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Characterizing
the Properties of Low Charging Materials Toni Viheriäkoski, ESD Specialist 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. The 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. Physical aspects 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 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. Experimentation The following 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 easily.
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 period. 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 (Figure 5).
Triboelectric chargeability 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 charging.
Conclusion 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. Considerations 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 of time? Suggested reading IEC
61340-5-1 Electrostatics - Part 5-1: Protection of electronic devices from electrostatic
phenomena - General requirements
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