Ghetto Grounding Strap? - Page 1
Forum section: electronics / testgear

author: jaunty
author_rank: Regular Contributor
timestamp: September 26, 2018, 04:54:58 am
content: So let's say I were in this position - dealing with some very fragile old CMOS logic - and didn't happen to have the foresight to purchase a new grounding strap - or say, lost my old one ... couldn't one simply tie some copper wire around one's wrist and attach it to a grounding source (or optionally with a 5K resistor attached)?? I mean - i can't see HOW the electrons would be able to tell the difference ...asking for a friend.Jon T.

author: DaJMasta
author_rank: Super Contributor
timestamp: September 26, 2018, 05:04:37 am
content: Yeah, or find yourself a grounded metal floor, take off your shoes + socks, and do your handling there. You just need a decent ground path for you to be discharged, though you want your work surface to be dissipative as well so that static doesn't build up on it. You could use a sheet of tin foil on cardboard with a ground wire as a mat, but it's also conductive so you'd short out anything touching it. It's also helpful to take off your socks, not wear fleece clothing, and generally stay away from synthetic fiber clothes that generate static. Also helps if your ambient humidity isn't too dry.

author: Gyro
author_rank: Super Contributor
timestamp: September 26, 2018, 10:00:31 am
content: Quote from: jaunty on September 26, 2018, 04:54:58 amNoooh!If you accidentally come into contact with something live with a grounded copper wire wrapped round your wrist, you're not going to survive long. Even with a 5k series resistor, the current is going to be pretty high. Standard wrist straps have a 1Meg safety series resistor and are insulated to prevent accidental ground contact after the resistor. Your friend would be better holding on to the mat surface or ESD foam that the ICs are hopefully sitting in, and not shuffling in his seat.

author: Fungus
author_rank: Super Contributor
timestamp: September 26, 2018, 10:58:21 am
content: Yep. The resistor is there for your safety, not because it's required by the "circuit" that you're part of.(in fact the CMOS chips are safer without it - definitely no static without the resistor!)If you go that route, I'd use a bigger resistor than 5k.You could also work on a piece of tinfoil that's connected to the GND of your (floating) power supply. If that seems risky, at least have a tinfoil area where the chips are stored and that you can touch before/while you handle them.nb. Some people have never zapped a chip in their life. I think it depends a lot on the chair and type of carpet.

author: Mr. Scram
author_rank: Super Contributor
timestamp: September 27, 2018, 01:56:52 am
content: A direct conductor is no use. Current would still flow violently, so you're still zapping the part. The million ohm resistor is there to protect the circuit.

author: Fungus
author_rank: Super Contributor
timestamp: September 27, 2018, 09:16:27 am
content: Quote from: Mr. Scram on September 27, 2018, 01:56:52 amI thought the idea is that there isn't any charge around to zap things with. Direct conductor will work just fine for that.

author: Gyro
author_rank: Super Contributor
timestamp: September 27, 2018, 10:17:58 am
content: That's all very well if everythingstartsat the same potential. The ICs might be at a different potential when you pick them up from whatever packaging they're stored in. Low impedance charge equalisation can cause high transient current on the first pin to make contact - hence the ESD Charged Device Model.It's far better to put them down on a resistive surface than a grounded metal plate. It's the same situation with the grounding strap.

author: Fungus
author_rank: Super Contributor
timestamp: September 27, 2018, 10:50:52 am
content: Quote from: Gyro on September 27, 2018, 10:17:58 amYeah, but the title is "Ghetto".

author: Gyro
author_rank: Super Contributor
timestamp: September 27, 2018, 11:08:36 am
content: ... or a rusty metal plate then.My point was that it applies to anything coming into contact with the device.

author: Fungus
author_rank: Super Contributor
timestamp: September 27, 2018, 11:19:47 am
content: Quote from: Gyro on September 27, 2018, 11:08:36 amAnd my point was (I think) that you're probably going to handle "whatever packaging they're stored in" before you touch any chips so in theory they'll already be at the same level as the rusty plate when they come out.And ... if they're ghetto-stored then all bets are off anyway.Fun fact: Some people go an entire lifetime with zero precautions and never zap a single chip.

author: Gyro
author_rank: Super Contributor
timestamp: September 27, 2018, 11:36:50 am
content: Hmmm, there's no reason that the ghetto solution shouldn't follow best practice, especially when it only involves a few 1Meg resistors and a bit of antistatic foam.QuoteThat's a bit like the elderly drivers who's never had an accident, but has seen plenty in his rear view mirror. Outright device failure is rare, subtly parameter degradation and reduced reliability is far more common.

author: Fungus
author_rank: Super Contributor
timestamp: September 27, 2018, 11:46:27 am
content: Quote from: Gyro on September 27, 2018, 11:36:50 amHave youseenthe price of that foam?Resistors? They takeweeksto arrive from China. I need to do it today...(can a CMOS chip be wired as a static-dissipating diode?)

author: Gyro
author_rank: Super Contributor
timestamp: September 27, 2018, 12:30:57 pm
content: Ok cheap brown cardboard on a conductive surface then.QuoteYou're kidding, right?I'm not going to play anymore.

author: Sceptre
author_rank: Regular Contributor
timestamp: September 28, 2018, 01:44:29 am
content: Quote from: Mr. Scram on September 27, 2018, 01:56:52 amNo, the resistor is there for safety.Quote from: Gyro on September 27, 2018, 10:17:58 amThat's what ESD mats are for!Quote from: Gyro on September 27, 2018, 10:17:58 amNo, not at all. In CDM, it's irrelevant what the resistance is between a conductive object (human, metal tool, surface) and ground. Unless you happen to be charged to the same voltage as the Charged Device, there will be rapid charge transfer (an ESD event) when the two objects get close enough. If you are wearing a ground strap, you will definitely be at a different voltage. A downstream resistor does no good. This is why some high-rel organizations require the use of ceramic-tipped tweezers - the dissipative material must be at the point of contact with the (possibly charged) device.Sceptre

author: Mr. Scram
author_rank: Super Contributor
timestamp: September 28, 2018, 02:00:20 am
content: Quote from: Sceptre on September 28, 2018, 01:44:29 amCan you prove it exclusively serves that purpose? It doing both makes sense, but people seem to be rather adamant it's one or the other.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 28, 2018, 02:11:22 am
content: The real advantage of a commercial strap is that it is easy to put on and take off and stays on when you have put it on.Every time I have been forced to use a ghetto strap I have trouble getting it to stay on, or if the wire is stiff enough and I wrap enough turns it flops around getting in the way, or if I tie it around my wrist it gets bound up tight and is hard to remove. And there always seemed to be a loose end stabbing me.Even the cheapest ones are effective, and I always found it worth the few bucks to get one.

author: Sceptre
author_rank: Regular Contributor
timestamp: September 28, 2018, 02:33:57 am
content: Quote from: Mr. Scram on September 28, 2018, 02:00:20 amPhysics dictates that the resistor can't be for ESD protection. The human is at 0V, the charged device is at, say 1kV. Volume(human) >> Volume(PCB track/device pin/IO gate silicon). Even if you remove your wrist strap and are floating, you will still rapidly drain virtually all of the charge from the device pin.Or else look at this page:http://desco.descoindustries.com/Articles/ESD-Around-High-Voltage.aspx"ESD Systems.com's Wrist Straps [and Foot Grounders] are Underwriter Laboratories listed. UL cautions that thes products are not recommended for use on equipment with operating voltage exceeding 250 volts recommending that the electrical current that the operator be exposed to be limited to 0.25 milliamp. At 250 volts, the 1 megohm [1,000,000 ohms] resistor does this."

author: Mr. Scram
author_rank: Super Contributor
timestamp: September 28, 2018, 02:37:27 am
content: Quote from: Sceptre on September 28, 2018, 02:33:57 amLooking around on the internet one finds people who should really know this stuff claim either or both, as your link does.

author: DaJMasta
author_rank: Super Contributor
timestamp: September 28, 2018, 02:59:29 am
content: Most of the static you're going to run into is built up on your body, though, so just grounding yourself (with or without resistor) will dissipate it and keep your parts much safer. The human body model is 100pF of capacitance, not too many sensitive CMOS devices are going to have that capacitance on a pin, even if they're the one with the charge built up.It's best to dissipate it slowly, but just grounding yourself to be neutral before handling goes a long way because where the buildup's likely to be. We still don't know what devices the OP is specifically using, but if they are basic logic gates then grounding yourself and conductive storage materials should be plenty. ESD requirements can be incredibly intensive, but while a lot of handling on the hobby end is fairly safe, basic precautions can add another 9 or two to the chance that handling it will be safe.A ground or recent ground of your body is better than nothing in terms of safety of the deviceNatural fiber clothes and stuff that doesn't build up static is better than othersHumidity in the air is better than notStoring parts in boards, ESD foam, or foam covered in tin foil is much better than notA 1Meg resistor in the wrist strap is standard for commercial/industrial equipment, not generally inline with an ESD mat (though the ESD mat is going to be 100s of k of resistance at minimum from the surface to the ground)

author: Sceptre
author_rank: Regular Contributor
timestamp: September 28, 2018, 03:08:46 am
content: Quote from: Mr. Scram on September 28, 2018, 02:37:27 amOK, try this one:http://www.dangelmayer.com/esd-myths-latency-success-stories.php"Myth 8: Grounded Metal Is a Safe Surface for ESD Sensitive Components or AssembliesTruth: Conductive materials are not safe surfaces for components that may be charged. This is true regardless of whether or not the conductive material is grounded. The CDM event occurs rapidly between two objects and the resistance to ground (including a 1-megohm resistor) has virtually no impact."A human is not as conductive as metal, but the principle still applies.

author: Sceptre
author_rank: Regular Contributor
timestamp: September 28, 2018, 03:28:23 am
content: Quote from: DaJMasta on September 26, 2018, 05:04:37 amI've tested myself in stocking feet (cotton or cotton-blend crew socks) a few dozen times in recent years, and usually measure around 150k(feet to my left hand). I once asked an instructor whether the heel strap ground ribbon needed to go inside the sock, and was told no, since most everyone perspires enough through the bottom of their feet to provide a conductive path. A thick enough acrylic or polyester sock may nonetheless be insulative.Sceptre

author: DaJMasta
author_rank: Super Contributor
timestamp: September 28, 2018, 03:36:51 am
content: That may be true, especially if you're on a solid floor, but I just think of wool socks on carpeting are the classic recipe for kids to generate static, so I sort of stay away from them as a matter of course

author: Sceptre
author_rank: Regular Contributor
timestamp: September 28, 2018, 03:49:37 am
content: Quote from: DaJMasta on September 28, 2018, 03:36:51 amYep, at a previous job I used to wince whenever a SW engineer brought me a board (unbagged), knowing that they just walked a hundred feet or so down the carpeted hallway. If they'd been in wool socks with no shoes, I'd have.

author: tautech
author_rank: Super Contributor
timestamp: September 28, 2018, 03:55:01 am
content: Quote from: Fungus on September 27, 2018, 11:46:27 amCan't find the RS listing of the stuff I use but it's so cheap you can chop up sheets of it and line your IC and semiconductor drawers with it.Comparable product:https://www.amazon.com/FOAM-CONDUCTIVE-ANTI-STATIC-INCH-x12/dp/B00B88717IQuoteYou haven't got 1M resistors ?

author: Sceptre
author_rank: Regular Contributor
timestamp: September 28, 2018, 04:07:24 am
content: Quote from: DaJMasta on September 28, 2018, 02:59:29 amI agree with this common-sense advice for the hobbyist, and basically follow that at home. I just wanted to point out that damage from discharging a charged device is not like electrical overstress that can be prevented by limiting current over, say, microsecond timescales. The CDM test waveform is only 3ns long - any damage is done long before a wrist-strap resistor sees whatever is left of the wavefront after it passes through the body.