Why do I get different resistances when measure the same (flat and film) material (such as dissipative mat) by using standard 2 point-probe and 2 x 5lb electrodes ?
I know I use two different kinds of electrons to make measurement, but what if the material under test is or isn’t homogeneous, it is supposed to happen?
If the material under test is nonhomogeneous (such as coating, laminate, loaded), how can I use instruments to identify?
Can I use 2 x 5lb electrodes (point to point measurement) to define material’s property?
When to use 2 point-probe to make measurement(except for size)?
You ask some very good questions. The differences in using most of the probes is really in accuracy. For a planer material, the concentric ring electrode is the most accurate. However, it does require a somewhat large sample size with a flat surface. It does have its limitations. If the material is not homogenous, the most conductive path may be the only one that is being measured.
The two 5 lb electrodes will measure point to point but is not typically used for surface resistance. These are used for worksurfaces and floors and will take into account multilayer constructions. If the materials are homogenous, then the ring electrode and 5 lb electrodes will be similar.
The two point probe is used when there structures are to small to use the ring electrode. If a material is not homogenous, then measurements can be significantly different between the ring electrode and the two point probe and different with the two point probe in different places on the material.
With that information, you can look at ANSI/ESD S20.20 for which method should be used for qualification. For worksurfaces, floors, garments and most ESD control items, the 5 lb electrodes are used. For packaging materials, the ring electrode is used. One reason for that is that the 5 lb electrodes are limited for the high resistance measurements that packaging uses.
The two point probe can be used in ANSI/ESD S541 along with the ring electrode for packaging material. For small structures there is no choice but to use the two point probe needs to be used.
For which one is correct or to be used for qualification, instead of comparing the measurements, see if the measurements meet the qualification limits. The limits are very broad and instead of comparing one measurement to the other, if both are within the qualification limits then the material can be considered qualified.
Really appreciate your detailed information and insight regarding these questions! Let me and our customers know more about S20.20 accurately.
For homogenous materials, the readings will be similar when use ring electrode, 5 lb electrodes or two point probe. In reality, most of material is non-homogenous making us hard to know the meaning of measurements/readings. As each instrument has its limits and application, and we have to understand what we’re trying to measure before apply any standard test methods according to ANSI/ESD S20.20.
We all agree that dissipative property material can lower down the rapid electrostatic discharge making sure ESD sensitive item protected. In other words, we “create” a path to let moving charge(current) slow down. Based on this concept, I wonder that:
How did ESD specialist formulate upper or lower limits (e.g. resistance, voltage) in ANSI/ESD S20.20?
For example, why the resistance of most ESD control items are recommended to lower than E+9 ohms? Packaging material below E+11 ohms?
How much peak current or duration(nanosec, milesecond or microsec) can we decrease when we follow all this requirements ? Have any approximate range?
Again, thanks for teaching me a lesson! Learn a lot!
For the first question, part of the answer is how standards are developed. In all cases, it is a consensus of a technical committee that determines the requirements. It also depends on applications. In your question, there are two applications.
The first one, resistance to ground of less than 1x109 ohms was set by how long it would take to drain a charge from a tote, device or anything with a specific capacitance (not sure what capacitance was used). At 1x109 ohms to ground, if you place a tote or device on a surface, it would take less than 1 sec to drain the charge. Once you get over this limit, the duration increased. This was selected, rightly or wrongly, as it would take a person to put down a tote, connect the wrist strap and then open the tote and if there were materials that would meet this limit.
The packaging limit of 1x10*11 ohms was based on materials available at the time and application. In this case, packaging is not meant to drain the charge but to limit the charging of the device and to prevent outside events from causing damage. At this time, before static shielding bags, a high resistance bag worked to limit external discharges and it was thought that resistance limits would correspond to charging of devices. That relationship has not held but the resistance of packaging materials has remained. Since many materials meet these limits, and there have not been any published issues using these limits, there is no need to change them.
If you would like to learn more about standards and how they are developed, I invite you to join our virtual meetings in January. You can find out about our meeting at www.esda.org under events.