd volt here or there doesn't really matter to most integrated circuits. ESD sensitive components are normally shipped in special packaging, which is slightly conductive. As soon as you grab the package, the voltage equalises.

4. The wrist strap will discharge any initial static when you first put the strap on. Then it will stop any new build up.

Answer 2

1) Actually the device and you should be grounded to earth potential so there is no potential between the device and any other object.

2) Earth ground has conductors that go to earth, in a real ESD system your required to make sure the resistance\impedance between you and earth ground is below a certain level. Ground on a schematic could be isolated and may not connect to earth ground. Usually chassis ground connects with earth ground.

3) Depends on the device chassis ground is usually the best, but in some devices may not connect with earth ground.

4) Packaging is designed to be a faraday cage, if you have a box surrounded by a conductor, the charge from external electric fields must be zero on the inside of the box (assuming that there is no net charge generated from the material). This means there are no potentials and no voltage gradients. The pink anti static bags are only suitable for one use as the conductive layer is easily damaged and does not prevent esd or triboelectric charging when damaged. If you really want to protect electronics, put them in the mylar bags or a metal case and make sure there are no openings. Materials (and packaging materials) such as foam near the device under protection should be ESD safe.

5) ESD wrist straps will zero out the potential between you and earth, if you have a charge and grab the strap there will be a spark (if you have a properly grounded wrist strap there will be no spark because of the 1M resistor). There are also strap monitors that make sure that the conduction between you and the strap is above a certain level. Sometimes (especially in dry environments) ESD lotion must be used to keep the conduction between you and the strap. In especially sensitive labs the humidity is controlled to a ~60% level because water mitigates charging. Secondly clothing also generates voltage potentials conductive lab coats minimize this effect.

JPL ESD Manual

Note on anti static bags from wikipedia:

Conductive antistatic bags are manufactured with a layer of conductive metal, often aluminum,[3] and a dielectric layer of plastic covered in a static dissipative coating. This forms both a shield and a non-conductive barrier, shielding the contents from static charge via the Faraday cage effect.

Answer 3

The resistance controls the SPEED of the discharge, which, yes, if you think of it this way the rate is what causes the damage. You are constantly making charge as you move around, move your arms breath in and out and your clothes rub. Not only is the strap and mat and the size of the resistance for the initial discharge as you walk up to the area and put the strap on, but it is also there to constantly maintain the charge you continue to make. the goal is to keep you and the unit at the same potential so you are not generating the kinds of steps in charge difference that cause damage (keep them slow and under control).

Yes touching the earth ground screw on the center of an outlet or light switch or the chassis of the computer nearby, etc will shed off the charge you built up walking over to the bench, and the reason you "feel" that discharge or even see it is because of the lack of control of the rate, no resistance you are just dumping it into that metal fed to a ground. And why you dont "feel" that zap when touching the mat or wrist strap after walking through the building to the lab bench.

It is all about controlling the rate of change of the charge, as well as the delta between you and the unit you are working on/touching. Heel straps are mostly useless, some folks think you can get away with only one, the idea is the same discharge to a treated floor, but you are likely walking producing a lot of charge, and not that you are skipping but if you only have one you are not maintaining control of that charge. If you are sitting, you probably dont have your heel on the ground anyway, now you rely on the chair to be grounding you if that works. If you really care about this at all, wrist straps are the only way to go, ideally the ones with beepers to tell you that you are connected at all times. All devices go in bags in boxes and then you can remove the strap and carry the box to another station, strap up and remove the items from the box and bags. Anything short of that why bother.

Answer 4

The way to prevent static damage is to prevent discharge into sensitive electronics like CMOS gates.

There are basically two ways to accomplish this:

1. prevent discharge into the part by ensuring that there are no charges around or ESD;
2. prevent discharge entering the device by shorting the pins to ground or clamping the pin voltage.

When modern circuits are connected to power supplies, most input pins have discharge diodes bonding the pin to the supply lines. This effectively routes the static charge to the supply to prevent damage.

The discharge into device in both cases then becomes a charged capacitor from the environment discharging into the circuit capacitance. If the capacitor in the environment is small enough or the voltage low enough, no damage occurs.

Remember the capacitance of a disconnected device is very small and it is very easy to cause a voltage of say more than 15 volt. When the device is in circuit the power supply circuits have much larger capacitance making damage less likely.

Hope this general description helps you to understand electrostatic damage.

Answer 5

To safely handle ESD sensitive devices or systems, simply touch the GND pin of

1) the PCB 2) the IC

And when someone wants to hand you an ESD sensitive "thingy", insist they physically touch you, first. That discharges the "thingy" also, and they can now hand you the "thingy"