CMOS DRAM chips and static

Wrap some aluminium foil around a small piece of expanded polystyrene, then you can just push the chips into the sandwich and keep the lot in a plastic box. The foil shorts all the pins together so there can be no potential difference between any.

Gareth.

Aluminium foil is a BAD idea. In the event that there is any appreciable charge on a given pin, pushing it into aluminium foil will discharge it *quickly* and the resulting current may kill it.

Always use high resistance material for storing ICs like the anyi-static black foam material. This allows any charge to 'leak' away slowly and safely.

Graham

I've just received some kits from Jaycar where they've used this very method. I must admit to being surprised - everyone else seems to use the correct foam.

--
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    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.

Are there any Data to show the actual risk involved using this method? I accept that best practice is always best, but what do we know about the reality of the situation? Don't forget we are discussing the storage of small numbers of IC's in somebodys workshop.

Gareth.

Eeyore wrote in news: snipped-for-privacy@hotmail.com:

It is NOT the current that kills CMOS, it is high VOLTAGE that punches holes in the insulating layers inside the chip. [I am not aware of ANY chip families where tiny CURRENTS would be a hazard].

As the conductors are quite small, it only requires a surplus/deficit of a "few" electrons to build up a high voltage.

When the chip is in a circuit, there will be much higher currents charging and discharging the

You want to avoid anything that may have accumulated a large surplus or deficit of electrons.

As long as the aluminum foil is not at a high static voltage, the chances of damage are quite small.

Hold the chip in one hand, the aluminum foil in the other and run your finger along the leads before you bring the chip into contact with the aluminum foil, if you want to 'slowly discharge' any charge that might have built up. (this assumes clean hands with normal skin resistance) If you want to be ULTRA safe, make sure you touch the Vcc and Gnd pins FIRST.

--
bz    	73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an 
infinite set.

bz+ser@ch100-5.chem.lsu.edu   remove ch100-5 to avoid spam trap

And how would this occur?

Aluminum foil is fine. The black stuff has a relatively low resistance anyhow. Check it with an ohmmeter.

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See manufacturers' advice.

A small sheet of conductive foam isn't expensive and I keep all the small pieces that ICs come shipped in. These often fit small storage drawers very nicely.

Graham

These are two different things. Those 'tiny' currents can be quite large when discharging a significant charge. Enought to damage the chip's internals through overcurrent.

Graham

charge

and

Because the ali foil provides essentially a short circuit path.

black

I have many times. You're plain wrong or using the wrong grade foam.

Graham

Eeyore wrote in news: snipped-for-privacy@hotmail.com:

Yes.

Yes, but you can not 'have a significant charge' on a chip without having a high voltage differential between the chip and the conductor! There are not enough charge carriers to produce a high current from the small internal capacitances of the cmos chip itself. If you have enough of a differential between leads on the chip to cause a high current, then you have already lost the chip due to the high voltage.

NO! The danger to cmos from static electricity is not due to over-current, it is due to high voltage differential between high impedance gates and the base substrate of the cmos chip.

The voltage punches a hole in the insulating oxide layer.

TTL chips and the gated devices in ICs can be damaged by excess current but that current is from a current source, not the kind of small static charge that is developed when you carelessly handle a cmos chip.

The use of aluminum foil for storage of static sensitive components is safe because there can not be an 'appreciable charge' on a given pin without the device already having been destroyed by the voltage!

Do some calculations and see what kinds of voltage one would need to produce enough coulombs of charge carriers to produce damage from excessive current on any IC. Remember, you have only the volume of the metal conductors involved to hold those charges.

Envision a capacitor, fully charged (the floating gate). Charge it to the MAXIMUM voltage that it can stand. Now throw a DEAD SHORT across the leads that feed that capacitor and look at the current flow as the cap discharges.

Compare that current with the normal charge/discharge currents that flow as pulses drive the gate when the IC is mounted and being used normally. Look at the rise and fall times. Look at the conductor materials used on the chip and connecting the chip to the lead. Find the weakest point along the current path and compute the maximum peak current that can flow in that conductor and for how long that current can flow before it causes damage. [remember, current causes damage by heat.]

Now, check to see if the max permissible voltage could possibly produce that current.

I think you will find that even chips that have built in weak conductors ['fuses' designed to be burned open by current flow] could not possibly be damaged by the small charge allowable between any two pins of a CMOS device.

Now, there MIGHT be some conditions where metal foil would NOT be a good idea, like those where electrolysis could develop, but we are not discussing those.

Another condition would be where there are high intensity ELECTRIC fields nearby, STRONG Pulsed magnetic fields or RF fields nearby. [such as EMP] But then the chip would be destroyed even if it were soldered into a circuit.

I have worked with components that are VERY static sensitive (point contact detector diodes used in radars) that were ALSO easy to damage with excessive current.

We sometimes had RF fields around that could cause excessive current flow. We kept the diodes wrapped in foil until we were installing them.

There are times when EM shielded rooms, anti-static mats and wrist straps are not available. When they are not available, I work on a sheet of aluminum foil and make sure I touch the foil and the component before the component touches the foil.

--
bz

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

bz+spr@ch100-5.chem.lsu.edu   remove ch100-5 to avoid spam trap

I do the same as I get lots of it with chips that don't even need it. The OP I presume doesn't have any. Anyway best practice is obviously to use the right stuff, in the meantime the foil should provide a degree of protection better than nothing. I don't think I will be recommending foil in the future though!

Gareth.

Yes. It seems strange that all chips seem to be packaged in the same way.

I must admit to never having taken any precautions when fitting any chip - but then I don't wear any artificial fabric clothes.

--
*If God dropped acid, would he see people?

    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.

"Dave Plowman (News)" wrote in news: snipped-for-privacy@davenoise.co.uk:

Perhaps you live in an area where the relative humidity is rather high. In which case, static electricity is seldom a problem.

If you lived in [for example] Wyoming, where RH is often in the low teens or lower, THEN you would need to be careful. And more careful in the winter [warming air dries it].

--
bz    	73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

bz+ser@ch100-5.chem.lsu.edu   remove ch100-5 to avoid spam trap

I always know when the humidity is low because then every time I get out of my car I get a shock when I touch the bodywork. The static comes from sliding myself off the seat on the way out. It only ever happens in warm weather. Probably not a good idea to be holding any RAM chips or sensitive Radar detectors during these times. Or a small animal.

Gareth.

You can damaged an IC at levels that won't generate a spark. OTOH, 'low humidity' in Central Florida is still over 50%. :(

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Michael A. Terrell
Central Florida

"Michael A. Terrell" wrote in news: snipped-for-privacy@earthlink.net:

Yep.

Likewise here, in Baton Rouge, LA.

--
bz    	73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an 
infinite set.

bz+ser@ch100-5.chem.lsu.edu   remove ch100-5 to avoid spam trap

You can certainly have a charge that does not exceed the breakdown potential of the IC oxide layer yet results in probably several AMPS of instantaneous current when shorted into close to zero ohms through an aluminium foil sheet.

Why do you NITWITS feel the need to argue about the BLEEDING OBVIOUS ?

Graham

I'm pleased to hear that.

Graham

The best advice around.

Only wear cotton and cotton mixes. Ensure a decent level of humidity.

THEN treat the floor with anti-static spay.

Graham