REQUIREMENTS ON A CLASS „0“ EPA – BASICS, STANDARDS,
As originally published in the SMTA Proceedings.REQUIREMENTS ON A CLASS “0” EPA – BASICS, STANDARDS,ESD EQUIPMENTS AND MEASUREMENTSDipl.-Ing. Hartmut BerndtB.E.STAT European ESD competence centreKesselsdorf, Germany, Saxonyhberndt@bestat-esd.comINTRODUCTIONLately, more and more publications report about ESDrequirements for an EPA „class 0“. What does this mean?This point is not described in the ESD standardsANSI/ESD S20.20 and the IEC standard IEC 61340-5-1.The electronic industry follows these rules when it comesto the protection of electronic components and assembliesagainst electrostatic discharges. Only one standard divideselectronic components into certain hazard classes, theHBM standard IEC 61340-3-1 (ANSI/ESDA/JEDEC JS001-2012).All electronic components and assemblies are exposed torisks of electrostatic discharges. Producers, suppliers,distributors and users have to perform the ESD controlsystem during the whole manufacturing process, themeasurements as well as during the applications. Allactive electronic components, beginning with simplediodes, transistors or complex inner circuits, require anextern ESD control system. In the next step, SMDresistors and condensers, and prospectively NEMS andMEMS are included in this danger category. Tests showthat these passive components can be damaged throughelectrostatic discharges.Table 1. ClassificationClassificationVoltage range (volt)0A 1250B125 to 2501A250 to 5001B500 to 1,0001C1,000 to 2,00022,000 to 4,0003A4,000 to 8,0003B 8,000The structures of electronic components become smallerand smaller. 5 volt of an electrostatic charge are alreadyenough to change the structures in small electroniccomponents. The structures will achieve such smalldimensions, so that electrostatic charges can causepermanent damages. In the year 2024 the sizes of theelectronic components will be less than 10 nm. Then,electrostatic charges of 0.1 nC and electrostatic fields of10 volt /cm (or 1000 volt /m) will be enough to damageESDS permanently.NotesAccording to this classification, class “0A” means amaximum electrostatic voltage of 125 volt. Class “0B”means an electrostatic voltage between 125 and 250 volt.The typical requirement for an EPA according to the ESDstandards (ANSI and IEC) is a maximum electrostaticvoltage of 100 volt. At the moment, the requirements forthe electronic industry are higher than for other industries.Most of the EPAs meet these requirements.Some semiconductor manufacturers demand a voltage of0 volt for their electronic components within the handlingarea of an EPA. Is it possible to implement suchrequirements? Most of the ESD equipment on the marketonly grants the requirements up to 100 volt. Thus, howdoes the material have to be developed? Currently, onlyspecial ionizers are suitable to meet the target. Typicalionizers only guarantee a minimum of 100 volt, highspecialized ones 10 volt (residual charge or balance). Anoptimized ESD Control System for machines with focuson cost-effectiveness is presented later.BASICSThe person is the greatest danger for electroniccomponents. Electrostatic charges of a person aretypically higher than 100 volt. The best way to reduce theelectrostatic charge of a person is grounding. We havetwo ways for personal grounding: wrist straps and ESDshoes. The wrist strap contact with the skin of the persondirectly. So, the final charge of a person can be smallerthan 100 volt. Many companies use ESD shoes forgrounding. In figure 1, the grounding resistance is showndepending on the body voltage. Typical values for systemresistance are higher than 10 MOhm. This means that thebody voltage is higher than 100 volt.
As originally published in the SMTA Proceedings.The first and best way for grounding is the connectionwith ESD equipment of personnel grounding. Groundingonly by table mats and floor materials is not enough. Staffis controllable and workstations are ESD conform.Nevertheless, how do machines, automated handlingsystems, packaging systems etc. work and whichrequirements they have to meet?REQUIREMENTS ON ESD - MATERIAL ANDDEVICESRequirements for a PersonPersons have to be equipped with ESD required shoes anda conductive garment. The most important measure is thewristband.Table 2. Requirements for personnel equipmentStepRequirements RATodayFuture7Wristband 3.5 10 1 106 ?8Shoes 1.0 10 1 106 ?(3.5 107 )Workingclothes 1.0 109 ?The figure 1 shows the important relationship between thebody voltage or electrostatic charge of a person and theresistance to ground. So, if we estimate a lower level forelectrostatic charge, we need a lower limit for themaximum resistance between the person and the floorgrounding connection. Figure 1 also shows that we needan alternate grounding path from the person to the floorgrounding connection in the future. If the resistancebetween person and shoes becomes smaller ( 1 x 106 Ω),the total resistance between person-shoes-floor material isgoing to be smaller at all ( 1 x 106 Ω).NotesHigher requirements are necessary, when the grounding happensexclusively over the floor. Is the maximum resistance from 3.5 107 to high? Yes, the system resistance must be smaller than 1 107 .The first value can only determinate the surface resistance. Thesecond important value is the charge decay time or chargedistribution time from the surface.The charge decay time must be smaller than 2 s (from 1000 v to100 v)1.Note: A new measurement method has to be developed to measure the static decay time of working clothes. The existing methods are not adequateany more.Voltage (V)10050001020Resistance (M)30IEC 965/07Figure 1. Body voltage relating to resistance to ground(IEC 61340-5-2 Ed. 1.0 TR, 2007)Only a wristband can guarantee a permanent discharge ofpersonal charge to a grounding point. Persons areconnected by their wristband, so, no electrostatic chargecan be generated. If a person has to walk aroundpermanently because of his/her activity, the discharge canbe performed through the shoes. In fact, the floor must beconductive. The shoes must have a defined resistance toground (see table 2). Garments prevent the transfer of allelectrostatic charges from normal daily clothes. If thesemeasures are realized persons/employees are equippedagainst ESD.Requirements for Working PlacesWorking places must be constructed like in the table 3shown below. If it is constructed like this, no electrostaticcharge can be developed. Furthermore, working placesurfaces must guarantee that electrostatic charges can beeliminated safely. Additionally, working places must beequipped with a central grounding point like earthbonding points and earth bonding boxes. The resistance toground of the working surface has a limited area. Table 3shows different requirements for working places.The resistance should not be too small. If it is too small ahard discharge (CDM method) can happen suddenly,which can damage ESDS. The limit of the upperresistance is defined in accordance to the fast andcontrolled, but still safe, discharge of electrostatic charge.At the same time the decay time is determined.
As originally published in the SMTA Proceedings.Table 3. Requirements for working place surfacesStepRequirements RATodayFuture5Working place 7.5 10 1 106 ?surfaceand 1 109 Decay time 2s (from 1000volt to 100 volt) 2s (from 100volt to 10 volt)Requirements for FloorsThe floor is an important part of an ESD area. It isnecessary for persons who do not wear any conductiveshoes and whose discharge mostly happens over the floor.The electrical characteristics are shown in table 4. Thereare many experiences with conductive floors. Basically,conductive coverings are suitable because hard coatings(epoxy) have additional problems with the contactRequirements for floorsRequirements RATodayFuture9Floor1 106 1 1 10 107 Higher 3.5 107 ?requirementsDecay time 2 s (from 2 s (from 1001000 volt to 100 volt to 10 volt)volt)Table 4.Stepbehavior. Some materials are not suitable. Previous testsmade many questions like: Do the measurement probesand sample are suitable at all? Do the probes reallyestablish the contact person-shoes-floor? Is the contactmaterial of the probe maybe incorrect? Some interestedparties have the opinion that contact material on probes donot agree with the reality. Other assumes that probes arenot the reflection of the contact person-shoes-floor. Afurther question cause quite a stir: Can a person bestandardized? Additional tests have been realized and willbe realized in the future [1].The basic requirements to conductive floors are notinfluenced through the tests. Electrostatic charges shouldbe discharged over a conductive floor.Extensive attempts show, that higher requirements have tobe fulfilled at working places, where people work bystanding. A higher resistance ( 3.5 107 Ω) woulddevelop electrostatic charge higher than 100 volt. Thereare different types of floors – floor coverings and floorcoatings, which can be thin or thick. But the contactNotes1 109 is too high and produce more than 1000 volt ofelectrostatic voltage.Surfaces with a resistance to ground about 1 106 leadto electrostatic charges higher than 100 v. Additionally, thedischarge behavior of the surfaces have to be determined.at a resistance value higher than 1 109 The measurement of the static decay time is necessary at 1 106 .resistance between person-shoes-floors is decisive. Thereason for it is the basic principle of the discharge ofcharged persons. Tests with different floor materialsshowed that only a few of them are suitable. [2]The additional measurements of the decay time areurgently necessary to qualify the material completely.NotesThe system resistance is the most important value in thefuture for personnel grounding.Today: at a maximum electrostatic charge of 100 volt from aperson, 10 volt will be the maximum in the future.The measurement of the static decay time is required abovea resistance from 1 106 .Requirements for an EPAFor having an optimized protection of ESDS, ESDworking places and working areas are necessary. Thebasic equipment: an ESD working place, which contains aconductive surface covering, a wristband and a groundingsystem. All equipment must be connected with agrounding point. That grounding point guarantees thesame potential at all points of the working place.The installation of ESD areas (EPA) is wiser. Because ofthe design of all materials and equipment, electrostaticpotential above 100 volt cannot be developed.Nevertheless, if some should be developed caused byunsuitable packaging materials, one can discharge themwithout any danger.After having equipped everything according to the ESDrequirements - all persons, working places and so on new sources of electrostatic charge will be seen. Personsand working places must be handled like the ESDrequirements. The charges can be controlled.
As originally published in the SMTA Proceedings.Requirements for Packaging MaterialThe packaging requirements correlate with the IEC61340-5-1 [4] (enumeration according to the standard)and will be introduced in the new packaging standard IEC61340-5-3 (see table 5 and table 6).concept guarantee a safe ESD control system as well asthe protection of ESDS against electrostatic charges. Wecannot find enough information and requirements for themachines in the existing standards.Table 5. Packaging material inside and outside of an EPA6.1 Inside an EPA6.2 Outside an EPAPackaging used within an EPA (that satisfies the Transportation of sensitive products outside of an EPA shallminimum requirements of ANSI/ESD S20.20) require packaging that provides:shall be:1. Low charge generation.1. Low charge generation.2.Dissipative or conductive materials for intimate contact.2. Dissipative or conductive materials for intimate3.A structure that provides electrostatic discharge shielding.contact.Items sensitive to 100 volt Human Body Modelmay need additional protection depending onapplication and program plan requirements.Notes: If electric field shielding materials are used to provide discharge shielding, a material that provides a barrier to current flow (insulator)must be used in combination with the electric field shielding material. Where this standard does not provide a test method, the user must determinethe electrostatic discharge shielding properties of the packaging. See Appendix G for guidance about determining discharge shielding properties.Table 6. Requirements for packaging materialMaterialResistance limitsCharge decaypropertytime2Electrostatic1 x 10 Ω RS 1 conductivex 105 ΩElectrostatic1 x 105 Ω RS 1 ( 1 x 109 )dissipativex 1011 Ω 2sElectrostatic 50 nJshieldingRequirementsforclass zero(1 x 102 Ω) RS 1 x104 Ω1 x 104 Ω RS (1 x1011 Ω)NotesLower resistance range, add chargedecay timeLower resistance range, add chargedecay timeShielding or field shieldingpropertiesNote: RS surface resistance (IEC 61340-2-3)A third requirement is the question of tailoring. In thefuture, companies will have more responsibility for the artof the packaging material.ESD Control ProgramThe introduction and the control of these 5 steps werealready described last year in the concept “5 Steps Plan ofan ESD Control System” [1]. The result is the followingESD control system:1. Analysis of ESDS, their damage limits and theexisting manufacturing process.2. Creation of a program and the introduction steps ofthe ESD control system.3. Personnel training4. Introduction of the ESD control systems5. Control and certification of the introduced ESDcontrol systems.The introduction of this ESD control system is morecomplex than the single system requirements of the IEC61340-5-1 and the control program of the ANSI/ESDS20.20. Only both standards and the additional existingRequirements for MachinesThe first and only requirements are demands for agrounding of all metal parts as well as for the avoidanceof plastic usage, which could generate electrostaticcharges and fields. Experiences show that this is notenough for the protection of ESDS in automated handlingmachines and systems. ESDS will not be damaged by theoperator, but by machines. The transport operation of anESDS in a machine can happen as following:1. Removal of the ESDS out of packaging is the firstsub-process. The ESDS has an isolating case, so it willbe electrostatically charged during the removal out ofthe reel or the tray.2. The electrostatically charged ESDS will be transportedto the PCB. Thereby a further electrostatic charge candevelop. The movement at high speed Pick-and-PlaceSystem should be enough of the generation ofelectrostatic charges.3. By placing it on the PCB, different potential betweenthe ESDS and the PCB exist. So, the potential
As originally published in the SMTA Proceedings.difference leads to a discharge, which will damage theESDS.These examples show that electrostatic charges alwaysdevelop when ESDS are parted or transported.Electrostatic charges will always generate because of thereason that components as well as PCBs are made of anisolating material. Other acts and production steps show,that this is not the only possibility for the generation ofelectrostatic charges in a production process. Furthercritical steps are for example: the printing of PCBs, thelabeling of PCBs and assemblies as well as testconstructions.Manual handling of individual components is not commonanymore. PCB assemblies are handled mainly byequipment and the final phases of mechanical assemblyare done by both humans and robots. In consequence ofthis the Human Body Model (HBM) is not valid ESDsimulation model as much as previous. The mainelectrostatic risk during automated manufacturing is withCharged Device Model (CDM) type of electrostaticdischarges. The additional model, but not standardizedyet, is the Charged Board Model (CBM).In the CBM type of ESD the assembled Printed WiringBoard (PCB) or some of the mechanics parts can becharged during handling. The discharge to ground orbetween the objects can happen at the same time. CBMtype of discharge is typically more seriously than othermodels for components due to high capacitance and highstored charge of PCB assemblies or mechanics.There are some main ESD control principles which areimportant in ESD Protected Area (EPA) as well as inautomated process equipment:1. All conductive and dissipative items are grounded.2. Materials or parts which are in contact with ESDSmust be made of electrostatic dissipative material.3. Non-essential insulating materials are excluded.4. Where insulating materials or parts are needed, thepossible charges must be minimized by specialmeasures, like ionization, shielding or coating.Enclosures of machines are normally made of conductivematerial. The conductive enclosure should have a straightand reliable connection to ground and the distance of theinsulating parts should be long enough in order not tocreate high electrostatic fields close to ESDS. Specialattention should be paid on grounding of parts which areseparated from the enclosure or are movable, likeadjustable conveyor.There are a lot of materials which can be in contact withESDS items. Components to be placed are stored in reelswith plastic tapes covered and nozzle picks the componentfrom reel. Components are placed on the PCB and PCB iscontacted with conveyor belts and possible support pins,gripper, clamps etc. All these materials should be made ofelectrostatic dissipative material at least in contact areaand a resistance to ground value should be between 1 106 and 1 109 .Components and PCB material have plastic, insulatingmaterial and they can become charged by tribocharging,e.g. by rubbing against conveyor belt, touching on otherproduct parts or in routing process. The charged ESDSitem can subject to CDM or CBM risk. All rotating andsliding elements form an ESD risk. The tribochargingduring automated manufacturing should be minimized andmetal contact to ESDS should be prevented. Normally,these preventions are not enough. Thus, an ionizer shouldbe installed in the area of rotating material.Ionizers are applied sometimes to remove electrostaticcharges from machines. Electronic components and PCBscannot be grounded. Thus, ionization is the only methodminimizing electrostatic charges at the moment.Intelligent ionizers are able to detect electrostatic chargesin machines and to generate equivalent charges for theirdecrease either.Requirements/Questions for an ESD control program formachines:1. Are all parts grounded?2. Is there no plastic material charged? Is only ESDplastic material used?3. How does delivery of ESDS and PCBs happen?4. Is the packaging material ESD conform?5. How are the requirements for the packaging materialfor non ESDS defined?6. How is the transport der ESDS inside of the machinedefined?7. If non ESD material is used (i.e. for high voltagewires), do the transportation ways of the ESDS haveenough distance from this?8. Are the PCB and the ESDS of the same potentialwhen they get in contact? (i.e. in a pick-and-placemachine)9. Are the PCB and the ESDS discharged enough?10. Are the transport conveyors, belts and systemsbetween the machines on the same electric potential?Based on these questions, ESD requirements will becreated for machines.MEASUREMENTSThe most instruments on the market for measureelectrostatic voltage, electrostatic charge and forresistance are not qualified for these correctmeasurements. Only special contact volt meters (CVM) orelectrostatic static volt meters (EVM) are able to measurethis very small electrostatic charge.
As originally published in the SMTA Proceedings.Figure 2. Measuring PC conductor with high animpedance contact volt meter [13]The first step is the measurement with a contact voltmeter. Furthermore, high sensitive electrostatic volt metercan be used. They do not damage ESDS during themeasurement.CONCLUSIONSThe requirements “0 volt” can be achieved, when diemaximum value will be required. Unfortunately, today wedo not have any ESD material, which requires theselimits.The biggest problems are machines and automatedhandling equipment (AHE), because very small chargesare generated in these machines, independent frompersons. These small and fast discharge procedures areenergy-intensive. They cause damages of electrostaticsensitive devices and assemblies. The grounding of allmetal parts does not suffice. New processes, which eitherdischarge very small electrostatic charges fast or preventtheses discharges, have to be developed.The only way to meet such requirements is precisionionization. All other ESD equipment have more than 0volt. Even with a limit value of 125 volt (Level 0A) it ishard to find suitable ESD material. A further attempt is toclassify ESD control areas in different zones.1.2.3.Basic control requirementsAdvanced control requirementsExtended control requirementsUntil now, there are not any different requirements forthese areas, defined in an ESD control program.REFERENCES[1] H. Berndt, Five Steps for the Introduction of an ESDControl System, Proceedings APEX 2004, Anaheim, CA,U.S.A.[2] H. Berndt, A study of the Variables of Electrodes usedin the Measurement of Table and Floor Materials andHow They Affect the Test Results, 23. EOS/ESDSymposium 2001, Portland, OR[3] H. Berndt; VDE-Schriftenreihe - Normen verständlich- Band 71 Elektrostatik - Ursachen, Wirkungen,Schutzmaßnahmen, Messungen, Prüfungen, Normung,VDE-Verlag, 3. Auflage, 2009[4] IEC 61340-5-1 Electrostatics - 08.2007: Part 5:Specification for the protection of electronic devices fromelectrostatic phenomena, Section 1: General requirements[5] IEC 61340-5-2 Electrostatics – 08.2007: Part 5:Specification for the protection of electronic devices fromelectrostatic phenomena, Section 2: User guide[6] ANSI/ESD S20.20-2007 ESD Association Standardsfor the Development of an Electrostatic Discharge ControlProgram for – Protection of Electrical and ElectronicParts, Assemblies and Equipment’s[7] ANSI/ESD SP10.1-2007 ESD Association Standardpractice for Protection of Electrostatic DischargeSusceptible Items – Automated Handling Equipment(AHE)[8] A. Olney, B. Gifford, J. Guravage, A. Righter, RealWorld Charged Board Model (CBM) Failures, 25.EOS/ESD Symposium 2003, Las Vegas, NV[9] JEDEC Standard JESD22-C101-E, Field InducedCharged Device Model Test Method for ElectrostaticDischarge-Withstand; Thresholds of MicroelectronicComponents, December, 2009[10] D.L. Lin, FCBM – A Field-Induced Charged-BoardModel for Electrostatic Discharges,” IEEE Transactionson Industry Applications, Vol. 29, No. 6, pp. 1047-1052,November/December uctors (ITRS), Factory Integration, Update 2011[12] H. Berndt, Electrostatic Discharge (ESD) and theTechnology Roadmapto2020,PanPacificMicroelectronic Symposium, January 2008[13] White Paper 2: A Case for Lowering ComponentLevel CDM ESD Specifications and Requirements, April2010
B.E.STAT European ESD competence centre . Kesselsdorf, Germany, Saxony . hberndt@bestat-esd.com . INTRODUCTION . Lately, more and more publications report about ESD requirements for an EPA „class 0“. What does this mean? This point is not described in the ESD standards ANSI/ESD S20.20 and the IEC standard IEC 61340-5-1.
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