First Published in EOS/ESD Technology Oct/Nov 1987

Charge Versus Voltage Measurements

When to use which

by Peter R. Bossard, Ph.D.
Vice President of R&D, Voyager Technologies, Inc.,
Langehorne, PA

In trying to quantify the effect of static on semiconductors or electronic devices, the question often comes up: should I measure the voltage or the charge? For the most part, although charge is the more basic measure, this is a matter of convenience, depending on the type of measuring technique. That it, the technique to use is the one most likely to be misinterpreted.

ESD problems arise when a net charge (static) is generated on an object due to the tribolectric effect, the root cause of all ESD damage. Measuring the net charge on an object gives a direct indication of the relative merits of one technique versus another in preventing charge buildup. Therefore, measuring the triboelectric charge generated on objects is a direct means of assessing the effectiveness of an ESD control technique such as a mat or a work-surface.

However, it is not always convenient to measure charge on people and objects. In these cases, voltage measurements may be easier to quantify and less subject to error than charge measurements. Consequently, a voltage measurement is better for determining the effectiveness of an established procedure for preventing charge buildup, providing that the relationship of voltage to charge is kept in mind. Therefore, the choice of a charge or a voltage measurement depends on the object and the environments.

For a reliable charge measurement, the object must be placed in a Faraday cup, where all its electric field lines are terminated. This technique is useful on small items such as ICs, wafers, masks and small-to-medium size circuit boards that can be conveniently placed in a Faraday cup. One of the nice things about measuring the charge using a Faraday cup is that it does not matter whether the object in the cup is conductive or nonconductive-- the net charge is measured in either case.

However, when the object is too large to fit conveniently in a Faraday cup, the voltage on the object should be measured. Gross errors can occur if you try to transfer the charge by conduction from the object to the Faraday cup. To successfully measure charge by conduction to the cup, the entire object has to be conductive. The object must be completely isolated from ground and electric fields. Also, its capacitance must to be small compared to the capacitance of the Faraday cup.

An example of a tempting but incorrect procedure for measuring the charge on people in a work environment is to have a worker touch a Faraday cup, attempting to transfer his or her charge to the cup. There is often a small and very weak EMF (less than 1 V) on people and their clothing due to salts and other ionic elements and compounds. A Faraday cup is a sink for charge and will attempt to drain down this small EMF. The net result is that charge can be transferred to the cup, giving the impression that the object was highly charged when it actually had very little charge, or that the object was slightly charged when it actually had a large charge. To avoid these types of problems when objects cannot be placed in a Faraday cup, measure the voltage on the object.

Measuring the voltage on an object that is not a voltage source requires a special type of voltmeter. Keep in mind that the object has a limited amount of energy at a given voltage, and care must be taken to ensure the measuring instrument does not grossly affect the result. The general requirements are that the voltmeter have a high input impedance (>10¹¹W), a low input capacitance (< 5 pF), and a wide dynamic range (> + 10 Kv). Noncontacting and specialty voltmeters can meet these requirements.

When the proper precautions are taken, the voltage or charge on an object can be used as data to determine the effectiveness of ESD procedures. An important point to remember is that an object's environment, along with its detailed immediate history (just prior to measurement), can greatly affect the charge or voltage reading. Therefore, it is necessary to take enough measurements to get a statistically significant average charge or voltage. If the average of the charge and voltage (plus four times the uncertainty) is consistently low (less than the voltage or charge specification for that environment), then the ESD procedures in use are effective.