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Aluminum foil as an antistatic bag substitute

I just received some memory modules wrapped in aluminum foil (eBay).  That's perhaps not as alarming as if they had been wrapped in standard bubblewrap (as was a processor I got a few weeks back), but I am wondering whether foil is the ultimate in ESD protection or poses a hazard to ESD sensitive components that may lie within.
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dserdu2
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dserdu2
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☠ MASQ ☠Commented:
I think the biggest difference is that ESD bags have the outside surface and inside surface insulated from one another.

All Al foil does is allow the static charge from the outside of the wrapping to reach any number of areas of the device contained inside!

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arthurjbCommented:
The aluminum foil can protect the memory from esd.

The reason it is not used for all components is that the foil is too conductive, and can short out the small batteries that some boards may contain.

I believe that masqueraid's response is incorrect, since if the component is wrapped properly, the static charge cannot travel within the device.  

All things have a static charge, the goal of proper esd handling is to make sure the charges of the devices are equalized and not able to damage the semiconductors on the devices.

The foil could be considered the "Ultimate" in esd protection, for devices that would not be damaged by the foil shorting out the components on the devices.
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dserdu2Author Commented:
Thanks for the comments so far; in my mind the jury's still out.

I ran across a reference to "triboelectric charging" and a minor concern that Al foil could cause problems related to it.  Apparently it's static charge induced by rubbing two dissimilar materials together, which I guess could happen in shipping if the foil were loosely wrapped and could move relative to the module.  The triboelectric series (see http://www.siliconfareast.com/tribo_series.htm) lists various materials and their propensity to gain or to lose electrons.  Interestingly (according to this site) triboelectric properties have nothing to do with conductivity!  If I'm reading this right, the ideal ESD container would among other things have the same triboelectric properties as the device it contained so that there would be no triboelectric charging.  Note that Al is slightly positive and gold is somewhat negative in the series, while polyester (a typical ESD bag outer layer) is somewhat nearer to gold in the series than is aluminum.

Independent of triboelectric charging between the container and the enclosed device, what effect would a non-insulated conductive material such as Al foil have on a differential charge occurring between the wrapped device and, say, a human hand that had recently been petting a cat in the dead of winter?  Would the device be insulated from the charge difference, would the charge be spread over a relatively large area and therefore attenuated, or would something else occur?
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PCBONEZCommented:
Using CONDUCTIVE material is a VALID form of ESD protection.
Anti-static bags actually are conductive due to the fact they have [very thin] metal in them.
You know those ESD bags with the grid pattern on them... The grid is thin metal foil.
The silvery ones are silvery because there is thin metal foil between the layers of plastic.

Read here under "Storage and Transportation of ESD sensitive component and boards"
http://www.allaboutcircuits.com/vol_3/chpt_9/1.html


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PCBONEZCommented:
>>>
Independent of triboelectric charging between the container and the enclosed device, what effect would a non-insulated conductive material such as Al foil have on a differential charge occurring between the wrapped device and, say, a human hand that had recently been petting a cat in the dead of winter?  Would the device be insulated from the charge difference, would the charge be spread over a relatively large area and therefore attenuated, or would something else occur?
<<<

By completely wrapping the modules in foil the shipper created a very effective Faraday Cage.
Just make sure you touch the foil to a ground before you open the package.
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arthurjbCommented:
I stand by my comments, that aluminum foil is a good esd prevention media for memory sticks that do not have batteries.
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PCBONEZCommented:
Good for you... Is someone arguing it?
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dserdu2Author Commented:
From the source PCBONEZ sent,  "Storage and Transportation of ESD sensitive component and boards"
http://www.allaboutcircuits.com/vol_3/chpt_9/1.html

"The most common method [of protecting components from ESD during storage or transport] is to use a variation of a Faraday cage, an ESD bag. An ESD bag surrounds the component with a conductive shield, and usually has a non static generating insulative layer inside. In permanent Faraday cages this shield is grounded, as in the case of RFI rooms, but with portable containers this isn't practical. By putting a ESD bag on a grounded surface the same thing is accomplished. Faraday cages work by routing the electric charge around the contents and grounding them immediately. A car struck by lightning is an extreme example of a Faraday cage.

"Static bags are by far the most common method of storing components and boards. They are made using extremely thin layers of metal, so thin as to be almost transparent. A bag with a hole, even small ones, or one that is not folded on top to seal the content from outside charges is ineffective."

I take from this that foil is OK ("*usually* has a non static generating insulative layer inside") from an ESD standpoint although the small battery scenario certainly seems a valid argument for having a non-conductive layer inside.  But elsewhere in the article it says plastics can be made conductive (ESD dissipative) and still have a huge resistance (millions of ohms per square inch) from the standpoint of a battery.  So a typical ESD bag doesn't truly have an insulator layer inside unless compared directly to foil.

What was news to me was that the ESD protected package needs to be grounded before opening it whether foil (as PCBONEZ asserted) or conventional ESD bag (as the article says), since until it's grounded it's not a faraday cage.   That would suggest not only laying hands on the PC's power supply before handling a CPU but actually placing the ESD container on the PC case before opening it.  Other news was that even small holes render an ESD bag ineffective.  Wonder how small is small?  The modules I got were loosely wrapped without fold-overs in the foil, hence there were holes.
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arthurjbCommented:
There are different levels of "Conductivity"  

If you measure foil with a meter, you will see that there is close to 0 ohms resistance.  (This is why it could short out batteries.)

If you measure a conductive bag with a meter, you can have 1 megohm resistance, and still be within compliance.

Here is a link to a pdf that explains a little about the measuring process;
http://www.sbpspace.biz/ATt%20files/ESD%20Training.pdf

Your text talks about a Faraday Cage,  the foil acts like a Faraday Cage.

The simplest part of this process is the part that you mention right at the end of the text, which is what most people don't understand.

ESD protection does not really keep electro static charges from building up on the components.  What the ESD process provides is a way to discharge the static charges without damaging the junctions inside the electronic parts.  The "Secret" part of the process is that you don't have to get rid of all the static charges, you only have to have all parts at the same static charge.

This is why Oldtimers who understand the process, may handle the parts differently, (but still safely!) than those folks who have just been taught all of the ESD process, without being taught the reasons and theory.
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PCBONEZCommented:
Nailed it --->> ""The "Secret" part of the process is ......""
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PCBONEZCommented:
If there is no difference in potential then there can be no current flow.
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arthurjbCommented:
Exactly, the current flow through the junctions is what damages the electronics.
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dserdu2Author Commented:
The complete quote was 'The "Secret" part of the process is that you don't have to get rid of all the static charges, you only have to have all parts at the same static charge.'

Perhaps this is semantics, but I'm trying not to gloss over this and instead to understand it intuitively.  Seems to me that it's not so much the potential difference between parts on a board or module that is problematic, although it would be were it to occur.  In practice the static potential difference develops between the module (and all of its parts, which would tend to be at the same potential) and a person walking around in silk stockings on a carpet.  The purpose of the ESD bag (or possibly foil) would be to shield the module and all of the components on it from the potential difference and nature's attempt to equalize it when the person picks up the bag; the energy will instead be stored on the bag's surface and dissipate as soon as the ESD bag is placed on a grounded surface.  The contents are safe to handle when the bag **and its handler** are brought to the same relative potential.  Potential difference between components on the module doesn't seem to be the issue.

But getting to the jist of ESD, there seems to be in this thread a consensus that it's current that kills the ESD sensitive components.  However, from the article PCBONEZ referenced:

"A static charge of millions of volts is common, however the reason it is not a threat is there is no current capacity behind it. These extreme voltages do allow ionization of the air and allow other materials to break down, which is the root of where the damage comes from."

But the article follows with "Damage to components can, and usually do[es], occur when the part is in the ESD path. ...if a part has a small or thin geometry as part of their [sic] physical structure then the voltage can break down that part of the semiconductor. Currents during these events become quite high, but are in the nanosecond to microsecond time frame. "

This is starting to sound like "folk knowledge" rather than basic understanding, and a muddy recitation of what has been heard rather than truly grasped.

Maybe I need to understand why a Faraday cage works and shields its contents from external static charge.  If part of the secret is not providing an "edge" for static charge to build up on then (getting back to the original question) the efficacy of aluminum foil may be influenced by the manner in which the module is wrapped.

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dserdu2Author Commented:
Thank you for the informative reference.
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