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First Published EOS/ESD Technology Europe Spring 1990 Going Beyond Surface Resistivity Depending on the material , surface
resistivity may be Stephen L. Fowler The misconception persists that low surface resistivity is required to dissipate static charge. That's true; however, only for certain kinds of materials, and reliance upon surface resistivity alone as a materials selection criterion can be misleading. The United States Federal Test Standard 101C, Method 4046.1 requires a static-protective material to have a charge-decay time of less than 2 sec. EIA 541 defines a static-dissipative material as one with a surface resistivity between 1 X 105 and 1 X 1012W/square. Unfortunately for the packaging specifier, the parameters of charge-decay time and surface resistivity correlate only for monolayer and homogeneous films. Both laminated materials and new multilayer coextruded films separate these two parameters. Although many experts have discussed this fact with regard to metallized laminates, low surface resistivity is still believed necessary for charge dissipation. (Ref 6). However, dissipation may be accomplished by volumetric conduction through a relatively thin, high-resistivity skin, across an inner conductive or dissipative layer and out of the film again through another thin, high-resistivity skin. Homogeneous Materials
Fig 1 shows the log of static decay versus the log of surface resistivity for many homogeneous samples. The scattering of the data points shows a weak correlation but a definite trend. A similar relationship has been reported by others (Ref 2, 4, 6).
Examination of the data shows that for a static decay of 2 sec, surface resistivities between 1011 and 10W/ square are plausible. Conversely, for a surface resistivity of 1012 W/ square, static-decay times of 0.6 to 9 sec are possible. A single layer, homogeneous material with a surface resistivity of 1012 will generally have a static-decay time of 2 sec. This justifies the two parameters typically specified for static-dissipative materials. Metallic Laminates
Fig 2 shows a "metal-in" laminated shielding material. This material has a dissipativepolyethylene layer on one side and a polyester terephthalate (PET) layer on the other. The surface resistivity of the polyethylene side is approximately 1011W/ square. That of the PET side is > 1014W/ square. However static decay requires < 10 msec regardless of which side is viewed during the static-decay test.
Fig 3 illustrates a "metal-out" shielding material. It has a dissipative poly layer on one side and a abuse coating over a metallization layer on the other. The surface resistivity is below 1012W/ square. Again, the static decay is < 10 msec regardless of the side viewed during the test.
The reason for this apparently fast charge dissipation, as measured by Method 4046.1, has been reported before (Ref. 6). Static decay time is dominated by voltage suppression, or more correctly, by the near-instantaneous decay of induced charges on the metal layer.
In Fig 4, the sensor detects this induced charge only while the high voltage is applied to the material's outer layers. Once the high voltage is removed and ground is connected to the electrodes, the polarized charge is relaxed, giving the appearance of rapid decay of surface charges. The voltage resulting from any charge remaining on the nonmetallic layer's surface is suppressed by the ground plane capacitively coupled to the metal layer. Therefore, for laminated materials with buried metal layers, the decay times measured in the Method 4046.1 test fixture are not related to surface resistivity. For laminates with surface resistivities < 1012W/ square, one can assume that the surface charges decay in < 2 sec. For those layers with resistivities above this level, no assumption can be made from the static-decay test as described in Method 4046.1 Multi-ply Coextruded Materials The application of coextrusion technology to static-control materials makes it possible to separate surface-resistivity and static-decay parameters without laminates containing metal layers. Coextrusion is the process of pressing multilayer materials from several extruders directly via one special die. This differs from lamination techniques, where individual layers of plastic film are bonded together after the fact to form a multilayer material.
In coextrusion, very thin layers of different materials with differing properties can be formed continuously. Fig 5 is a cross section of a five-layer, coextruded ESD-packaging material. Each layer can be made from different base polymers to optimize such attributes such as moisture or vapor barrier capabilities, flame retardancy, sealability, stiffness, and strength. Where ESD-protective properties are desired, each layer can be compounded with antistatic additives. The loading levels, polymer, and additive types can be varied for desired effects. The outside layers, for example, might be designed with nontribocharging, dissipative, or noncontaminating attributes. The internal layers might add barrier, strength, or dissipation. If all layers have dissipative properties, the material will exhibit volume conductivity. If internal layers are made dissipative by additives and the external layers are left as pure polymers, then the total structure will exhibit reasonable static-dissipative properties without the 1012W/ square surface resistivity that is usually required. If the external layers are compounded with properties that are anti-static/dissipative, then the total structure will exhibit the normal relationships of conductivity, decay, and antistaticity. Where Resistivity Misguides Surface-resistivity measurements assume that a material is homogeneous. When a material is a laminate or a coextrusion, surface resistivity becomes invalid as a selection criterion. As reported by G. Bamgartner, the contributed volume o the resistance helps determine the ability of a material to conduct charge (Refs. 3, 4). When a coextruded material has clean surfaces and a gradient of antistatic additives in the bulk of the structure, its actual ability to dissipate charges is related to many more factors than resistivity. Among these are:
Each of these play an important part in charge dissipation, and surface-resistivity measurements also give poor estimates of the total effect of the various parameters. This is not to say that a material with a surface resistivity of < 1012W/ square does not dissipate charge. On the contrary, if the surface resistivity is within the limits specified by the EIA, charges will be dissipated. Also, the static-decay numbers will probably be within specification.
However, if a material has a surface resistivity higher than 1012W/ square, it may not accumulate charges and dissipate them through the mechanisms just stated above. Its static decay may also be within specification due to charge dissipation. The static decay of a coextruded material with a sub-surface dissipative layer may also be rapid due to relaxation of induced charges through the resistance of internal layers as described for metallized laminates.
Measurement Caution
Even the measurement of surface resistivity on coextruded materials must take the material's nature into account. Fig. 6 gives a close-up view of a surface-resistivity measurement of a 3-mil coextruded material. In this view the relative dimensions are obvious, and while electrode separation is only 1/32 in, the layer thickness are much smaller. Therefore, if the outer layers have a relatively high resistance and the inner layers are more conductive, charge dissipation is related to volume as well as surface conduction. If dipole motion is considered, overall dissipative ability is made clearer. In surface-resistivity measurements, neither the voltages used nor the electrode configuration is standardized. Therefore, the gradient of the measurement voltage is an important variable in determining a material's impedance to charge dissipation. Measurements are made with voltages ranging from 10V to over 1,000V. Table 1 describes the variation in voltage gradients encountered in surface-resistivity measurements using annular electrodes.
As in the measurement of surface resistivity, real-world charge dissipation is dependent on voltage gradient. Fig. 7 illustrates the principle of voltage gradient relative to a charged conductor. If a conductor like the one in Fig. 7 is charged, the charge density, or quantity of charge per unit area, is greatest at the point of greatest curvature as in the intensity of the electrical field near the conductor. The field intensity at the sharply curved point may be strong enough to cause the medium surrounding the point to become ionized and produce corona discharges. The measurement of surface resistivity does not approximate the actual discharge or dissipation of charge by a material. A person's finger or a sharp tool can cause the voltage gradient to become very high for relatively low voltages. This allows more charge removal per unit time than can be accurately inferred from surface-resistivity measurements. During static-decay tests and actual discharges to or from a material, the voltage gradient of the discharge configuration, the dipole motion of the material, and the combined resistance of the bulk of the material all play a part in the material's ability to dissipate or not retain charges. Static events are not purely DC phenomena; they are a complex combination of AC, DC, neutralization, and RF mechanisms. Therefore, surface-resistivity measurements, being DC in nature, don't correlate with decay or dissipation in all situations and are limited indicators of a material's static safety. A more accurate indicator is the charge left on a material after it has been grounded for some period.
Three different situations for determining this retained charge can be demonstrated. Fig 8 shows that if a material is charged to some magnitude, Q, then placed on a grounded plane, it will conduct or dissipate the charge to some lower level Q'. The actual amount of residual charge is dependent on original charge, voltage gradient to ground (Q=CV), and combined resistance to ground.
In Fig 9 a charge conductive block is placed on a grounded material. Its residual charge is dependent on the parameters above plus the geometry of the charged block. If the block has sharp edges, its charge density and voltage gradient will be relatively high, allowing more charge dissipation than predicted by the material's surface resistivity. For both volume-conductive or volume-dissipative materials, retained charge is nearly zero.
If a charged material is discharged by touching it with a grounded finger as in Fig 10, the residual charge is dependent on the same variables as before. In this case, if the material is some distance from ground, its capacitance to ground is much smaller than in Figs 8 and 9. This means the voltage gradient due to the total charge is greater. Therefore, the ability to dissipate the charge is greater for a given material structure and combined resistance. The charges will flow from the material until the driving forces and impedances equalize. Again, for conductive or dissipative materials, the residual charge is essentially zero. For multilayer materials with high surface resistivities, the residual charge can be very low. To measure the retained charge in Fig 8, the material is isolated from ground, then charged by electrode contact or tribocharging to a specific level. The material is placed on the grounded plane for a predetermined amount of time. It is then lifted off the plane and its charge or voltage measured. For Fig 9, the charged block must be isolated from ground by an insulating string. It is charged, placed on the grounded material for a predetermined time, then lifted an dits residual charge or voltage measured. The material in Fig 10 must be isolated from ground as it is charged to a specific level. Then, it is touched by a grounded finger for a predetermined amount of time and its charge or voltage measured to determine the retained charge. Dissipation Threshold Dissipation of charge from or through a coextruded ESD-packaging material is sensitive to what might be called a dissipation threshold. If the multilayer structure is totally compounded to allow volume conductivity as well as surface conductivity for all layers, the material will exhibit a minimum dissipation threshold, in which case Ohm's law clearly applies. However, if the conductivity of the structure varies from the inside to the outside, its dissipation ability is dependent on the voltages or charge concentrations involved. This means that for charge concentrations above a certain level, the material will dissipate those charges down to an equilibrium level. Ohm's law is more complicated. Dissipation threshold is not a new concept. Most insulators exhibit such a threshold at high charge densities. When the charge density or voltage gradient on an insulator such as Teflon reaches the ionization potential of air (or any medium in which it is immersed), the charge will mostly avalanche off the insulator. This is the insulator's threshold for the removal of static charge. If a polymer has its threshold modified by coextrusion and antistatic additives, this threshold may be designed to exist at a more suitable level. Dissipation thresholds of 50V are achievable. If a 50V threshold is achievable with pure polymer surfaces, then a clean material can exhibit reasonable static properties. A clean-skinned material cannot meet all specifications set by the United States' EIA or by the military. However, many applications require minimal additives on the surface of materials and tolerate the reduced electrical characteristics for this physical benefit. References 1. "DC Resistance or Conductance of Insulating Materials," ASTM Standard Test Method D-257-78, 1983. 2. G.R. Berbeco, "Passive Static Protection: Theory and Practice," EOS/ESD Symposium, EOS-2, Sept. 1980. 3. G. Bamgartner, "Measure Resistance Instead," Parts I & II, EOS/ESD Technology Magazine. December/January 1989 and February/March 1989. 4. G. Bamgartner, "Static Decay Versus Surface Resistivity Measurements," Lockheed Tech Notes. 5. S.L. Fowler, "Triboelectricity and Surface Resistivity Do Not Correlate," EOS/ESD Symposium, EOS-10, Sept. 1988. 6. J.R. Huntsman, "Triboelectric Charge: Its ESD Ability and a Measurement Method for its Propensity on Packaging Materials," EOS/ESD Symposium, EOS-6, Sept. 1984. 7. D.M. Yenni, et al, "The Deficiencies in Military Specification Mil-B-81705: Considerations and a Simple Model for Static Protection," EOS/ESD Symposium, EOS-1, Sept. 1979.
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Surface resistance of the material's outer layers,
