Fowler Associates Labs

First published in EOS/ESD Technology Feb/March 1989

Measure Resistance Instead: Part II

Resistivity and decay measurements can not only be misleading,
but can also be sometimes confused by misunderstandings
and misused terminology. The simple, unambiguous test described
here can provide much more reliable materials characterizations.

Link to Measure Resistance Instead: Part I

Ben Baumgartner
Lockheed Missiles & Space Co. Inc.
Sunnyvale, CA

Part 1 of this article (EOS/ESD Technology, Dec./Jan. 1989, p. 20) discussed some of the technical problems of materials characterization using resistivity measurements. Part 2 continues this discussion and proposes a simple, almost foolproof test for characterizing ESD-protective materials and packaging in the real world.

Circuit Models of Materials.

Considering only the resistive component, a circuit of an ideal material would look like the example in Fig 1a. The surface cell resistivity, R_s, for this ideal material is much less than the volume-cell resistivity, R_v. A surface-resistivity measurement would show RS cells as 40 W (adding up surface cells) without a large error because RS is little affected by the shunt effect of RV

Figure 1. Models of insulative (a) and buried-layer (b) materials.

The horizontal volume cells add up to 400 W and a path from the top to the bottom surface along the bottom and back to the top would add up to 120 W (four volume units X 10 W + 40 surface units X 1W + four volume units X 10 W). The current through this resistance path to te guard electrode is subtracted from the surface-resistivity measurement. When the value of RV becomes lower than ideal (insulative), it will affect the surface-resistance measurement. If the resistivity of the volume cells is changed to 1 W, the volume cell can be seen to dominate the measurement.

In terms of the number of cells, the length-to-thickness ratio of volume cells is 10 to 1 and wold translate into a 0.040-in. electrode spacing for a 0.004-in. (4-mil) material. ASTM D-257 requires a gap (g) of at least twice the material thickness, and in most circular-electrode designs this ratio is much larger when measuring 4-mil material.

If a buried layer of lower resistivity, or conductive material, is added, the example circuit could look like Fig. 1b. The resistivity value of RV per cell would not change, but the dominant path would be drastically altered.

The path from the surface of the material to the buried layer is short (two cells- 20 ohms), so the resistance is very low to conductive layer C. The predominant path has changed the point-to-point surface resistance to 40 W shunted by 44 W (2 V X 10 + 40 cell X 0.1 + 2 V X 10). The measurement is no mostly a volume measurement and is almost independent of electrode spacing.

But this is only an illustration; in any real material, resistance is more complex. For example, the surface-resistivity concept becomes extremely difficult to understand if applied to tabletop and garment materials.

Volume measurements encounter similar difficulties when different resistivity layers are present in a material. The layer with the highest resistivity dominates the measurement, and yet the thickness of all the layers will be used in calculating the resistivity. Wires or other conductive materials that may or may not be confined to a layer can result in measurements that simply can't be related to the concept of volume resistivity.

Resistance, Protection and Circuit Packaging

The reason for measuring package materials can be seen by following the two possible current paths in Fig. 1. (The three figures here show the equivalent circuits of ESD paths.) One possible discharge path is through the package wall, after which the IC pin carries the current (which goes through the component) to a pin on the opposite side of the circuit board. Finally, the current passes through the package to ground. The magnitude of this current must be considered for two types of sensitive devices. Diode junction or bipolar devices are protected if junction currents are limited to a dissipation rate that the devices can handle. If these currents are small, the voltage drops in the devices will not produce arcs across metallizaed IC traces, nor will these small currents melt IC traces or silicon in the junction areas. These type of devices are energy sensitive. The package resistance can be specified and provides protection from low-frequency current. The metal-oxide semiconductor in Fig 2 is voltage sensitive. A voltage-sensitive device passes no current thorough the oxide at voltages below oxide-breakdown potential. Even if a large amount of energy is available from a source, no energy is dissipated in the oxide below the breakdown voltage. However, when oxide breakdown voltage is exceeded, the device can be damaged by an exceedingly small amount of energy. Because voltage breakdown is necessary for small currents to flow, unprotected oxide is considered voltage sensitive. (When an oxide device is protected by a diode, the device acts like an energy-sensitive device and can be classed as such). For voltage-sensitive devices, resistances have been measured as high as 1 X 1014, including package-header resistance (which may account for much of what is measured). This high resistance causes extremely small currents passing through the bag to generate an voltage drop across the gate oxide. Therefore, currents caused by tribocharging the top surface could damage high-resistance, voltage-sensitive devices. Proper volume resistance measurements can determine the package resistance required for a given surface potential to limit the voltage drop across such a high device resistance. A drop of half the source voltage would require a total package resistance equal to the resistance of the device being protected. This is not sufficient to protect voltage-sensitive devices without the package providing an external shunt resistance to drop the source voltage to a manageable level. The resistance paths through the material and the conductive (metallized) layer shunt resistance, 2Rs (both sides), including the seam resistance, are shown in Fig 3. Package protection can only be estimated using resistances because the high-frequency effects of inductance, Ls, and seam bond capacitance, Cb, are not considered.

Figure 1. A pictorial of circuit paths in an ESD bag. Figure 2. The path through an ESD bag and a sensitive IC. Figure 3. Equivalent circuit in a metallized layer bag.

Problems with Methods

The point-to-point resistive components of a material are not given by Method 4046, and are not easily determined by the method in ASTM D-257 due to interrelated problems with materials and methods. An illustration of the problems was seen in Fig 4 and 5 of Part 1 for a buried-layer material, and applies to both measurement methods. Neither instrumentation method can separate surface and volume current for all ESD-protective materials.

If surface resistivity is measured without a guard electrode, the measurement is essentially a volume measurement, as discussed earlier, but, frequently, unguarded measurements occur when the electrodes cannot conform to irregular or molded materials, and, therefore, make poor contact.

There are a large variety of unguarded electrode sizes and designs that also affect the volume-current component of such a measurement. Electrode-contact areas change with the use of knife edges, braid or rubber electrodes of various configuration and should be expected to cause differences in measured values, as shown in the table. This table's measurements were made on tabletop materials with five different electrode designs.

[EOS/ESD Technology Editor's note: Although tabletop materials were measured as an example, this method is not proposed for tabletop measurements. Though similar, this method is different from the EOS/ESD standard worksurfaces measurements in that resistance is measured rather than surface resistivity. Baumgartner supports the measurement methods used in the EOS/ESD Association's worksurface standard, and is a member of the worksurfaces committee. The two measurement methods should not be confused.]

For volume measurements, multiple possible current paths can prevent accurate measurements. The presence or absence of a guard is an unknown variable, unless described in a material's data sheet, and affects the surface currents, I_s, present in the measurement. Surface current may also be present in volume measurements made on small samples-- samples that will usually differ from large flat specimens of the same material.

Volume current will be increased by a buried conductive layer or composite cloth with conductive-thread groups. The lower electrode in Fig 5 of Part 1 could receive increased current due to the material's conductive buried layer. This is significant if the E1 area is used for calculating volume resistivity, but the true area actually being measured is the unknown area contacting the lower electrode.

There may be as many electrode-configuration interactions with ESD materials as there are material types. Certainly no user can be expected to research or understand all of these problems. So resistivity measurements, while proper for ideal materials, are usually inappropriate for characterizing ESD materials in actual use.

In fairness, surface- and volume- resistivity measurement problems are not due simply to the instrumentation involved. When used properly on the right material, good instrumentation will produce correct values.

A Simple, Two-Point Resistance Measurement

Although resistivity measurements are useful during development of barrier materials and design of packaging systems, conditions outside the laboratory lead to the sort of measurement inaccuracy and confusion just described. However, ESD-protective bags, totes, and boxes need to be tested in the workplace, both before and after use. Blister packs and other specially formed packages must also be tested nondestructively.

Obviously, a test method suited to real-world use is needed. Therefore, we propose a simple, two-point measurement of resistance for use with dissipative-range materials.

The method uses 0.125-in-dia electrodes 0.75 in. apart for surface-type measurements. Surface resistance is measured with the electrodes on the same side of the material, while volume-mode measurements are made with electrodes on opposite sides of the material, and in line with each other.

Using this approach, any finished material can be measured in a specific location. Materials that have visible cells or pores, such as foams, can be characterized as well as more homogeneous samples, such as sheets, molded sheets, laminates, composites, cloths, and bubble packs. A measurement protocol would have to be established for some composite materials.

Figure 2. Diagrams of a two-point surface measurement (a) and a two-point volume measurement (b).

Fig 2a shows the equivalent circuits for surface and volume measurements. Any size, shape, or construction of ESD-protective dissipative material can be measured with this approach, including foams, composites, laminates, dissipative cloth, etc.

The two-point, surface-resistance (RS) measurement shown in Fig 2b produces consistent measurements. Interelectrode distance is fixed for all surface measurements so that consistent readings result, regardless of the presence or absence of a metallized resistive film R_M. Also, electrode voltage is fixed to produce consistent values since resistance values may be nonlinear with voltage.

Electrode spacing for surface-mode measurements is 0.75 in; 100 V is the applied open-circuit electrode voltage. The same voltage is used for volume-mode measurement. This voltage was selected because 100 V is often the specified maximum allowable potential between surfaces in ESD-safe work areas.

In addition, 100 V is a reasonable voltage should a material's resistance become nonlinear below this level, halting current flow. Based on 100 V and 0.75-in. Electrode-spacing values, the linear IR drop of points spaced 1 ft. apart would be equivalent to 1600 V (100 V/0.75 in. X 12 in.). Buried-layer materials are nonlinear and, therefore, have a lower equivalent voltage for a 1-ft separation depending on the volume conductivity to the metallized layer.

This method involves the use of a 100-V power supply to place a voltage on the material or the exposed electrodes. To avoid shock, the supply must be limited to safe currents preferably below the shock-sensation level. If this is done, the equipment may be used like any other low-voltage instrument. A 100 V source with current limited to 0.1 mA would be able to measure down to a resistance of 1 MW.

Significance and Use

The purpose of this procedure is to evaluate the surface and volume resistance of materials used to protect ESD-sensitive devices. Surface-mode resistance is used to characterize the ability of a material to dissipate static charge from its surface, based on a fixed electrode size and spacing. Electrodes for surface and volume measurements are shown in Fig. 3.

Figure 3. Surface and volume resistance of materials used to protect ESD-sensitive devices are measured via a two-point electrode method.

Volume-mode resistance characterizes the ability of a material to dissipate static charge throughout itself, based on a fixed-electrode-size measurement for the actual material's thickness. Volume resistance is particularly important in evaluating packaging for voltage-sensitive devices.
Because resistance, and not resistivity, is being measured, this method has none of the material restrictions required of ASTM D-257 resistivity measurements. In a straightforward resistance measurement, it is clear that both surface and volume currents are involved.

This method evaluates the current path resistance for dissipative materials of all forms and types and accurately simulates worst-case conditions.

When volume resistance is measured using this procedure, a portion of the measurement current may flow on the surface of the materiel. Likewise, when a surface charge is dissipated from a material, a portion of it may flow on the material's surface. Therefore, even though the resistance measured by this procedure includes both surface and volume resistance, the current paths measured are a closer simulation of the actual path through which a charge is dissipated.

Extremely accurate measurements of volume or surface resistance for ESD-protective materials are not required because actual discharge paths are uncertain. However, measurements to within an order of magnitude are possible-- an improvement over the present methods using a variety of electrode configurations.

The resistance values yielded by this method and resistivity as measured by ASTM D-257 are not comparable. Although both this method and ASTM D-257 produce about the same numerical values for ideal surface measurements, the values differ more for volume measurements because this method does not normalize the measurement for the thickness of the specimen involved. Also, no guard electrode is involved in the measurement, so unrestricted current paths can be similar to the unguarded static-charge paths in a finished package.

The values given by this method are not intended to classify material as dissipative. However, if desired, finished products can be determined to be in the dissipative range using this method's measured values rather than those determined by ASTM D-257.

Note that simply measuring a resistance value in the dissipatvie range does not necessarily qualify a material for an intended application. On the other hand, a value as measured by this procedure can be used to provide a more desirable finished product instead of just meeting the blank criterion of being in the dissipative range.

Comparing resistance measurements for two products of vastly different thickness prodeces a comparison of volume-mode charge-dissipation time. This can be accomplished only with a resistance measurement of an actual material thickness, and not with normalized ohm-cm measurements.

For data sheets describing materials, resistance can be reported for a 3- X 3- in. (7.62- X 7.62-cm), flat standard specimen. ESD-protective products (finished goods) may be tested "as is." All these items, when measured as lab qualification specimens, must be conditioned at a stated humidity.

Summary and Conclusions

Specifications and measurement of dissipative materials must give an accurate representation of products used to protect sensitive devices. Unfortunately, this is not always true.

Resistivity measurements (ASTM D-257) are useful for engineering data, but inappropriate for characterizing ESD materials in use. The static-decay test (FED-STD-101C, Meth. 4046) lacks the ability to cover the dissipative range, and can thus give a confused impression of surface-resistivity.

Circuit models explain some of the variations in resistivity and dissipation-time measurements, and the cause of these variations is a combination of instrumentation and material interactions. The two-point resistance measurements described here can reduce the confusion and improve the intended package protection.

Protective designs for energy-sensitive and voltage-sensitive devices must be assessed differently, and require realistic measurements. This test method determines the surface and volume resistance of static-dissipative materials by measuring current when a voltage is applied on the surface, or across the volume, of a dissipative material.

References

1. D.C. Burdeaux, C.L. Mott, "An Analytical Approach to Surface Resistivity Measurement," Evaluation Engineering, November 1986 (Table 3, p. 87; Summary, p. 97).
2. B.N. Stevens, "Determining the Surface Resistivity of ESD-Protective Cellular Packaging Materials," Proceedings, 1986 EOS/ESD Symposium.
3. S.L. Fowler, "Triboelectricity and Surface Resistivity Do Not Correlate," Proceedings, 1988 EOS/ESD Symposium.
4. Packaging Material Standards for ESD-Sensitive Items, Electronic Industries Assn., EIA STD IS-5-A (now EIA-541; 2.2.2, 4.2.3).
5. V.E. Shashous, "Static Electricity in Polymers-- Theory and Measurement," Journal of Polymer Science," 1958, 32:65-85.
6. G. Baumgartner, R. Havermann, "Testing of Electrostatic Materials-- Fed. STD 101C, Method 4046.1," Proceedings, 1984 EOS/ESD Symposium.
7. G. Baumgartner, "Static Decay Test versus Resistivity Measurements," EMC EXPO, 1987 International Conference on Electromagnetic Compatibility.

This article (Parts I and II) is based on a paper that was originally presented at the 1987 EOS/ESD Symposium and is reproduced here in revised and updated form with the permission of the EOS/ESD Association.