05 Dec

Q: I have one question/problem that I would like to ask regarding an experiment to test a bench top air ionizer. From the attached documents, there are 2 graphs of decay time versus effective distance, one for decay time on negative charges and the other one for decay time on positive charges. As you can see, the closer the bench top air ionizer (effective distance), the lesser the decay time will be. My question is, why is it during the distance of 20 to 24 inch the graph line become a straight line (saturated) and not growing linearly like the other points?

A: I see you’re using the Bench Top style air ionizer. The “PC” means that it is “targeted” coverage as opposed to “extended” coverage. The posted operating range appears to be from 1′ x 5′ or 12″ to 60″.

Your graphs range in distance from 4″ to 32″. I’d consider using a test method recommended by EOS/ESD S3.1 if you haven’t already.

Was your humidity really at 80%?

This ionizer also has a heater function. I’m not familiar with that particular feature, whether or not it is a factor here.

Ionizers have two properties that ANSI EOS/ESD S3.1-2000 defines through the use of a CPM; discharge time and offset voltage.

Ionizers increase the electrical conductivity of air, which is especially useful in environments that use insulators which cannot be removed from the EPA (such as PC boards). Grounding an insulator doesn’t remove it’s electrostatic charge. They also reduce the effect of the earth’s field, which increases with the altitude above the work surface of the DUT(device under test).

Now things get complicated.
Discharge time:
Ionizers decrease the charge on the CPM exponentially with the time constant RC.
R = resistance of air (Keep in mind that air resistance increases with the distance that ions must travel).
C = capacitance of the plate. ( Keep in mind that smaller objects have lower capacitance and the time to discharge them maybe shorter than the discharge time).

Offset Voltage:
This deals with induced potential on objects. It has been found that the mobility of negative and positive ions are different. Thus, there is a small electric field generated which is zero at the work bench surface (if dissipative ESD mats are being used) and largest as you get closer to the ionizer. We use an isolated system, which reduce this effect by a simple law of nature- charge cannot be created or destroyed in an isolated system. With the more sensitive devices, such as an MR head on a disk-drive, I’d be very careful here as no system is perfect. Will electrical potential damage a device, or will the rate of current discharge do the damage?

Unfortunately, there’s other things involved with your chart, such as the mere dimensions of your plates and the distance from them. Really close to the plates, the effects of decay time are linear and are affected by a plane source. Further out to some point, you may be dealing with a line source which drops off as the function of L/2 where L = length from the plate, and ultimately, when you get 7 times that length or 7L, you are dealing with an inverse square or L / 4 equation- or point source- out there you are parallel to the source or detector. An example of this equation is to calculate the surface area of a sphere with a diameter of 2 units as opposed to one with a diameter of 4 units. Suffice to say, nature is too complicated to be linear.

I’ve only scratched the surface on your question here and I’d like to give it more thought. I’d run the experiment myself here, but I currently have some equipment in for calibration. For now, let’s move in the direction of testing per EOS/ESD S3.1.