Assessment of ESD threats to electronic components

Electrostatic discharge (ESD) is a major source of failures in electronic devices and products detected during manufacturing. Reduction of semiconductor element dimensions as well as implementation of new product and production technologies have made many devices extremely vulnerable to disturbances of electrostatic origin. Effective ESD damage prevention requires that ESD threats are carefully assessed and understood. This paper reviews new research results on electrostatic discharges as well as tools for the assessment of ESD threats to electronic components. Influences of the new results on the ESD control are discussed. There is a need to modify existing standards for the ESD control in electronics industry in order to meet challenges related to the manufacturing of future electronic products.     

Electrostatic testing of a rigid intermediate bulk container (RIBC) intended for use in explosive atmospheres

Rigid intermediate bulk containers (RIBC) of 1 m³ volume, consisting of an HDPE body almost entirely surrounded by metal sheeting, are readily available commercially and are used across a wide range of industries with both flammable and non-flammable liquids. Regarding their use with flammables, these containers have been claimed to be suitable for use in Zones 1 and 2 (IIA and IIB) according to CENELEC R044-001. A draft of IEC 61340-4-6 also implies that this style of RIBC is acceptable for use with flammable liquids with regard to electrostatic ignition hazards. However, testing of one such RIBC showed that discharges of several hundred nanocoulombs could be obtained from the interior plastic surface even when the metal frame was earthed, and also from the cap and around the top opening. In subsequent tests, these discharges were shown to be capable of igniting typical flammable atmospheres. Therefore, it is important to recognize that such containers are not inherently safe with regard to electrostatic ignition hazards, and that additional precautions may be required in order for them to be used safety with potentially explosive atmospheres.     

Helmut Krämer memorial lecture: Electrostatic assessment of products and processes – A view backwards and forwards

In the last few decades, electrostatic knowledge as laid down in standards and regulations has developed from lean national documents to two comprehensive international IEC documents with more than 200 pages together. In this paper, the development of the knowledge on the electrostatic assessment of products and processes is given as well as expected future trends. As the work of Helmut Krämer was focused on electrostatic hazards of solids and liquids, the emphasis of this paper is placed on this topic too. Additionally, a curriculum vitae of Helmut Krämer and his contribution to this development is given in an annex.     

Induced-charge errors in charge-transfer measurement: Brush discharges between charged,insulating discs and earthed,conductive spheres

Charge-transfer measurements made with fast-response, unshielded probes will, in principle, underestimate charge-transfers because of induced-charge errors. It is, however, difficult to know when the degree of underestimation is significant or to allow for it quantitatively because information on the magnitude of these errors is fragmented. Consequently, when measurements are done for electrostatic hazard assessment, unsafe equipment could wrongly be classified as safe. To rectify this situation, and thus contribute to improving charge-transfer test methods in static electricity standards, we have systematically calculated induced-charge errors in brush discharges between insulating discs and earthed spheres. Errors are expressed as the charge-collection efficiency, which is the ratio of charge transfer in the external measurement circuit to charge transfer across the discharge gap. Key results are the worst-case efficiencies at the charge-transfer ignition-thresholds suggested for different classes of flammable material (60 nC, 30 nC and 10 nC for gas group IIa, IIb and IIc materials respectively). For 60 nC transfers and with our best estimate of post-discharge conditions on the disc surface, the worst-case charge-collection efficiency is 0.56, which indicates the error-allowance needed for safe assessment. For 30 nC or 10 nC transfers, the charge-collection efficiency is both smaller (～0.2), and considerably more sensitive to gap geometry. Consequently, it is not recommended to use charge-transfer measurements with fast, unshielded probes to assess electrostatic hazards for gas group IIb or IIc materials.     

Learning lessons from five electrostatic incidents

It is well known that electrostatic discharges can ignite mixtures of flammable gases with air and suspensions of combustible dust in air. For this reason, the prevention of electrostatic discharges is an essential part of measures to prevent explosions and fires. Incendive electrostatic discharges occurred in five cases in various chemical plants.The incidents involved were:

• 1.Spark type electrostatic discharges from a leaking steam pipe.
• 2.Spark type electrostatic discharges inside a charging chute for a solid.
• 3.A spark type electrostatic discharge whilst transferring a solid powder from a bin, which led to ignition of the powder.
• 4.A propagating brush discharge during pneumatic transfer of solid, which caused a severe electric shock to a process operator.
• 5.A brush discharge inside an electrostatic precipitator which caused ignition of an aerosol of hydrocarbon.

To avoid explosions and fires it is important to prevent incendive electrostatic discharges from occurring in industrial facilities. To help managers of chemical plants to do this, the incidents above are presented in the form of learning lessons including:

• •the mechanism of the generation and separation of electrical charges,
• •the mechanism of the electrostatic discharge,
• •the root causes of the incident,
• •the safety measures which are necessary to avoid a repetition.

     

Monitoring of electrostatic fire and explosion hazards at the inlet to electrostatic precipitators

The paper deals with the continuous monitoring of electrostatic fire and explosion hazards that can occur at the inlet to electrostatic precipitators (ESPs) when highly charged dust particles are transported by a gas carrier that can be the mixtures of both incombustible and combustible flue gases. The risk of ignition and even explosion is especially high in the presence of an explosive mixture of oxygen and, e.g., hydrocarbons, carbon monoxide, etc. To avoid the danger of electrostatic discharges and their consequences for a whole installation including an electrostatic precipitator a method and a specially designed and built system should effectively enable the continuous monitoring of the hazards and should immediately manage any automatic control system or some other control elements. Some theoretical considerations concerning the method proposed, the physical quantities that must be measured, and the derivation of a novel dynamic safety criterion for assessing the risk of hazardous electrostatic discharges are presented. Finally, the author presents and discusses the possible practical application of the microprocessor-based measuring system verified experimentally in the past to the continuous monitoring of the hazards and to the management of an automatic control system to be put into operation. The paper presents a certain idea and proposal of the problem's solution based on the author's many years' experience in the field of pneumatic transport of dusts, powders and granular materials, of the electrostatic measurements of electric and mechanical quantities characteristic of the particulate transport, and of the risk and prevention of discharges of static electricity in transporting pipes and silos, vessels, etc.     

Transferred charge of brush discharges in explosive gas atmospheres – A verification of the ignition threshold limits

A wide series of experiments has been performed to check the incentivity of hydrogen/air, ethene/air and propane/air mixtures due to brush discharges. Thereby, the transferred charge as a criterion to judge the ignition potential is determined to verify the thresholds of transferred charge given in the standards IEC 60079-0 and in EN 13463-1. These thresholds have never been examined directly in an experiment before. It is stated that the thresholds for explosion group IIA, IIB and IIC represent different levels of safety. Using adequate thresholds the criterion of transferred charge is suitable for a judgement of potential electrostatic ignition sources.     

Incident case histories on mobile containers

While handling liquids in rigid intermediate bulk containers (RIBC), the assessment of ignition sources due to electrostatic discharges is of especial interest since it is often difficult to rule out all possible conditions which can lead to the generation of an effective discharge. Far from suggesting new criteria for the selection or construction of RIBC or standards for its design, this paper has the aim of describing three incidents where an electrostatic discharge caused a flammable mixture of vapours with air to ignite. Situations presented could usually be rated as highly remote on its probability of occurrence by operators since they are common and never caused an incident, but on the other hand it is rather easy, from a theoretical point of view, to explain the phenomena observed by applying the basic assessment rules for the generation of electrostatic ignition sources.If existing guidelines were to have been followed (mainly CENELEC 50404 and BGR132), those incidents would have not occurred.     

The need to certify products to CDRH Regulations

Laser products that are sold in the United States are required to comply with the regulations published by the Center for Devices and Radiological Health (CDRH) within the US Food and Drug Administration [1]. The need to certify products is straightforward in most instances, however, the applicability of the regulations is not as clear for: lasers sold only to Original Equipment Manufacturers (OEMs) as components for incorporation into end-use products, end-use products that contain lasers which have already been certified by their manufacturers, and end-use products that are being imported into the US and that already comply with international laser safety standards. This article will discuss these applications in an attempt to clarify the need for certification by laser product manufacturers and importers. The discussion applies equally for lasers and laser systems.     

Study on the test method of the electrostatic sensitivity of the non-initiating explosive devices

The application of the non-initiating explosive devices may cause the problems of the electrostatic hazard. With the support of the fitting test apparatuses, the anti-electrostatic ability and/or the electrostatic sensitivity of the tested samples can be accurately measured and assessed by adopting the test method of the initiating explosive devices. The related results could provide a scientific evidence for the establishment of safety standards and the adoption of preventive and protective measures.     

Characterization of electronegative plasmas

The characterization of the plasma state is of great interest in industrial applications based on plasma enhanced chemical vapour deposition (CVD) processes. We have performed experiments on a capacitively coupled radio frequency discharges in air and SF₆. The use of gases containing electronegative components, such as oxygen or fluorine, leads to quite peculiar discharges, due to the presence of negative ions which affects the transport properties of such a plasma. Plasma parameters have been measured by means of movable electrostatic Langmuir probes. The implementation of a suitable numerical model of gas-phase chemistry and transport phenomena allow us to predict the amount of negative ions. In particular we show that the ion to electron density ratio strongly depends on the diffusion process and on ion recombination rates. Thus measuring it leads to a better understanding of ion diffusion and in particular of the ambipolar electric field.     

Effective method for measuring the energy of propagating brush discharges

Due to their very high energy propagating brush discharges are the most dangerous electrostatic discharges. As it is known, they usually occur on thick insulating layers on which significant charge can be accumulated. Discharge energy is a function of several parameters, like the surface charge density (influenced by the thickness of the layer and its relative permittivity), the size of the charged surface, etc. This paper represents a laboratory model to examine the energy of propagating brush discharges. Based on these measurements list of results were evaluated to estimate the energy of the generated propagating brush discharges. This energy is especially important to determine how incentive these discharges are. By the help of the model the effect of the polarity of the original charging, the surface charge density of the sample and the geometry of the sample have been examined.     

Observation of closed loops in high-voltage discharges: a possibleprecursor of magnetic flux trapping

In multimegavolt electrostatic generators, the accidental sparkover discharges change in character as the voltage and stored energy increase. The usual forked, unidirectional discharges are replaced by complex discharges containing numerous closed loops. It is suggested that this is an example of the formation of closed-current loops that generate magnetic fields in astrophysics     

Risks of ignition from ‘Type D’ ungrounded fibc

Avoidance of risks of ignition with unearthed Type D FIBC requires that electrostatic discharges cannot quickly access large quantities of charge via any conductive features of the material. The present studies show how the effective resistivity of conductive features within FIBC type fabrics can be measured, without electrical contact, from the variation of shielding performance with frequency. From such measurements an empirical acceptance criterion has been derived that matches well to the results of gas probe ignition tests.Studies are proposed to provide reliable criteria for assessing FIBC Type D bag materials and to help optimize FIBC fabric design.     

Increasing the reliability of risk assessment for systems endangered by electrostatic discharges

Risk assessment for systems endangered by electrostatic discharges is especially important in industrial electrostatics. Different methods are widely used for the approximation of the risk. One of them is the fuzzy logic based fault tree analysis. The reliability of the calculated probability of the top event in the fault tree (in our case fire or explosion) strongly depends on the accuracy of the probability values connected to the basic events. This paper deals with that problem, how the reliability of risk assessment can be increased by the application of a complex model for the determination of initial probabilities for the basic events. A case study is analysed to overview reliability problems for risk assessment in such an industrial process where risk of fire or explosion is present due to the electrostatic discharges.     

Study of magnetically enhanced corona pre-charger

In this paper, experimental investigations of the discharge characteristics of magnetically enhanced corona discharges, for the purpose of capturing fine aerosol particles, are presented. The discharge mechanism during such a process is analyzed as well. The effects of magnetic enhancement under different magnet flux densities, and in positive- or negative-corona discharges, were experimentally compared. The magnetically enhanced effects in different inter-electrode regions were studied. Experimental results demonstrated that the magnetic field could efficiently increase the concentrations of both the negative ions and the free electrons during negative-corona discharge. The dominant mechanism of magnetic enhancement in a corona discharge involves the Larmor precessions of free electrons which enhance ionization of the gas molecules near the discharge electrode. A convenient configuration for enhancing corona discharge was formed by placing permanent magnets with a local strong magnetic field near the discharge electrode. A magnetically enhanced negative-corona (MNC) pre-charger was assembled in front of an electrostatic enhancement filter. The influence of the MNC pre-charger on the efficiencies of an electrostatic enhancement filter was measured and compared with that of a conventional corona pre-charger. The free-electron-charging mechanism of the MNC pre-charger was preliminarily analyzed. Our results show that the new pre-charging technique is promising for capturing fine aerosol particles in electrostatic enhancement filters or electrostatic precipitators.     

Ignition of a cloud of dry powder using super brush discharges

Ignition of a cloud of dry powder is a major concern in the field of industrial process safety. The different types of discharges are already defined (spark discharges, brush discharges, propagating discharges, cone discharges, corona discharges) such as their ignition properties in a gas or a dust atmosphere. For example, it is known that a classic brush discharge cannot ignite a cloud of dry flammable dust [6,13]. Glor and Schwenzfeuer performed direct ignition tests using brush discharges and defined that even if the energy released by this kind of discharge equaled the one of a spark, the power released by the brush discharge is too low to trigger an ignition.However, some doubts remained for super brush discharges. A brush discharge as a super brush discharge occurs between a charged insulating object and a conductive electrode. The main difference lies in the surface charge density reached on the insulator that is much higher for a super brush discharge than for a brush discharge. A high charge density can be reached for example using pipes of polyethylene individually charged by tribo-charging piled one above another. Such a configuration was evocated by Lüttgens [12] and tested by Larsen [11] who performed direct ignition tests in oxygen enriched atmospheres.This study is relevant with the actual safety problems since pharmaceutical and chemical powders are well known to generate electrostatic charges during their transport or handling and since the same configuration of independent polyethylene fibers can be found in flexible bulk containers that are one of the most common solutions to package this kind of powder.This paper presents the experimental set-up and the results of direct ignition tests performed with a polyethylene wax whose MIE is lower than 1  mJ at ambient conditions. The electric field reached at 1 m and the charge transfer were also registered and are described. Finally, numerical simulations are carried out to define the original surface charge density in order to help to understand the phenomenology of this discharge and its frequency of occurrence in industry.     

Electrostatic ignition hazards in the process industry

Fires and explosions continue to be triggered by the accumulation of electrostatic charge in different branches of the process industry. This is the case even though nowadays the electrostatic phenomena that give rise to ignition hazards in industrial practice are generally well known. This paper presents various reasons as to why this should be so and draws attention to the conflicts of interest, which arise, when other requirements need to be met. In addition, current trends and new results with respect to the assessment of electrostatic ignition hazards are reviewed. They include (i) incendivity and characterisation of discharges from insulating surfaces with and without contact to a conductive surface, (ii) incidents affecting the present assessment of occurrence and incendivity of propagating brush discharges in practice and (iii) characterisation, measurement method and ignition hazard assessment of modern packaging systems such as flexible and rigid intermediate bulk containers (FIBC and RIBC). Finally, the consequences of Directives issued at the European and national levels are appraised with respect to electrostatic ignition hazards.     

Study of field-induced explosive electron emission mechanisms ofpseudospark superemission cathodes

Various mechanisms of electron emission, including the field, field-enhanced thermionic, and explosive electron emissions from pseudospark cathodes, are discussed and compared. The mechanism of the field-induced explosive electron emission due to microstructure on the cathode surface is considered to be more likely the pseudospark superemissive mechanism. A high-mean electric field up to 3-5 MV/cm on the cathode surface in the end of hollow cathode phase is enough to initiate the mechanism. The cathode spot initiation delay time (<10 ns) and explosive emission threshold current (~10⁸ A/cm² ) prior to the high current conducting phase are given by solving the initial value problem of the one-dimensional heat conduction equation, thus explaining the existing experimental data of the pseudospark cathode superemission. In the case of multigap discharge, the above mechanism occurs on nearly all cathode and interelectrode surfaces. Experimental evidence in single- and multigap pseudospark discharges supports the suggested explanation     

Induction charging risk assessment: Charged board alike discharges to metal and human body

Charge can easily be induced on electronics or on other conducting parts if they are exposed to external electrical fields. In production facilities where sensitive electronics are handled, strong electrostatic fields should be avoided due to the risk of causing electrostatic discharges (ESDs) that could damage components. In electronics manufacturing this is usually achieved by grounding all conductors and removing all insulators from an ESD Protected Area (EPA) in the facility. However, it is not always possible to remove all insulators from the EPA as they are sometimes an essential part of the production processes. In this case, a method of risk assessment is necessary to evaluate safe operation. We have studied induction charging of a dummy PWB (Printed Wiring Board) through a grounded MOSFET transistor, by grounding it directly to metal or through the human body, when the PWB is exposed to a static electric field. The experimental setup can easily be turned into an induction charging probe by changing the MOSFET transistor to a low leakage current, high voltage capacitor of suitable size and measuring the voltage over this capacitor.