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