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Question I have one question, how is the surface resistance calculated? Why is its unit ohm per square? -Anonymous, Tianjin, China
Answer The unit for surface resistance is the Ohm. The unit for surface resistivity use to be the Ohm per square, but we now refer to it as the Ohm also to minimize confusion. The way surface resistance is calculated is to take two electrodes (probes) and place them on the planar material to be measured, then apply a voltage (either 10 volts or 100 volts, depending on the materials actual surface resistance) and then measure the resulting current between the two electrodes. Now, assuming that the material is linear you can then apply Ohms Law and calculate the surface resistance (Resistance = Voltage/Current). Resistance: property between two points on a material that indicates its ability to conduct a current (charge flow) between the points. The higher the resistance, the harder it is for a current to flow through it. Resistance can be measure several ways. For ESD, because we tend to measure very high resistances, we need to use a Megohmeter. Typical resistance measurements involve two 5 lb probes on top of a planar surface (mat) that will give you the RTT value (resistance from point-to-point). Another resistance reading is RTG (resistance from point-to-ground) which uses only one 5 lb probe and the other connection goes right to the common point ground snap. Resistivity: a basic property of a material that is independent of where the measurement occurs. it has a geometrical relationship to resistance. Can be measured with a concentric ring probe or using the back of the meters parallel rails. Concentric Ring Probe:one way to measure surface resistivity in ESD, using a 5 lb. probe that has two conductive rings on the measuring surface. One ring is the size of a women’s bracelet, the other like a silver dollar. These two rings share the same "center point" and hence are concentric. A voltage is applied between the two rings and (because they are concentric) almost all of the current is collected and registered into the meter. Knowing the current and voltage, you can derive the resistance (Ohms law....see attached). The difference (relationship) between resistivity (using the concentric ring probe) and resistance is typically one magnitude (x10). Resistivity is/should be one magnitude higher than resistance for homogeneous materials. Deviations:Deviations will occur in your readings. I don’t know why you had yours, but here are some causes of those inconsistencies:- not using 5 lb probes, but just clips - probes are dirty (poor contact resistance) - banana/phone plugs are not snuggly installed into the jacks - low battery - dirty substrate (mat or floor surface) - measuring multi-layer materials (tunneling effect) - very low/high humidity - crossed wires (from meter)- pressing down to hard or not hard enough (when making railing measurements) - measurements are made over a conductive base (i.e., a mat on top of a metal -surface) - at 10 volts instead of 100 volts and vice versa - using different meters for your readings - when using the probes vs. rails, make the proper adjustment on the reading - a puddle of coffee exists between the two probes - etc....
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