THE CONTROL OF STATIC ELECTRICITY
J. N. Chubb - John Chubb Instrumentation, Unit 30, Lansdown Industrial Estate,
Gloucester Road, Cheltenham, GL51 8PL.
Paper presented at the "Electrostatics Summer School '85" held 11 - 13
September 1985 at University College of North Wales, Bangor.
The aim of this paper is to describe approaches to control static electricity and overcome the problems static can cause.
2. TYPES OF PROBLEMS.
Static can be a cause of problems in many areas of industry. It presents a prospective source of ignition for flammable gases, liquids and powders. It can cause fires and explosions in tankers, aircraft and petrochemical plant and in the printing, pharmaceutical, food products and explosives industries. Shock risks to personnel can arise in the plastics, paper and packaging industries and in handling powders. Problems with mechanical handling of thin web and sheet materials and from the attraction of airborne dust can arise in the plastics and packaging industries. Static damage can occur in the manufacture, handling and application of sensitive semiconductor devices and static discharges can disrupt operation of microelectronic systems. Static is hence a problem in many very different areas of industry.
3. SOURCES OF STATIC.
In general terms static charges are generated/separated wherever insulating materials (as webs, sheets, mouldings, powders or liquids) rub, slide or separate from other surfaces - whether these other surfaces are insulating or are earth bonded metal. The more rapidly this occurs and the finer the particles involved the greater are the amounts of charge likely to be generated. Charge generation will also occur when metal or relatively conducting materials are rapidly separated or disrupted and one part is electrically isolated - for instance in the break-up of a high pressure water jet. Thus operations such as sieving, pouring, sliding, spraying, filtering, grinding, pipe flow, pneumatic transport are likely to generate static charges (1). If the materials are either very dry or hydrophobic so that they have very high volume and surface resistivities, or if charge leakage to earth is otherwise restricted, then it becomes likely that high levels of static charge will be present.
4. BASIS OF HAZARDS.
The hazard that static presents in the case of flammable gases, vapours and powders relates both to the capacitively stored energy in relation to minimum ignition energies and to the breakdown voltage of the minimum gap from which an ignition will propagate. Typically, the minimum ignition energies of common hydrocarbon gas/air mixtures is 0.2 mJ (2) with a few kV minimum breakdown voltage. With powders minimum ignition energies start at a few mJ (1). Shocks from electrostatic discharges become discernible around 1 mJ, are likely to be uncomfortable in the 10 to 100 mj range and will cause major muscular contraction above 1J (3). Even if this is not fatal it may cause a serious accident. Mechanical handling problems arise when electrostatic forces become comparable to gravitational or other constraining forces. This relates to the strength of local electric fields and hence on insulators to surface charge density. In general electrostatic forces are weak but dust will be attracted to surfaces at charge densities of a few x 10-7 C m-2 (3). Static damage to semiconductor devices is very dependent on the device type and design and CMOS devices and fine geometry structures tend to be the more susceptible (4). The risks are usually expressed in relation to the voltage involved with a 'human body model' discharge - although damage is more likely related to discharge energy and voltage. Damage sensitivities down around 50 volts may be experienced. Problems with the upset of microelectronic systems are also usually expressed in relation to a human body model discharge and system immunities of several kV are likely to be needed (preferably over 15 kV for uncontrolled environments) as high potentials can readily be generated on personnel in normal working environments by movements across flooring etc.
4. TACKLING ELECTROSTATIC PROBLEMS.
4.1 General aspects.
Fortunately there is an appreciable amount of experience on static available and a good deal of work has gone into understanding static problems and finding practical solutions (3,5,6,7). A number of simple rules are listed below which provide a good general guide for tackling static problems:
a) enhance system design to prevent influence of static - for example ensure proper earth bonding of all metal parts of equipment and plant so that hazardous quantities of charge cannot be accumulated where they are available for easy and rapid discharge, use of inert gas with flammable vapours to render atmosphere non-flammable, improve mechanical handling of webs and sheets for more positive action, provide on-chip protection against static discharges, build microelectronic systems to have adequate discharge immunity.
b) enhance system design to accept the risk of direct consequences of static problems if the generation and discharge of static cannot be controlled - for example provide explosion venting and/or explosion suppression facilities, avoid manual loading of powders into reactors where flammable gases or clouds may be present.
c) change system design or operation to minimise the generation of static for example avoid sliding of insulating webs on stationary surfaces and on stalled rolls, minimise use and size of high resistivity plastic surfaces, limit speeds of pumping of high resistivity liquids.
d) change system design to enhance rate of leakage of static so that high charge levels and high potentials cannot occur - for example using antistatic additives in high resistivity liquids, use antistatic coatings on web surfaces, use earthing rods when loading powders into bins with insulating liners, use conductive flooring, footwear, clothing, work surfaces and wrist straps to prevent charges building up on personnel.
e) use only BASEEFA or similarly certified electrical equipment in areas where flammable gases may be present. Earth bond all instrumentation and portable equipment before introduction into hazardous areas.
f) charge neutralisation - for example using passive or active units or air ionisation to limit charge levels on any highly insulating surfaces.
The above list is a simple outline of basic actions which are generally applicable and which should be taken wherever static is likely to be present in industrial plant and processes. Fuller advice on approaches to control undesirable static are given in a recent British Standard (3) and in a number of published papers which relate particularly to industries concerned with handling flammable materials (1,8,9,10). If there is any doubt about the level of importance of static in particular situations, if there is some doubt about the application or economics of standard approaches or if there is doubt about the effectiveness of control equipment then measurements (11) and/or professional advice are needed.
The following sections describes a number of approaches for controlling static problems in areas not so well covered in published papers.
4.2 Static control in handling and application of semiconductor devices.
Concern about electrostatic damage to semiconductor devices has been increasing in recent years as device geometries ge t finer and the requirements for reliability become more stringent. The levels of 'static' presenting risks to semiconductor devices are much lower than those giving rise to problems of ignition, shock and mechanical handling so not only are more sensitive and capable instruments required but these need to be used with greater care and understanding. Some types of electrostatic damage can be monitored at final testing and damaged devices replaced. However there are types of damage which cannot be detected by direct characterisation testing and the 'walking wounded' type of damage shows up as reduced reliability and shortened life. The only sure protection against the risk of this latent electrostatic damage is the endurance that at no time during the fabrication, handling or assembly of static sensitive devices are the devices exposed to electrostatic fields or discharges beyond some defined values.
Many firms have taken basic precautions against static by the installation of conductive work surfaces, by personnel earth bonding via wrist straps and by using 'antistatic' or conductive bags for storing and transporting components and assemblies. Such proprietary approaches are fine as far as they go - the danger is that having taken such precautions it may be assumed that all static risks have been eliminated. There are many ways in which static can still present significant risks - for example from clothing (10), from passive component and instruction note packaging, from inappropriate IC storage tubes, from insulating tool handles and from supervisory and visiting personnel not bonded to earth via conductive footwear and flooring, etc . The only reliable way to find out where risks are present and to design and prove appropriate control techniques is to measure and monitor electrostatic conditions throughout production operations from goods inwards to goods out. The actions which may be needed will vary from simple rearrangement of the positioning of operations to changes of material and/or working procedures - and may include the use of air ionisation for charge neutralisation.
Protection against static based on proper measurements will ensure remedial efforts are concentrated where they are needed and this should enhance both internal confidence and the confidence of critical customers.
4.3 Protection of microelectronic systems.
In the manufacture of semiconductor devices, the kitting of printed circuit boards with devices, and in the testing, repair and servicing of such boards it is reasonable to expect control of electrostatic aspects of the working environment. Such control is either not easy or not possible for many microelectronic systems - for example those destined for office and domestic use and for portable and body mounted equipment. For such situations it is desirable or necessary for the equipment to be tolerant of any electrostatic discharges which may arise nearby or directly to any external part of the equipment.
Electrostatic charge will be generated on people by such normal activities as walking across carpets, getting up from chairs, rubbing clothing against surfaces etc (10). The levels of charge will be higher in low humidity e nvironments and where artificial fibres are extensively used. Body potentials up to 15 kV may be expected. Electrostatic discharges will occur from charged fabrics, from the charged body and from any metal objects held in the hand etc. These electrostatic discharges may involve high potentials and so may be able to jump gaps of several millimetres of air through gaps in equipment casings directly to internal circuitry. The discharges can involve currents up to several amps and involve frequency spectra extending up to several hundred MHz - particularly where a metal conductor acts as the source of the discharge (12). Such discharges thus have timescales for change of current and of electric field which cover the range involved in the operation of digital circuitry, microprocessors etc. The interaction of electrostatic discharges with equipment operation may arise by direct discharge or by capacitive, magnetic or electromagnetic coupling processes.
The general way to avoid electrostatic discharges affecting microelectronic systems is to mount the equipment within an enclosure providing good electrostatic and magnetic shielding and to suitably decouple all input and output connections. In practice complete shielding is often not feasible and the following points should then be noted:
- circuits should as far as possible be encased in a fully conducting enclosure with the minimum area and exte nt of any apertures.
- all connections to and from circuits within the enclosure to external equipment should be decoupled directly to the enclosure boundary at the entry point - or if decoupling is not feasible then contained within a shielded cable with the shield bonded directly only to the enclosure at either end. Connection to the conducting enclosure should be directly at the socket and not via the circuit board and/or lengths of wire. (Circulating earth currents around interconnected shielded units may be avoided by single point earth bonding and careful cable layout. The shielding of cables should not be taken to a signal earth point on a circuit card).
- if ribbon cables are to be used for interconnecting equipment it is best to use at least one track as an earth bonding line with this treated in the same way as the cable shield described above.
- mains power input (and output) lines may well need filtering and the decoupling should be directly to the general conducting enclosure and as close as possible to the point of entry into the enclosure.
- it needs to be remembered that 'earth' connections via the mains lead are actually rather long and tortuous. They will have resistance and inductance and the opportunity to act as an extended aerial to pick up capacitive, magnetic and electromagnetic interference.
- the conducting enclosure around equipment usual ly provides a convenient local self capacitance to which the charge involved in electrostatic discharges and in decoupling actions can be arranged to flow with suitably low resistive and inductive impedance and without interaction with circuit signal or earth lines. If the enclosure does not adequately provide this then a suitable local self capacitance can be provided by a large metal plate.
- if there is a risk of direct discharge to a circuit (for example via key-button apertures) the discharge needs to be intercepted on an intermediate conducting boundary which protects the circuit and shunts the charge flow as directly as possible to the conducting enclosure with minimum interaction with the main circuit tracking.
The actions needed to achieve immunity against electrostatic discharges are similar to those needed to prevent the emission of electromagnetic radiation but they are not equivalent and need to be checked separately.
4.4 Charge neutralisers and static 'Eliminators'.
Where static accumulates on high resistivity surfaces to levels which cause problems, in spite of such actions as described in section 4.1 (a) to (e) as may be feasible, it is necessary to consider charge neutralisation. This approach is most frequently encountered in the processing and handling of web and sheet materials - such as plastics (13) and paper.
Charge neutralisers are of fo ur main types: passive units, active units, air ionisers and radioactive units. In their different ways all four approaches produce air ionisation and achieve charge neutralisation by migration of air ion countercharge to the charged surface.
In general terms charge neutralisation should be applied to web fed materials as near as possible to the point at which actual problems are encountered. It may be appropriate also to neutralise close to the main source of charging and this needs to be judged on a proper appreciation of the electrostatic conditions in relation to the processing system. Depending on the ty pe of problem experienced it may not be too important whether charges on webs are fully neutralised on each side of the web or whether only nett neutrality is achieved. If full neutralisation is needed, for instance to reduce binding in wound up reels, it will be necessary to use a neutraliser on the correct side of the web. The level of charge on individual sides of webs may be assessed using a sensitive fieldmeter to measure charge density as a web passes over an earthed roll (11,13).
4.4.1 Passive neutralisers.
Passive neutralisers operate by using the charge on the surface to be neutralised to generate an electric field at nearby earthed surfaces and by so concentrating this electric field at a number of points of small radii of curvature to cause local electrical breakdown of the air close to these points. The electrical breakdown is localised as a corona by the small radius of curvature and is unable to form a spark channel to bridge the gap to the surface. The breakdown processes however produce numerous air ions and these move in the electric field between the corona discharge region and the surface so that ions of opposite polarity to the charge on the surface will move to the surface and tend to neutralise the charge there. This process will operate so long as the voltage difference between the surface and the points is above the minimum of the Paschen curve (about 350 volts for air) and so long as the electric field at the points is locally above the breakdown strength of air - about 30 MV m-1, but higher with small radii of curvature. The threshold voltage for initiation of corona will be affected by any accumulation of dust and dirt on the points - so these do need to be kept reasonably clean.
Practical passive neutralisers often use an array of fine wire mounted from a support bar but the design is a compromise between the fineness of wires for good field concentration and the mechanical robustness required to withstand industrial use and maintenance. The use of carbon fibre brush, in which the fibre diameter is around 10 micron, is an attractive alternative to wires.
The two main disadvantages of passive neutralisers are their inability to operate at very low levels of surface charge density (corresponding to surface potentials of a few kV) and their tendency to 'overcompensate'. This overcompensation is associated with the aerodynamic displacement of the ion current flow to the surface by the air motion induced by motion of the web surface. The effect may be minimised by use of a second neutraliser, preferably with a heated discharge wire or points (14).
4.4.2 Active neutralisers.
These units have similarity to passive neutralisers in that the concentration of electric field at fine points or a wire is used to provide a source of air ionisation. The main difference is that a separate a.c. high voltage power source is used to generate this electric field. The generation of ions is thus independent of the level of charge on the web surface and the field generated by this charge on the web is only involved as the means of transporting ions of appropriate polarity and quantity to achieve neutralisation. Voltages of 6 to 12 kV are commonly used at normal mains frequency. For webs moving at speeds above about 2 m sec-1 (14) a higher frequency energisation is needed to achieve uniform neutralisation.
The risks of shock to personnel and of ignition of flammable atmospheres can be avoided with active neutralisers by low value capacitive coupling of the power supply to each discharge point or by earthing the discharge points and having the nearby high voltage electrodes embedded within suitably robust encapsulation. A claimed advantage of this later approach is that the unit can operate as a passive neutraliser if the high voltage power supply should fail.
4.4.3 Air ioniser units.
These are similar to active neutraliser units but use an air flow or air blast (15) to project air ions of both polarities to a region in which there are surfaces which may have charges needing to be neutralised. Ideally the units produce a neutral mixture of positive and negative air ions from which ions of appropriate quantity and sign may be extracted by the electric fields generated by any surface charges. A balancing quantity of ionisation will flow to other nearby earthy surfaces to maintain neutrality. In this way the air ions make the air weakly conducting in the region treated. The approach is at present mainly used in semiconductor manufacturing plant but could find application in other areas where complex or changing geometries (for example reel-up and unwind stands) make it difficult to use more conventional neutralisers.
The main problems with this approach are the difficulty of ensuring good neutrality of the air ion mixtur e, the risk of introducing ozone into the working environment and the danger that electric fields from shielded charges will cause charge to be deposited on nearby otherwise neutral insulating surfaces. At the present state of this art it is wise to monitor the electrostatic conditions in the region to be protected.
4.4.4 Radioactive neutralisers.
Radioactive units use an alpha particle emitting source as a means of generating air ionisation from which the charges on the surface to be neutralised may extract an appropriate quantity and polarity of air ions to achieve neutralisation. Po 210 is the usual radioactive isotope used because it produces high energy alpha particles with little gamma particle emission and has a convenient half life. Neutralising currents around 9 microamps m-1 are available (16).
Radioactive neutralisers do not provide as high neutralising currents as are available from passive or active units and their performance does fall off over a period of several months (138 day half life). They are however simple and compact, require no external power source and should be able to be used in flammable atmospheres with precautions to prevent risk of sparking if highly charged webs can move to touch earthed surfaces.
If materials are being hand led or processed which are likely to generate charge there are a number of precautions which need to be taken to avoid risk of ignition of flammable atmospheres and shocks to personnel. These are well described in the recent British Standard (3) and papers by Gibson (1,7). If there are any doubts about the charge generating capabilities of particular materials or processes or about the ability of standard approaches to adequately and economically handle problems then it is necessary to make appropriate electrostatic measurements and/or seek professional advice.
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