Fixture for measuring surface resistance - Page 1
Forum section: electronics / projects

author: skipjackrc4
author_rank: Regular Contributor
timestamp: April 29, 2016, 03:43:43 am
content: I have a sheet of some resistive foil and I'm trying to measure the surface resistance (units of Ohm/square). The spec is around 1 kOhm/sq., which is well within the range of a standard DMM.My initial thought was to use two narrow 1" long electrodes and place them 1" apart on the sheet and then measure the resistance between the electrodes. This method would work if I could cut the sheet into an appropriately sized square, but that's not an option in this case. I need to make the measurement in the middle of the sheet. My concern is that the resistive material surrounding the immediate measurement area will appear in parallel with the measured current path and result in an artificially low resistance measurement.Has anyone on the forum done such a measurement? Is there a better way for making a surface resistance measurement in the middle of a sheet?

author: Cerebus
author_rank: Super Contributor
timestamp: April 30, 2016, 06:53:21 pm
content: I'd offer a potted answer but I don't have it at my fingertips and this stinking cold I have is making my mind work like it's in treacle so I'm currently incapable of more sophisticated maths than 2 + 2 = ATishoo.This is just the old "infinitegridof resistors" problem except it's a "finitesheetof resistors". Look up (Google is your friend) the former and you'll be able to work out how to correct your suggested measuring method to account for the 'extraneous' resistive sheet element. Dave has a video where he explores the resistor grid problem and also one on sheet resistance. My guess is that the answer is 'double your reading' but I don't trust my memory under the current viral onslaught.

author: FrankE
author_rank: Regular Contributor
timestamp: April 30, 2016, 08:28:41 pm
content: Why use old imperial units for distance when you are using SI and SI derived units (ohms, Siemens ...)?

author: DimitriP
author_rank: Super Contributor
timestamp: April 30, 2016, 08:58:46 pm
content: The setup of Fig 1 looks promising without going circularhttp://www.trekinc.com/pdf/1005_Resistivity_Resistance.pdf

author: TimFox
author_rank: Super Contributor
timestamp: April 30, 2016, 09:23:52 pm
content: To avoid the shunting from the rest of the sheet, why not use a central disc and a concentric annulus to define an annular region with no "fringing" problems?

author: Cerebus
author_rank: Super Contributor
timestamp: April 30, 2016, 10:48:06 pm
content: Quote from: FrankE on April 30, 2016, 08:28:41 pm1) Because he's 'merkin.2) It's ohms per square. The square doesn't care what units it's in as long as it is square. Could be bee's dicks, could be inches, could be angstoms, just as long as it's the same number of them on each edge.

author: sarepairman2
author_rank: Frequent Contributor
timestamp: May 01, 2016, 01:01:14 am
content: be sure to standardize force over area for your experiment.an esd mat is done with 5lbs for instancesearch user "robrenz" threads, he has made a thread related to a white paper on surface resistance which illuminates many behaviors that we see. many of his resistance topics are very interesting

author: richard.cs
author_rank: Super Contributor
timestamp: May 01, 2016, 08:49:09 am
content: Make a four-wire measurement with four equally spaced point contacts in a line. The contacts need to be far apart compared to the sheet thickness and many spacings from the edge of the sheet. You can then calculate sheet resistance - see here:http://four-point-probes.com/four-point-probe-equations/This means you don't have to cut the sheet or worry about it's exact shape.