The development of new antistatic packaging materials, principally those intended to protect sensitive electronic components from static electrical charge, created a need for new terminology, uniform definitions for this terminology, and usable quantitative standards for the industry. Some key professional organizations have developed and started to promulgate standards [EOS/ESD Assoc., 1994]; however, as one should expect for any rapidly expanding technology, agreement on them is hardly universal.
The theory of charge relaxation in bulk conductors was developed in some detail by Woodson and Melcher . Their basic result is that, in a uniform ohmic conductor with electrical resistivity r and dielectric constant k, volume charge will decay exponentially with a time constant: t = k eo r, where eo is the permittivity of free space. Note that this time constant for bulk charge depends only on intrinsic properties of the homogeneous material containing the bulk charge. When the material is inhomogeneous or when interfaces exist between different media, the time constant becomes dependent on geometrical factors such as the density of layers, etc. [Jones and Chan, 1989]
Definitions for antistatic and static dissipative packaging materials have been proposed. In the making of these definitions, there is on the one hand the tendency to rely on familiar terminology and measurable quantities that the engineers expected to use these standards feel comfortable with. The specification of antistatic materials is often given as a bulk resistivity, r in W-m, or a surface resistivity, rs in W/sq. Note that the bulk resistivity is an intrinsic property of the material while the surface resistivity, in its simplest form, is related to the thickness of a conductive layer on an insulating substrate.
where t is the thickness of a layer having uniform resistivity r. These resistivity quantities are relatively easy to measure with common instruments and so they are used quite extensively.
On the other hand, there is the requirement that static charge control standards be general in their applicability and related as clearly as possible to the electrostatic phenomenology that is to be controlled. In this light, some dispute the use of resistivity measures, and instead recommend that charge dissipation times should be the quantitative basis of static charge control technology. There are several arguments favoring a time-based specification [Chubb, 1998]. First, many antistatic materials are not really ohmic conductors. They can exhibit non-linearity or charge-carrier dependent behavior. This nonlinearity is acknowledged in the specification that electrostatic ground testing should be performed at a voltage of ~500 V. Second, it is often difficult to distinguish between volume and surface conduction. Third, the charge dissipation time is a more direct measure of the efficacy of an antistatic material. Being able to state that an antistatic bag for holding computer boards will dissipate 90% of its charge within 1/10 of a second is clearly more illuminating than saying that the bag has a surface resistivity of 1011 W/sq. Finally, using time as a measure has the advantage of consistency with industrial safety practice in the manufacture and processing of petrochemicals, pharmaceuticals, and polymers, where electrostatics has long been recognized as an ignition hazard.
To gain some idea of how the various measures relate to one another, refer to Table 1 which lists several quantitative measures for EOS/ESD exposure of electronics components. In some cases, both bulk and surface resistivity ranges are provided for packaging materials. A number of different, somewhat overlapping classification schemes exist for these materials.
|insulative||ND||>1012 W/sq*||ND||>0.5 sec||ND|
|antistatic||ND||109-1014 W/sq||1010-1014 W/sq||ND||ND|
|static dissipative||102-109 W-m|
|105-109 W/sq||102-108 W/sq||0.01-0.5 sec||102-109 W-m|
|<105 W/sq||ND||ND||<102 W-m|
|electrostatic shielding||ND||ND||ND||ND||<10 W-m|
|EMI shielding||ND||ND||10-2-1.0 W/sq||ND||ND|
The various resistivity- and time-based measures in this table can be reconciled as long as agreement is made about the system or systems under consideration. In the assembly, handling, and packaging of electronic components and boards, the bags and other containers are available in a narrow range of sizes and usually have the same geometries. Thus, the charge storage capacity, that is, the capacitance, will always fall within some predictable range of values so that effective charge dissipation times will be reliably correlated to some effective resistivity measure.
Consider a 10 cm by 10 cm plastic antistatic plastic bag laying flat just above a grounded work surface in an electronics assembly workstation. Ignoring finite resistivity and assuming that the average spacing between the bag and the work surface is s = 1 mm., then the capacitance will be approximately
The resistance of a square bag is approximately equal to the surface resistivity rs, and the static charge dissipation time t may be calculated as an effective RC circuit time constant:
Using 109 W/sq as an upper limit for surface resistivity from the table above yields a static dissipation time of ~0.1 seconds, which corresponds closely to the acceptable upper time limit for dissipation of undesirable static electricity [Chubb, 1998]. This dissipation time is common for items such as antistatic mats.
For a larger bag of the type used to protect a computer board, the capacitance C will be higher, possibly necessitating use of a bag with lower resistivity so that the charge still dissipates rapidly.
J. Chubb, unpublished data, 1998.
J. Kanarek and W. Tan, "A Primer on Static Control Plastics - part 1," Wescorp Corporation Newsletter, WescorpWorld, Summer, 1998.
EOS/ESD Association, Glossary of Terms, ESD-ADV1.0-1994, 1994.
Compliance Engineering magazine.
H.H. Woodson and J.R. Melcher, Electromechanical Dynamics," part II, (Wiley, New York), 1968, section 7.2.
T. B. Jones and S. Chan, "Charge relaxation in partially filled vessels", J. Electrostatics, vol. 22, pp. 185-197, 1989.