Resistance of component bag made of "reddish" ESD plastic?

[treez]

Hi
Due to lack of funds we are doing electronics in an office with a carpet which is not ESD safe.
Often we need to carry a PCB with ICs etc on it….walking across the carpet as we do so.
The typical thing we do is place the PCB on a plastic component bag made up of a piece of that “reddish ESD plastic” .
We wish to test whether such “reddish plastic” really is ESD safe.
What Ohms value should we get if we clip to it with crocodile clips (about 2cm apart) to measure the resistance?

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the following datasheet doesnt give the ohms/cm, so how will we measure it?
https://www.farnell.com/datasheets/...MImKjpuabA6AIVF-DtCh0mEA5REAAYASAAEgLyR_D_BwE

..there is great scope here for fake ESD plastic bags to come on the market, and result in loads of ESD fails.

 

[dick_freebird]

Somebody needs to do the cost / benefit analysis of
proper ESD protection (floor, surface, ionizers, RH
control) vs the labor and material costs of blowing sh!t
up over and over and then chasing your a$$ to figure
out why.

That is, at minimum, how many wasted engineer-hours
would have paid off the infrastructure cost?

I would opine in meetings that failing to use proper ESD
procedures will not only be a nagging low level yield
and "experiment contamination" problem, but on sold
product also infant mortality, reliability, field returns
and of course customer audit failure problems of the
sort that deliver reputational damage that can't be fixed.

You could ask to see the analysis that supports this
"costs too much" story, which might reveal that no
such thing was ever done and whoever's saying it is
just trying to avoid work and cost locally without regard
to the actual best practice and outcomes or even any
supporting facts / analysis.

 

[FvM]

The datasheet has proper specification of surface resistance, apparently you don't understand its meaning. See https://en.m.wikipedia.org/wiki/Sheet_resistance

Antistatic bags are designed to dissipate charges deposited at the surface, not to shield the contained components against ESD on unpacking.

Correct ESD flooring, tools and worktables as well as shoes and work coats are the solution.

 

[BradtheRad]

Consider building this static charge detector. Made from a 4011 quad NAND gate. Red led lights for positive polarity, green led for negative. The article below has professors' names associated with it. (A simpler circuit can be made from a jfet and led.)

www2.ece.rochester.edu/~jones/demos/ahern.html

-------------------------------

Staticides are listed at Amazon. (However check whether it can be shipped outside US.)

http://www.amazon.com/s?k=staticide

The Staticide brand is described as effective to protect electronic equipment.

------------------------------------

There are anti-static sprays for garments. Some reviews say it works on a sofa or chair for a few days. Evidently it leaves conductive residue. Many complain about the vinegar odor.

Static guard
'cling remover'
'wrinkle release'

 

[pjmelect]

There are two types of anti static material, one red or pink and the other type black. The black materials are conductive and the red or pink ones are not. The red or pink material work by being a plastic that does not allow a static charge to be generated or carried on its surface. The reason for the two types is that if the board or component has a battery then discharge in the red or pink material is impossible where as the black material is very conductive and will discharge the battery.

 

[c_mitra]

Re: Resistance of component bag made of "reddish" ESD plastic?

What Ohms value should we get if we clip to it with crocodile clips (about 2cm apart) to measure the resistance?

That may not work well.

They are electrically conducting but the conductivity is not much. That is a feature so that discharge current stays low with a reasonable time constant (0.1 to 2 sec or so).

Most likely you will see open circuit if you just use a croc clip to make contacts. Better way is to draw two parallel lines using some conducting paint and measure the resistance.

If the specified value is 1.0E10 (ohms), that means the resistance will be between two lines 1 cm long and 1 cm apart (or if it pleases you, 1 m long and 1 m away).

there is great scope here for fake ESD plastic bags to come on the market, and result in loads of ESD fails

Does that really make sense? It does not cost much to make these ESD bags? Who will make fake bags?

Just spraying some graphite powder can make them conducting (but should not be too conducting).

 

[std_match]

Watch the ESD bag/mat videos by Dave Jones:

https://www.youtube.com/watch?v=imdtXcnywb8 EEVblog #247 - Anti Static Bag Myth Revisted
https://www.youtube.com/watch?v=7FFrZl9ymFw EEVblog #250 - Anti-Static Mat Myth
https://www.youtube.com/watch?v=27VrJnvzP3E ESD Bags: Static Dissipative vs. Antistatic vs. Static Shielding

There is also info from 3:34 in this video:
https://www.youtube.com/watch?v=kLJOI9HC-tQ EEVblog #3 - Static Myths, PIC Micro, Pocket Multimeter

 

[c_mitra]

Consider building this static charge detector. Made from a 4011 quad NAND gate. Red led lights for positive polarity, green led for negative.

It is really interesting; I saw the link you gave and it does detect movement of charges (that is a current) but not a stationary charge.

It is not the static charge that is the culprit: the sudden discharge of the static charge (that produces a current) that causes the most damage.

A static charge is associated with an electric field and a potential. But you can only measure potential differences (but not absolute potentials).

You have a component inside a bag that is charged to a high potential. The component is safe as long as it is inside.

When you take out the component (or suddenly discharge the surface charge on the bag), the sudden potential difference causes the fatal damage.

 

[treez]

Thanks, i see that if i were to carry a populated PCB across a non ESD safe carpet, then the pink bag ("anti-static)" would be of little use, but just very very slightly better than me holding the PCB circuit in my hand.
We need to use the silver bags ("static sheilding").....or , but just a bit less good, the "static dissipative bag"

 

[dick_freebird]

There exist ESD-safe work totes / tubs for carrying WIP
around from station to station. You'd only need a couple,
maybe a cart with drag-chains to keep at "floor potential",
and the discipline to use them appropriately.

 

[treez]

Thanks. ill look into static shielding tubs because static shielding bags are pricey, and to test if they are genuine you need a $460 surface dc voltmeter, and a $3 ignitor, as in top video of kindly supplied #7 post
https://www.alphalabinc.com/product/svm2/

 

[pjmelect]

Thanks, i see that if i were to carry a populated PCB across a non ESD safe carpet, then the pink bag ("anti-static)" would be of little use

The pink/red bags are good enough for the M.O.D to protect their equipment. The pink/red bags are effective at protecting components from static.

 

[c_mitra]

Thanks, i see that if i were to carry a populated PCB across a non ESD safe carpet, then the pink bag ("anti-static)" would be of little use, but just very very slightly better than me holding the PCB circuit in my hand.

I do not get it; can you please elaborate?

I presume that by static shielding you mean a surface that blocks electric field (any conducting surface will do). However, if the electrical conductivity of the surface is low, it will respond poorly to a changing electric field (when the static is being discharged).

Static dissipative surface are also conducting electrically: but with a higher surface resistance they will prevent a rapid discharge of the surface charge.

 

[treez]

...the top video of #7 above shows why pink bags would be useless for carrying PCBs whilst walking on a non esd safe carpet

 

[betwixt]

All of this goes around in circles. Static is a fact of life, it isn't the voltage that causes harm, it is the voltage difference when charged items touch each other.
If voltage alone did harm, consider say 200V/metre rise in voltage above ground level in dry air, would mean all aircraft electronics got fried at take off and probably vaporized at higher altitude.

Placing things in static dissipative 'pink' bags just equalizes the charge around the device. The bag might be at high voltage but everything inside it is at the same potential and therefore completely safe. This is like the aircraft analogy, they can withstand high altitudes and even lightning strikes but everything inside the fuselage is at the same potential so no harm is done.

Also consider that static charges do not magically appear out of nowhere, they build gradually if they can and even the tiniest of leakage paths will discharge them, including when the charge is trying to build up. The properties of 'anti-static' plastics are designed to prevent the charge building. Try rubbing two pink plastic bags together and see how long it takes to make sparks!

Brian.

 

[c_mitra]

it isn't the voltage that causes harm, it is the voltage difference when charged items touch each other.

I am trying to say the same thing; the culprit is the rapid movement of large amounts of charge under significant potential gradient. The associated energy kills the chip.

 

[c_mitra]

...the top video of #7 above shows why pink bags would be useless for carrying PCBs whilst walking on a non esd safe carpet

The experiment (in the above video) has not been done (planned) properly. All experimental design has one focus: prove an hypothesis. The outcome of the experiment may be pass or fail.

In my personal opinion, the video mentioned appear to be a sponsored one: the experiment is not planned well in advance.

 

[betwixt]

Video #7 is quite honestly nonsense. Did anyone notice the comparison between pink plastic and metal flashed bags wasn't done properly? Pink plastic is (I think it's an amide additive) designed to prevent the build up of charge, the metal bags are designed to conduct charge by virtue of the high density of the metal sputtering on the surface. When the piezo zapper was placed inside the plastic bag, it predictably raised the potential around the wires and was detected by the meter, when the test was repeated with the metailized bag, he held it in his hand which was also resting on the anti-static mat so it conducted the charge away through his body. As c_mitra points out, the test is not done properly and if anything confirms our belief that the bags are perfectly safe when used properly.

Brian.

 

[treez]

Why is it an ESD wrist strap alone is not enough?...

I once worked in a company where batchs of PCBs would be brought out of stores, given small PCB modifications (eg a track cut with an all metal scalpel and a resistor changed) by assembly staff, then put on soak test. These PCBs had a 10% death rate from presumably ESD…….even though the workers were wearing ESD wrist straps, and were on a floor which was (according to the datasheet) non- ESD generative.
The failures were typically FETs going short or ICs going short.
Of the “same circuit” PCBs which did not have this hand modification…the soak test death rate was nil.
Is it possible that the “anti-static ESD bags they were stored in were fake ones?

 

[betwixt]

Many years ago I was called in to a factory to investigate high failure rates in 'prone' devices like microwave mixer diodes and MOSFETS. I identified two major fail points, one was a rack of PCBs passing a CRT monitor, almost touching the glass. This is less likely to be a problem now LCD has largely replaced CRTs. The other was a vacuum based test rig, as the air rushed over the test fixture it developed a large static charge on it's plastic surface. The PCBs passed test but as they released from the fixture it discharged and killed the devices. That was a hard one to find as the boards passed the test but failed if retested but only if they had been released from the fixture.

We keep trying to impress on you that static alone is not in any way harmful, it is the discharge when surfaces at different potentials contact each other that does the damage. If the wrist straps and the PCBs were at the same potential there would be no damage. If you are sure the damage isn't from other actions, look at the route the PCBs take from start to finish and see all the places where they could touch something or someone at a different potential. For example, no matter how conductive a floor is, someone walking over it with synthetic shoe soles could charge the whole rack then cause the damage by touching the boards while grounded.

Brian.