can we anticipate if we will have problems from static electricity

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CAN WE ANTICIPATE IF WE WILL HAVE PROBLEMS FROM STATIC
ELECTRICITY?
JOHN CHUBB
John Chubb Instrumentation, Unit 30, Lansdown Industrial Estate,
Gloucester Road, Cheltenham, GL51 8PL, UK
(Tel: +44 (0)1242 573347 Fax: +44 (0)1242 251388 email: [email protected])
Most people know that static electricity can present risks and cause problems. The electronics
industry is particularly susceptible because of the low energy and low voltages at which damage
can occur. While much attention is paid to ‘body voltage’ in relation to direct discharges, it
needs to be appreciated that indirect effects from charges induced by electric fields from nearby
charged surfaces can cause damage via the ‘charged device’ model. These questions are
discussed at the annual EOS/ESD Symposium run by the ESD Association and are the subject of
a number of Standards (for instance IEC 61340-5-1).
‘Static’ is a materials question - and risks and problems arise if materials are not ‘suitable’.
This article is about methods of measurement to give confidence in the assessment of materials.
‘Materials’ may be static protective clothing, worksurfaces, packaging, jigs and fixtures –
anything that may come into proximity to sensitive devices or assemblies.
'Static electricity' arises when surfaces in contact are separated. If the charge that
arises from differences between the surfaces cannot run away to earth quickly enough
then it is trapped - it is 'static'. The 'quickly enough' relates to the time for the charge to
spread out over the surface of a material and/or to leak away to earth. If this time for
charge movement is very short the material is described as a ‘conductor’ - if very slow
then an ‘insulator’. Traditionally the performance of materials in this range has been
assessed by measuring ‘resistivity’. As we shall see this is NOT an appropriate approach.
Electrostatic charge retained on the surface of a material creates a potential at the
surface. It is this potential that creates electric fields at nearby items. These fields cause
attraction of dust, dirt and thin films, induce charges on items nearby and that can cause
direct electrostatic discharges. Thus it is ‘surface potential’ that is the parameter of
prime practical importance.
The two features of importance to end users are:
1) the surface potentials that may arise
2) the time that significant surface potentials are present
Many materials are non-homogenous. Paper is, and a prime example is the fabric used
for cleanroom garments and for static contolled clothing. Such fabrics are basically
polyester with an included stripe or grid pattern of conductive threads. If these threads
are surface conducting then it is easy to see that a ‘resistivity’ measurement only gives
information about the conductive threads – it tells you nothing about the opportunity to
retain static charge on the fabric between the threads and create an average high surface
voltage. The way to assess whether significant surface voltages will arise on materials
and for how long they are present is to put some electrostatic charge on the surface and
see what happens. This can be done experimentally by rubbing a local area, measuring
how much charge has been transferred at measuring the initial peak voltage created and
how quickly this decays away.
The quantity of electrostatic charge transferred by rubbing and sliding type actions is
limited by the intensity of the mechanical operation (speed and pressure) and by the
character of the materials involved. What is important then is the surface potential
created for the maximum quantity of charge likely to occur in practice.
Experimental measurements have been made on a variety of materials. These studies
have shown that the initial peak voltage at the end of a sharp rubbing action is
proportional to the charge transferred [1,2]. The volts per nanocoulomb varies greatly
between materials and, as one might expect, the voltage on some cleanroom garment
fabrics is strongly suppressed by the capacitance provided by the conductive threads.
From these studies one also of course gets information on how quickly the surface
voltage decays as the charge migrates over the surface and away to earth.
Testing materials by rubbing surfaces is fine, even if not easy, for laboratory studies.
It is quite unsuitable for industrial testing. Instrumentation based on the use of corona
charging has been developed over the last 15 years or so that is compact, portable and
easy to use (see Figure 1). Studies have shown that the capacitance effects and the charge
decay time performance exhibited when surfaces are rubbed are well matched by those
observed with corona charging [1,2]. Test methods based on corona charging are well
established – and there is now an IEC Standard in place [3]. Simple addition of charge
transfer measurement enables calculation of the capacitance effect experienced by surface
charge.
At a recent ESTECH Conference [4] it was reported that voltages of several hundred
volts can arise on the surfaces of some commercial cleanroom garments when these are
rubbed. (The test set-up is illustrated in Figure 2). It was also reported how these voltages
could be predicted from corona charging measurements on the basic garment fabric.
Getting appropriate performance features for materials is often not easy. In many
cases antistat additives or topical antistats are used. These usually work by absorbing
atmospheric moisture. In this case their performance will deteriorate at low humidity.
Hence it is important in purchasing materials to know they have been properly tested over
the full range of likely application environments.
Air ionisation is often used as a way to control static surface voltages in work
environments. This is fine, but has its limitations. It does not dissipate charges quickly
and can be subject to shadowing at the workbench. It is hence best used as a secondary
rather than primary way to control static.
CONCLUSION:
The combination of charge decay and capacitance effect measurement provides an
appropriate way to assess the suitability of materials for avoiding problems and risks
from static. Such measurements are best made by suppliers, so that materials and items
can be purchased by users with confidence to appropriate performance specifications.
Users may feel the need to cross-check performance to ensure requirements are met.
It is important that the right test methods are used. For instance: there are many
‘standard’ methods for charge decay measurement around - and they give very different
results. They cannot all be right! In the work we have done and reported we have taken
care to ensure that our observations match to end user requirements to validate the test
methods used.
References:
[1] J. N. Chubb “Measurement of tribo and corona charging features of materials for assessment of risks
from static electricity” Trans IEEE Ind Appl 36 (6) Nov/Dec 2000 p1515-1522
[2] J. N. Chubb “New approaches for electrostatic testing of materials” J. Electrostatics 54 (3/4) March
2002 p233 (Presented at ESA meeting, Brock University, Niagara Falls, May 2000)
[3] “Measurement methods – Ability of materials and products to dissipate static electric charge” EN
61340-2-1: 2002
[4] J. N. Chubb, P. Holdstock, M. Dyer ”Predicting the maximum surface voltages expected on inhabited
cleanroom garments in practical use" ESTECH 2003, Contamination Control Division, Phoenix,
Arizona.18-21 May, 2003
Figure 1: Corona charge decay measuring instrument, JCI 155v5
Figure 2: Set-up for measuring surface voltages on inhabited cleanroom garments
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