Resistivity and Resistance in ESD Control

There is a lot of confusion out there as to what the difference is between resistivity and resistance. We get asked questions on a regular basis so hopefully this post will put an end to any misunderstanding – we’ll explain the difference between the two and will point out the measurements you really need to worry about when it comes to your ESD Control Programme.

The difference between Resistivity and Resistance
Resistance or resistivity measurements help define the material’s ability to provide electrostatic shielding or charge dissipation.“ [Source]
However, resistance and resistivity values are not interchangeable. Let’s get a bit technical here to illustrate the difference between the two:

1. The resistance expresses the ability of a material to conduct electricity. It is therefore related to current and voltage. In fact, the surface resistance of a material is the ratio of the voltage and current that’s flowing between two pre-defined electrodes.
With a pure resistive material,

Resistance Calculation

where:

– R is the resistance (expressed in Ohm Ω),
U is the voltage (expressed in Volt) and
I is the current (expressed in Amp).

The unit of measure for surface resistance is W. It is important to remember that the surface resistance of a material is dependent on the electrodes used (shape as well as distance). If your company implements an ESD Control Programme compliant to the ESD Standard EN 61340-5-1, it is therefore vital to carry out surface resistance measurements as described in the Standard itself.

2. The surface resistivity of a material describes a general physical property. It is not influenced by the shape of the electrodes used or the distance between them. “Surface resistivity, ρ,  can  be  defined  for electric current flowing across a surface as the ratio of DC voltage drop per unit length to the surface current per unit width.” [Dr. Jaakko Paasi, VTT Industrial Systems: “Surface resistance or surface resistivity?”]
As Dr. Jaakko Paasi describes in his research note, surface resistivity can be expressed by using a concentric ring probe as  

Resistivity Calculation

where:

– k is the geometrical coefficient of the electrode assembly,
rcentre is the outside radius of the centre electrode and
router is the inside radius of the outer electrode.

For the electrodes recommended by EN 61340-5-1, the coefficient k = 10.
The unit of measure for surface resistivity is W but in practice you will often see W/square (which technically is not a physical unit).

As previously explained, the surface resistivity does not depend on shape or distance of the electrodes used when performing the test. You can compare results freely – no matter what type of electrode was used to get the measurements in the first place.

Converting from Resistivity to Resistance
Values of surface resistance and surface resistivity become comparable if the measured surface resistance value is multiplied by the geometrical coefficient of the used electrode fixture.” [Dr. Jaakko Paasi, VTT Industrial Systems: “Surface resistance or surface resistivity?”]
If you measure surface resistance according to EN 61340-5-1, then the corresponding surface resistivity can be calculated by multiplying the resistance value by the geometrical coefficient factor k = 10. Likewise, surface resistivities can be converted to surface resistances by dividing the surface resistivity value by 10.

Per User guide EN 61340-5-2:1999 Clause 4.1.1 “Point-to-point resistance has been discussed, rather than the surface and volume resistivity which was found in previous standards and reports. This change has been made to cater for non-homogenous materials, which are becoming increasingly common in these applications, as well as ease of measurement.
Particular care is needed in interpreting results when measuring non-homogeneous materials such as multilayer mats or conductive-backed synthetic fibre carpeting containing a small amount of conductive fibre. Buried conductive layers can provide shunt paths. Be clear when stating what you have measured!

A few notes in regards to measuring surface resistance and resistivity:

  • On large surfaces, such as bench-mats, readings will sometimes vary with increasing time of measurement. This is due to the ‘electrification’ of the mat beyond the area measured. It is therefore important to measure properly and to keep the duration of measurement constant. Fifteen seconds is an arbitrary but practical duration for measurement time.
  • Moreover, the materials needing to be checked in an EPA are most of the time, non-conductive polymers that have been made conductive or antistatic by addition of conductive particles or by special treatments during manufacture. The resistivity of such materials may vary from one point to another or they may be direction dependent (anisotropic).
  • EN 61340-5-1 goes some way to specifying the procedures to be followed and test probes to be used, so that the results can be compared, at least roughly.
  • Also, the resistance of some materials may vary with humidity level and temperature. It is therefore good practice to take a note of these two parameters when measuring.

So now that we’ve identified what the difference is between surface resistance and resistivity, there is one more thing we want to cover in today’s post: the different types of surface resistances you will come across when dealing with ESD and how to measure them:

1. Resistance to Ground (Rg)
Resistance to Ground is a measurement that indicates the capability of an item to conduct an electrical charge (current flow) to an attached ground connection. The higher the resistance in the path, the more slowly the charge will move though that defined path.” [Source]
The Resistance to Ground is measured to ensure that surfaces in an EPA are correctly grounded. This is certainly one of the most useful measurements in an EPA.

Performing a Resistance to Ground TestPerforming a Resistance to Ground Test

To perform the test:

  • One 2.3kg cylindrical probe is required for this measurement.
  • Connect the probe to a megohm meter and place it on the surface to test.
  • Connect the other ohmmeter lead to earth or to an ESD ground point.
  • Measure the resistance at 10V for conductive items and 100V for dissipative items.

2. Resistance Point-To-Point (Rp-p)
A point-to-point measurement used during the qualification process evaluates floor and worksurface materials, garments, chair elements, some packaging items, and many other static-control materials.“ [Source]
Resistance Point-To-Point is used to assess the performance of an item used in an EPA.

Performing a Resistance Point-To-Point Test

To perform the test:

  • Two 2.3kg cylindrical probes are required for this measurement
  • Connect the probes to a megohm meter.
  • Place the material to be tested on an insulative surface such as clean glass and place the probes on the material.
  • Measure the resistance at 10V for conductive items and 100V for dissipative items.
  • Move the probes so as to measure in a cross direction and repeat the test.

Point-to-point measurements are important during the qualification process for proper evaluation of flooring and worksurface materials. After installation, the resistance-to-ground measurement is more applicable since it emulates how the material really behaves in practice.” [Source]

3. Volume Resistance (RV)
Although this is one of the less common measurements when it comes to ESD, it’s still worth to mention the volume resistance here. You would measure the volume resistance when a non-grounded item such as a container is to be placed on a grounded item, such as a mat. The volume resistance will indicate whether the item can be used in the desired manner.

Performing a Volume Resistance TestPerforming a Volume Resistance Test

To perform the test:

  • Two 2.3kg cylindrical probes are required for this measurement
  • Connect the probes to a megohm meter.
  • Put the first probe upside down and ‘sandwich’ the test sample between it and the second probe placed on top.
  • Measure the resistance.

So hopefully we have put an end to any confusion in regards to surface resistivity and resistance and answered all your questions. If there is anything else you’d like to know, let us know in the comments.

References:

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About descoeurope

Desco Europe is the newest brand in the Desco Industries family, consolidating our two previous UK-based brands, Charleswater and Vermason. Charleswater, originally located in W. Newton, MA, was acquired in 1990. After several years of offering both Desco and Charleswater products in the US, Charleswater was set up to be the leading brand in the UK with the acquisition of Conductive Products in 1998. Vermason was a manufacturer of ESD protection products and was founded in Letchworth in 1979. In April 2007 Desco Industries, Inc. of Chino California acquired 100% of the shares of Vermason Ltd. Vermason is now a division of Desco Industries, Inc. Today, as Desco Europe, we retain our manufacturing capability in Letchworth, Hertfordshire, with sales and administration being split between Letchworth and our Crowborough, East Sussex office. Desco Europe sells the full range of DII products, manufactured in our facilities in the USA and UK, servicing the European market via trained distributors who will sell the Desco Europe value package and complete ESD solution to all ESD users in their territory, leading with hi-end solutions that mark us out from the competition. Desco Europe’s UK-based inventory coupled with that of our distributors means that we are in a unique position to support our Customers across Europe no matter what their needs.

Posted on 2017-07-20, in ESD Information, How-To, Question & Answers, Resources. Bookmark the permalink. Leave a comment.

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