Resistance Point-To-Point

Definitions of Resistivity and Resistance in ESD Control

Image scsstaticcontrol August 10, 2017 1

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 Program.

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:

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, where:
– R is the resistance (expressed in Ohm W),
– U is the voltage (expressed in Volt) and
– I is the current (expressed in Amp).The unit of measure for surface resistance is ohms (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 Program compliant to the ESD Standard ANSI/ESD S20.20, it is therefore vital to carry out surface resistance measurements as described in the Standard itself.
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
where:
– k is the geometrical coefficient of the electrode assembly,
– r_centre is the outside radius of the centre electrode and
– r_outer is the inside radius of the outer electrode.For the electrodes recommended by ESD TR53 (Compliance Verification of ESD Protective Equipment and Materials), the coefficient k = 10.The unit of measure for surface resistivity is ohms (W) but in practice you will often see ohms/square (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 ESD TR53, 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.

Care is needed in interpreting results when measuring non-homogeneous materials such as multilayer mats or conductive-backed synthetic fiber carpeting containing a small amount of conductive fiber. 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 worksurface 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).
ESD TR53 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 (R_g)
“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.

To perform the test:

One 5lb cylindrical probe is required for this measurement.
Connect the probe to a megohmmeter 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 (R_p-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 5lb cylindrical probes are required for this measurement
Connect the probes to a megohmmeter.
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 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 (R_V)
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.

To perform the test:

Two 5lb 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:

Jaakko Paasi, VTT Industrial Systems: “Surface resistance or surface resistivity?”
David E. Swenson, Affinity Static Control Consulting: “Electrical Resistance and Resistivity”
ESD Association, Inc.: ESD Fundamentals – Part 3: Basic ESD Control Procedures and Materials

Checking your ESD Control Products

Image scsstaticcontrol July 27, 2017 0

Last time we explained how to easily create a compliance verification plan and why it’s important to have one in place. Today’s post will elaborate on the subject of periodic verification and highlight common products in your EPA that should be regularly verified and more importantly how they should be checked.

Why periodic verification
Compliance verification is a requirement of ANSI/ESD S20.20:
“The Organization shall prepare an ESD Control Program Plan that addresses each of the requirements of the Program. Those requirements include:
– Training
– Product Qualification
– Compliance Verification
– Grounding / Equipotential Bonding Systems
– Personnel Grounding
– ESD Protected Area (EPA) Requirements
– Packaging Systems
– Marking” [ANSI/ESD S20.20 clause 7.1 ESD Control Program Plan]

Installed ESD Control products must be checked regularly to ensure they meet the required limits per the ESD Standard. “Compliance verification records shall be established and maintained to provide evidence of conformity to the technical requirements. The test equipment selected shall be capable of making the measurements defined in the Compliance Verification Plan.” [ANSI/ESD S20.20 clause 7.3 Compliance Verification Plan]

Below, you will find a list of the most common ESD Control Products in your EPA and how to test them:

Worksurface Matting
The purpose of ESD workbench matting is to ensure that when charged conductors (conductive or dissipative) are placed upon the surface, a controlled discharge occurs and electrostatic charges are removed to ground. However, this only occurs if the ESD worksurface is connected to ground. If the matting is out-of-spec, not grounded at all, or the stud on the mat has become loose or if the ground cord has become disconnected, charges cannot be removed.
Many companies use a daily checklist, which includes the operator having to verify that ground cords are firmly connected.
Remember to regularly clean your workbench matting to maintain proper electrical function. Do not use cleaners with silicone as silicone build-up will create an insulative film on the surface.
The company’s compliance verification plan should include periodic checks of worksurfaces measuring:

Resistance Point-to-Point (Rp-p) and
Resistance-to-ground (Rg)

A surface resistance tester can be used to perform these tests in accordance with ANSI/ESD S20.20 and its test method ESD TR53; if these measurements are within acceptable ranges, the worksurface matting and its connections are good.

Wrist Straps
As discharges from people handling ESD sensitive devices cause significant ESD damage, the wrist strap is considered the first line of ESD control.
Before handling ESD sensitive items, you should visually inspect the wrist strap to see if there are any breakages etc. The wrist strap should then be tested while worn using a wrist strap tester. This ensures all three components are checked: the wrist band, the ground cord (including resistor) and the contact with the operator’s skin. Records of each test should be kept. Wiggling the resistor strain relief portion of the coil cord during the test will help identify failures sooner. Analysis and corrective action should take place when a wrist strap tester indicates a failure.

It is recommended that wrist straps are checked at least daily. An even better solution to daily wrist strap checks is the use of continuous monitors. They will alarm if the person or worksurface is not properly grounded.

A note on worksurface matting and wrist straps: if you are using common ground points to ground the operator and/or work surface matting, remember to measure resistance to ground regularly as well (every 6 months for example).

Floor Matting
A flooring / footwear system is an alternative for personnel grounding for standing or mobile workers. Foot grounders quickly and effectively drain the static charges which collect on personnel during normal, everyday activities. Foot grounders should be used in conjunction with floor surfaces which have a surface resistance of less than 10¹⁰ ohms.
As ESD floors get dirty, their resistance increases. For optimum electrical performance, floor matting must be cleaned regularly using an ESD mat cleaner. Do not use cleaners with silicone as silicone build-up will create an insulative film on the surface.
Dissipative floor finish can be used to reduce floor resistance. Periodic verification will identify how often the floor finish needs to be applied. As the layer(s) of dissipative floor finish wear, the resistance measurements will increase. So, after some amount of data collection, a cost-effective maintenance schedule can be established.
Floor matting can be checked using a resistance meter. A surface resistance meter is designed to measure resistance point-to-point (Rp-p) or surface to ground (Rg) in accordance with ANSI/ESD S20.20 and its test method ESD TR53.

Footwear
ESD Shoes or foot grounders play an essential part in the flooring/footwear system.
Before handling ESD sensitive devices, visually inspect your ESD footwear for any damage. Just like wrist straps, footwear should be checked while being worn using a wrist strap/footwear tester.

Records of each test should be kept. Analysis and corrective action should take place when a footwear tester indicates a failure. Footwear needs to be checked daily.

ESD Packaging
Re-using shielding bags is acceptable as long as there is no damage to the shielding layer. Shielding bags with holes, tears or excessive wrinkles should be discarded.

An operator packing an ESD sensitive item into a Shielding Bag — *Make sure your ESD shielding bags are un-damaged*

It is up to the user to determine if a shielding bag is suitable for re-use or not. The testing of every bag before re-use is not practical. Many companies will discard the shielding bag once used and replace it with a new one. Others will use a system of labels to identify when the bag has gone through five handling cycles:

Non-reusable labels are used that require the label be broken to open the bag.
The bag is then resealed with a new label.
When there are five broken labels, the bag is discarded.

The same principle applies to other ESD packaging, e.g. component shippers.

Ionizers
Ionizers are intended to neutralize static charges on insulators thereby reducing their potential to cause ESD damage. However, poorly maintained ionizers with dirty emitter pins and out-of-balance ionisers can put a charge on ungrounded items.
Remember to clean ionizer emitter pins and filters regularly. You can now even purchase ionizers that will alarm when emitter pins need to be cleaned or the ionizer is out of balance.

Charge plate monitor and static decay measurements using 963E ionized air blower

Static neutralization (the ability to reduce or eliminate a charge on a surface) is an important quality for ionizers. Static decay time is defined as the time interval needed to reduce a defined voltage potential on an object to a defined lower potential by means of applied ionized air. Another important aspect for ionizers is the ability to produce a balanced stream of positive and negative ions. A charged plate monitor or equivalent can be used to accurately measure both of these parameters.
For more detailed information on measuring the performance of ionizers refer to the ESD standard ANSI/EOS/ESD-S3.1 for Protection of Electrostatic Discharge Susceptible Items-Ionization.

Wrist Strap/Footwear and Resistance Testers etc.
So, you check your wrist straps and/or footwear and workbench and/or floor matting regularly. But have you remembered the testers themselves? What good do all the checks do, if the testers you use are out-of-spec and show you incorrect results?
Yearly calibration is recommended – many manufacturers offer a calibration service or alternatively you can purchase calibration units from them and perform the calibration yourself.

There you have it – a list of the most commonly used products in your ESD Protected Area (EPA) that you should check on a regular basis.
Questions for you: Do you have a verification plan in place? If so, how often do you check your ESD protection products?