Nylon (polyamide) PCB standoffs reduce ESD problem?

[treez]

Hi
Presumably the reason these PCB standoffs are made of nylon (polyamide) is that it has lower resistivity than most other plastics?
-And in turn this means less ESD problem when using them?

Nylon (polyamide) standoff:
https://uk.farnell.com/wurth-elektronik/971200365/standoff-hex-male-female-28mm/dp/2988262

Resistivity of various plastics and nylon:
https://www.professionalplastics.com/professionalplastics/ElectricalPropertiesofPlastics.pdf

 

[Easy peasy]

you get what you get with a low cost nylon, that fact that it is highly resistive is a good thing if you are supporting LV boards off high voltage mains boards ...

 

[betwixt]

Nylon 6.6 is normally used because it is 'self lubricating', it slides easily and is easy to fit a nut to the thread. It DOES generate static under friction conditions but as a PCB spacer that shouldn't be a problem. They are cheap to make and versatile, that's why they are used, it has nothing to do with ESD and in many instances wouldn't be touching a ground at either end anyway. With a resistivity of around 10,000M per square cm it wouldn't have much effect on a static charge!

The carbon loaded variants ARE conductive and might be of some use in ESD situations but only if carefully placed. As a general purpose spacer they are no better than plain Nylon ones.

I use them to mount PCBs in watertight enclosures because the slight elasticity allows a tightened nylon nut on a nylon threaded pillar to deform slightly around the mounting hole and seal around it.

Brian.

 

[c_mitra]

PCB standoffs are made of nylon (polyamide) is that it has lower resistivity than most other plastics?

Cost the most important factor for business.

There are several kinds of nylon available: they are all useful for socks to innerware to standoffs.

Nylon 6 is made from one molecule: NH2-CH2-CH2-CH2-CH2-CH2-COOH

Nylon 66 is made from two: NH2-(CH2)6-NH2 and COOH-(CH2)4-COOH

And there are more.

It can be mixed with MoS2 (a lubricant like graphite) - looks black- to make plastic parts that is self-lubricating.

Or it can be filled with carbon fibers.

Measuring conductivities of plastics (both surface and volume) have not been trivial. And getting reproducible values are harder.

But cheaper alternatives will be PVC (useful for shoe soles and wind cheaters) but they do not machine well (don't know why).

Also PVC can take up lots and lots of fillers (from clay to coal).

 

[treez]

Hi,
We are trying to work out which has the lower resistivity......

1…this floor tile, described as with a “surface resistance” of 10^11 “ohm per metre squared”
https://www.antistaticesd.co.uk/shop/anti-static-esd-mats/heavy-duty-esd-7mm-interlocking-tiles/

or,

2…Nylon 6, 6 standoffs, said to have a “surface resistivity” of 10^11 “ohms/sq”
https://www.professionalplastics.com/professionalplastics/ElectricalPropertiesofPlastics.pdf

Also, there seems no uniformity of units….resistivity surely has units of Ohm.Metres?

Also, if the Nylon 6,6 and that floor tile both have the same resistivity, then how can the floor tile be useful in ESD protection?

 

[d123]

Hi,

there seems no uniformity of units….resistivity surely has units of Ohm.Metres?/QUOTE]

There's a great A D or LT app note that explains, amongst other things, how to use pcb copper as sensing resistor, and the formula is basically that copper-clad squares on an e.g. 1oz pcb that are respectively n and 10n size/dimensions will have the same resistivity.

This article explains it:

Counting squares: A method to quickly estimate PWB trace resistance

- - - Updated - - -

Nice quote from article:

"We now know, for example, that a square of ½ oz. copper has a resistance of about 1 mΩ. This is regardless of the size of the square."

I think that may be why the nylon standoffs say ohms/sq.

 

[treez]

Thanks yes, as you know thats because the ratio Length/Area is the same for each of those squares, and resistance = resistivity.Length/Area

But i am not sure what the unit "ohms/sq" means for the nylon 6,6 in the above.

Yes thanks d123, i suspect it means resistance from one side of a square tile to the opposite side...at 10^11 ohms, it makes me wonder how its useful in ESD protection.?
It surely wont stop static charges from being generated?

 

[Easy peasy]

ohms/square is commonly used, as in pcb track of a fixed thickness, it will have so many ohms/square, for 20 squares in a row, you will get 20 x R

for a 4 x 4 arrangement of sqaures you just get R again, as 2 long but 2 wide also ... this makes it easy to work out R's for track lengths ...

BTW a square 1cm and a square 1m have the same R

for ESD dissipation, total resistance must be 10 Meg or less ....

- - - Updated - - -

1 Oz Cu, 35um, is 0.5 milli-ohm per square ...

 

[c_mitra]

each of those squares, and resistance = resistivity.Length/Area

That is for volume or bulk resistivity; consider a unit cube and you measure the resistance across two opposite faces.

For surface resistivity, you consider an unit square and measure the resistance across two opposite sides.

When you see unit like ohms/sq you know that you are dealing about surface resistivity. The sq is superfluous but kept as a convention. It just tells that you are talking about surface resistivity.

And yes, at 10^11 ohm sq, it is not useful for ESD protection.

 

[treez]

And yes, at 10^11 ohm sq, it is not useful for ESD protection.

Thanks, but this ESD floor tile has a "surface resistance" of 10^11 ohm per metre squared...are we saying its no good?
https://www.antistaticesd.co.uk/shop/anti-static-esd-mats/heavy-duty-esd-7mm-interlocking-tiles/

...i understand it wont be ESD dissipative, but will possibly be ESD non-generative?

 

[FvM]

this ESD floor tile has a "surface resistance" of 10^11 ohm per metre squared

That's a nonexisting unit. Obviously a typo. Correct unit is ohms/sq, see https://en.wikipedia.org/wiki/Sheet_resistance

 

[c_mitra]

understand it wont be ESD dissipative, but will possibly be ESD non-generative?

It takes two to produce some ESD. If both the materials have similar work function, they won't produce much static when rubbed together.

Even if one of them is conducting, it can produce a lot of static. A steel tanker carrying a liquid fuel (petrol for example) can produce lots of static in the fuel (the tank can be easily grounded)- simply due to fuel sloshing in the tank.

I do not have a ready reference, but it was taught in our school that rubbing one (list of various insulating materials) with another (say another list like flannel, silk, wool etc etc) can produce lots of static.

Rubbing the same material with itself will not be useful.

You can easily produce static by rubbing a plastic film on a metal table.