electrostatic protected workstation or area (EPA). In that issue, we
focused on the conductive items and materials, including personnel,
which enter or are present in the EPA. This month we look at the
necessary nonconductors in the EPA and the mitigation of electrostatic
charges that may be present on those items.
Industry guidelines and standards for setting up and managing an
EPA suggest that only process-essential insulators be permitted in the
protected area. This means there must be a thorough and complete
policing of the EPA to ensure that no nonessential insulators
(nonconductors) enter the work environment of the EPA. While setting up
an EPA would be simpler if everything were conductive or electrically
dissipative, there will always be a need for insulating materials in
electrical and electronics assembly. Component and device packaging,
circuit cards, flex circuits and the insulative coating on wires are
examples of process-essential insulators necessary for assembly of most
electrical or electronics products. However, the nonessential insulators
that must be excluded in a properly set-up EPA include personal items,
food wraps, coffee cups, handbags, etc. In addition, conventional
packaging materials, such as expanded polystyrene, bubble wrap, plain
plastic, etc., should be eliminated from within the boundaries of the
EPA whenever possible.
ANSI/ESD S20.20 offers guidance regarding the maximum electrical
field level from the necessary nonconductors in the EPA. Electrical
field measuring instruments usually report in terms of surface voltage
at some predetermined distance, generally 1" from the surface being
measured. Any item or surface of a material that measures [greater than
or equal to]2000V at 1" should be kept 12" from unprotected
parts sensitive to [greater than or equal to]100V when tested according
to the Human Body Model (HBM). This suggested level was determined by
empirical testing in several different labs during the development of
S20.20. An insulative material surface with a measured 2000V electrical
field at 1" will not transfer enough energy either by induction or
direct contact to harm a 100V HBM device. A charged conductor will
transfer nearly all its stored energy upon contact with a grounded
conductor while an insulator can only discharge from a small area. Thus,
a charged insulator is far less hazardous in the environment than a
charged conductor. In fact, an insulator is not technically classified
as being charged since it cannot discharge in the conventional sense.
However, an insulator that emanates an electrical field can charge up
conductors that are momentarily grounded while within the influence of
the electrical field (induction). Therefore, reducing the electrical
field strength from insulators is an important aspect in the design of
an EPA.
Mitigating Electrostatic Charge
Reducing the electrical field strength of a charged process
essential insulator requires careful forethought. Topical treatments may
be applied to the necessary insulators to make them somewhat dissipative
or low-charge generating (antistatic). Items treated in this manner are
no longer insulative in the technical sense. It is logical, whenever
possible, to apply dissipative/antistatic coatings to clear plastic
windows on process equipment, and also to plastic fixtures and tools,
templates, test beds and other insulating surfaces involved in an
assembly or test operation. Numerous topical treatments are available,
but finding the most suitable formula may require some trial and error.
While some coatings and treatments are relatively permanent once applied
to a surface, many conventional topical treatments wear off or change
performance with time and must be reapplied with some regular frequency.
This may add an unreasonable burden to maintaining the EPA, so care
should be taken in selecting the topical treatment.
Ionized air can reduce the electrical field from the necessary
insulators while they are in-process. Ionizers create positive and
negative charges in air in approximately equal numbers. Opposite sign
ions are attracted to electrostatic charges on a surface to neutralize
the charge one for one. Positive ions will neutralize negative charges
and negative ions will neutralize positive charges. To make certain an
ionizer does not charge a surface, the ionizer must produce a balanced
number of positive and negative ions. However, a reasonable off-set
voltage for most ionizers is in the 10 to 50V range when measured at
12". Nuclear (alpha) source ionizers and electrical ionizers with
feedback loop systems are available with lower offset voltage levels if
needed in critical applications. A properly positioned air ionizer can
reduce the electrical field strength of an insulator to minimal levels
if given enough residence time. Any ionizer system should be checked in
the intended application to ensure it is capable of reducing or
eliminating the problem. Ionized air is not a cure-all but can prove
beneficial when used appropriately. Ionized air may be the only
available solution for many applications, especially where insulating
items must be in motion within the process.
David E. Swenson is cofounder and president of Affinity Static
Control Consulting LLC, a member of the ESDA standards committee,
ANSI/ESD S20.20 task force and board of directors, and ESDA senior VP
and professional certification manager. He spent 35 years in ESD control
material development at 3M (mmm.com).
This column is a regular contribution of the ESD Association
(esda.org).
[ILLUSTRATION OMITTED]
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