of Low Charging Materials
Toni Viheriäkoski, ESD
Lassi Sutela, Degree student (Electronics)
ESD laboratory, Nokia Networks
The phenomenon of chargeability is difficult to
understand. A lot of confusion is caused by introducing the "low
charging" term to the general packaging requirements. The term
is useless and meaningless without characterization. A standardized
term must have a reason for it's existence.
What kind of material is the low charging one?
The term "low charging" is meant to be
used for materials which exhibit properties that minimize charge
generation. Some materials get charged more easily than others.
Charging depends on the charging method. In many cases by changing
the test method the charge generation properties of materials acts
in a different way.
rule of thumb is that the method has been chosen incorrectly whenever
noticeable charging is not evident. Generally all materials can
be charged very easily. Measurements and assessments drawn from
them should be verified using several methods. Even if the test
arrangement were implemented carefully, we still would have a question:
"Is the material under the test a low charging one or not?"
An acceptance test based on a single method of evaluation of chargeability
is an illusion.
Triboelectric chargeability is an extremely difficult
phenomenon to evaluate. In accordance with a simplified theory the
quantity of charge depends on the movement of objects, friction
between them, stray capasitances and electrical properties of the
material such as a dielectricity and polarization. Unfortunately
this is not all about it. Chargeability depends also on thermoelectric
and electrochemical effects. Piezoelectric effects, light, humidity,
etc affect it as well.
There are also other well-known mechanisms for electrostatic
chargeability: ionization such as a corona discharge, direct connection
and induction. Evaluation techniques and experimental studies for
these mechanisms are based on the following principles:
The charge is given by equation
Charge can also be obtained by integrating
the current over duration
An electric field and potential
have a relation, which is
E = electric field, V = potential, Q = quantity
of charge, I = current, t time, C = capacitance, U = voltage
In general two measuring methods for chargeability
of a material exist: the quantity of charge and the electric field
caused by a charged object. Several variations of these methods
are based on the equations above.
Even if we could fully understand the phenomena
of chargeability, there were a huge amount of uncertainty factors
related to the measuring techniques. The evaluation of standard
uncertainty can be based on statistical methods or on some other
scientific knowledge. A measurement result is meaningless whenever
a standard uncertainty is not defined. Uncertainty factors are not
completely taken into account in related standards. Characteristic
of chargeability is always an estimation.
Obviously if we could determine the triboelectric
properties, there were still excessive problems to acquire knowledge
from all other charging mechanisms. There are no reliable methods
to measure chargeability in practice because the evaluation techniques
in genenal are not readily available. Also a repeatability of widely
known methods is very weak. Typically the person making the measurements
affect more results than the test arrangements.
experimental study is based on comparison measurements. Results
are indicative and valid with a prevailing test arrangement and
conditions. The measurements are based on the chargeability and
the surface potential as a function of time. The repeatability of
these measurements is satisfying although the surface potential
of dielectric can not be measured accurately.
Comparison of dissipative and
conductive samples using an induction model
Surface potentials of the samples were measured
in two phases. In the 1st phase the sample was brought to an electric
field. In the 2nd phase the sample was removed from that field.
A dissipative sample was grounded continuously. A conductive sample
was discharged after polarization. Both samples got charged very
Comparison of four charging methods with a single sample
The analyzed sample was an ordinary 297 x 210 x
0.1 mm copier paper (Xerox, 80g/m2). The sample was grounded continuously
when using the corona and induction models. In a case of the triboelectric
and direct connection the sample was grounded after the charging
A sheet of paper is defined as a neutral material
in the triboelectric series. It is dielectric in low humidity conditions
and dissipative in conditions of higher humidity. Aluminum is right
above the paper in the series. Despite this fact paper can be easily
charged using an aluminum plate. It is not reasonable to define
the neutral level of triboelectric series, because "neutral"
material can also be charged easily. Decay times are also affected
by the charging methods. For example the stray capacitance has a
strong effect into charge decay with the direct charging method
Triboelectric chargeability was studied by charging
two dissipative PE material and paper. The charging was carried
out using a PVC plate. PVC is located in the negative side in the
triboelectric series. It is far from "zero point", so
it should charge materials easily. The charge was measured using
a charge analyzer. These results are only indicative because the
repeatability could not be achieved. From the table 1 it can be
seen that each material was charged. According to these tests there
are no noticeable differences in the charging properties of these
materials. The definition of low charging material can not be based
on a single method of triboelectric chargeability.
Table 1. The measurements of triboelectric
Number of test
Triboelectric behavior can be experimentally evaluated
by the specific method. The problem is that we cannot always verify
the evaluation using another method.
When analyzing a low charging objects there are
a huge amount of unknown factors with a wide scale of variations
related to mechanisms of chargeability and the test methods. It
is an extremely challenging task to determine unknown correlation
of uncertainty factors.
Mainly it is impossible to estimate an uncertainty
of the chargeability for low charging objects.
Protection against ESD should follow the common
sense and obey the keep it simple principle. Electronics industry
require simple evaluation methods with clearly defined specifications
and characterization. Would it be reasonable to replace a low charging
with another term, which could be characterized by the function
IEC 61340-5-1 Electrostatics - Part 5-1: Protection
of electronic devices from electrostatic phenomena - General requirements
IEC 61340-5-2 Electrostatics - Part 5-2: Protection of electronic
devices from electrostatic phenomena - User guide
IEC 61340-2-2 Electrostatics - Part 2: Measurement methods - Section
2: Measurement of chargeability
EA-4/02 Expression of the uncertainty of measurement in calibration