When I'm working on computers, I want to avoid electrostatic discharge. I have a wristband, but I'm not sure that I'm attaching it to the right thing.
ESD rule number 1 - The aim is to have all objects being worked on at the SAME potential.
If this is ground potential, so much the better.
Computer ground and wrist-strap clip should be connected.
Connecting both to ground is a desirable bonus.
The aim is to have you and it at the same voltage.
- Usually, I'll clip it to the case of the computer. However, there are two cases in which I'm not quite sure what the proper procedure is.
As above. The aim is to get you and ALL the electronics that you are working on at the same "potential". This should ideally be ground potential - but it is more important that you and the work be at the dame voltage than that you be at ground. In fact, if you are at ground and the circuit you are working on is not, you MAY do more damage by having the earth strap than by not having it. May.
If there are two cases, ground or connect them both in some way.
- First, sometimes laptops seem to have a all plastic case until you take them apart enough.
They will often have a ground connection or a jack etc that has "grounded" metal. If none of these then see below.
- Second, things like harddrives are ESD sensitive, but when you get them they aren't in any computer, so what should you attach to? That got me to wondering if it would work for it to be attached to a ground (like the third prong in a US outlet, which is attached to a grounding rod or water pipe). Could someone clear up for me what will work to avoid ESD and what will not?
You need an "ESD safe" work area. Typically this is a grounded work surface that is mildly conductive. An ideal material is "butyl rubber" which is used for roofing and waterproofing. This has carbon black included in it which is what makes it conductive.
Price is reasonable compared to almost any alternative. You can sometimes get scrap roll ends or sheet covers used to cover bales which are even cheaper.
Ideally avoid a high conductivity surface such as a metal sheet if you are going to work there with circuits with power on (magic smoke happens) and if you have a PCB with makns or high voltage on even butyl rubber sheet may end up smoking if you apply enough voltage :-(.
A common metal sheet is better than nothing at all - just keep ALL power away from it.
ESD will discharge to ground quit quickly via a 1 megohm resistor. Connecting one og these in the wriststrap gound is wise - unless there is one there already.
ESD precautions:
I would like to get an anti-static mat for my bench top
Buy the cheapest one in the size you need.
Suitable materials for ESD desktop sheets: Any somewhat conductive sheet of material that is grounded via a 1 to 10 megohm resistor will work as an anti electrostatic work surface.
(Sheets with very low resistance per square risk conducting current when PCBs etc with exposed terminals are placed on them. (Ask me how I know :-) ).
Sheets need not be more than a trace conductive, although very conductive sheets will work. eg a thin sheet of galvanised steel would work well but be immensely inconvenient and potentially very dangerous as it allows instant conduction from anywhere on a work bench to anywhere else.
Better cheaper faster ... A potentially much cheaper than "real" solutions product is Butyl Rubber sheet sold for roofing and pool liner purposes. The conductivity in this material is caused by carbon-black which is integrated in the rubber and it is wise to check that at least some conductivity is present. Take an ohm meter set to say 10 megohm range with 2 sharp probes. Press probes into sheet just not touching so that probes penetrate sheet surface. If there is even a trace of resistance measurable then the sheet will work as an anti ESD sheet. Results may vary from megohms to a kilohm or so depending on carbon black loading.
Measured resistances: Notionally the "resistance per square" of a sheet of conductive material is the same at all scales so the resistance across say the diagonal of a square of material should be the same for a square of any length side. So if a 1cm side square measures 100k across the diagonal then so too should squares of 10cm or 100cm or 1m length sides (given the same thickness in each case). In practice results may vary somewhat, but this is a good enough gyide for ESD purposes - if you can see a reading in the megohms of less range at any distance (small or great) the sheet will probably work OK.
Wood: An older worn wooden desk will probably be an adequate ESD protective surface in its own right. A new coat of varnish may destroy this. An older wooden floored building will also probably be ESD safe. A new concrete floored building will not be and linoleum or carpet add their own potential problems.
Carpet: Some carpets generate enough "static electricity" to cause user shocks from sparks generated. Your electronics will die even when you just wave your hand near them if not well protected. Use of an antistatic spray on the carpet may stop users receiving shocks but induced voltages may still destroy ICs.
Especially sensitive devices include: LEDs, Schottky diodes, GaAs devices, MOSFET gates, unprotected CMOS, Gunn diodes (who?). Many modern devices have ESD protection built in - especially those which are especially sensitive if unprotected, but suitable care should stoll be taken. I have seen less sobvious ESD damage over decades than the salessmen would have led me to expect BUT I have seen certain ESD failures and in some cases could virtually guarantee device failure by following ceryain unsafe procedures.
Safety: Mats and wrist straps should have a high value resistor - usually 1 megohm but not critical - to avoid grounding users for power voltages and so creating a shock hazard.
Best Answer
My entire office is carpeted, and we constantly debug/test raw PCBAs at our desks. I take care to ground myself on metal objects around me, and I have an ESD mat on my desk. When I have to transport boards, I put them in ESD bags. I also take care to ground myself after getting up from my chair, or upon sitting down. When passing off boards to others, we always do the "EE's handshake", where we touch hands before exchanging the PCBAs.
This has worked well for us, and I have yet to see a PCBA damaged due to unintentional ESD.