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A Guide to ESD
By Dr Jeremy Smallwood, Electrostatic Solutions Ltd
How to use The ESD Guide The ESD Guide gives an introduction to the main requirements of the EN 61340-5-1 Electrostatics. Protection of electronic devices from electrostatic phenomena – General Requirements and it’s User Guide EN 61340-5-2. We’ve also included some background information as to the principles behind these standards.
These documents are now standards in Europe and have replaced EN100015. They are available in the UK as BS EN 61340-5-1:2001 and BS EN 61340-5-2:2001. (See "Where to get" at the end of this article). These two BS ENs are technically identical to the original IEC documents. IEC 61340-5-1 & 2 are expected to remain in their current form until 2005.
Whilst The ESD Guide gives many of the key requirements and recommendations, it is not comprehensive and we recommend that you obtain copies of the 61340-5-1 & -2 documents for full guidance.
What is ESD? ESD stands for ElectroStatic Discharge.
Static electricity is an everyday phenomenon - there can be few of us who have not experienced a static shock after walking across a room and touching the door knob, or on getting out of a car.
Other static nuisance effects include the cling of some fabrics to the body, the sticking of a plastic document cover, or the attraction of dust to a TV or computer screen.
While we can feel some of these effects, static electricity is normally present at lower levels that we cannot feel, hear or see, but may nevertheless damage sensitive electronic components. It can build up rapidly on objects, in unexpected ways, to produce surprisingly high voltages.
If two objects that have different voltages approach each other closely enough, charge may pass from one object to the other in a fast electrostatic discharge. While this only lasts a microsecond or less, the peak discharge current can be several Amps and the peak power can be in the kiloWatt range!
Why worry about ESD? ESD can cause unseen damage to electronic components during manufacture of electronic assemblies and equipment. If the damaged component fails immediately, the result can be a board that fail tests and requires rework. This represents lost production and additional manufacturing costs.
Worse than this, a component may be partially damaged and weakened. It may suffer a change or drift in characteristics. It may remain within specification, but fail later when in use by a customer. It has been estimated that 90% of damaged devices may be discovered in this way. This is the most expensive type of failure, as it represents:
q Customer dissatisfaction, and the possibility of loss of product reputation and future sales
q Customer service personnel and facility cost
q Engineers time, possibly for on-site repair with travel, and parts replacement
Analysis of non-conforming or defective devices showed that 60-75% were damaged by EOS (electrical overstress) or ESD. This rises to 90% for newer technologies. About 70% of these failures were attributed to damage from incorrectly grounded people.
Toshikazu Namaguchi, Hideka Uchida. EOS/ESD Symposium EOS-20 1998 pp 245-251
In manual assembly most ESD arises from charged personnel, if they are not grounded.
Most people do not feel an ESD shock unless they are charged to over 4000 V (the sensitivity threshold varies between people, and even over parts of the body!). This voltage is quite common in the uncontrolled environment - how many of us have not felt the occasional electrostatic shock in everyday life?
Overview of 61340-5-1 & 2 The IEC 61340-5-1 Technical Report Protection of electronic devices from electrostatic phenomena – General Requirements was developed from earlier Standards including EN100015. It is accompanied by a User Guide IEC61340-5-2, which gives a lot of additional information to help the non-specialist implement IEC61340-5-1. As a IEC documents, they have world-wide applicability and are planned to be developed into full world-wide IEC standards in the future.
In Europe, the 61340-5-1&2 documents have been adopted by CENELEC to replace EN100015. We recommend that the guidance given by the 61340-5-1 General Requirements document is considered mandatory, while the additional guidance given by the 61340-5-2 User Guide may be considered to be recommendations.
Parts of 61340-5-1 may be found on technical grounds to be unnecessary or irrelevant to a particular installation or processes, and may be omitted. Where only partial compliance is required in this way, this should be made clear to all who use or audit your ESD facilities. A good way to achieve this would be to document it in an in-house ESD Programme or the EPA Certificate of Conformance.
IEC61340-5-1 specifies general requirements for the design, use and control of a protected area so that electrostatic sensitive devices (ESDS), having sensitivity of 100V (Human Body Model test) or higher, can be handled with minimal risk of damage from procurement through to end of life. The Technical Report covers:
q Signs and markings
q The electrostatic protected area (EPA) including requirements for protective equipment, construction of the EPA, working practices, and field work
q Protective packaging
q Training requirements
q Quality responsibilities
q Periodic Audit instructions
An Annex describes test methods to be used to verify the performance of equipment, materials and packaging for use in protection of ESDS.
The Technical Report does not cover health and safety requirements, and compliance with local regulations and practices should always be observed in this respect.
The Technical Report warns that special care may be required in implementing static damage prevention measures under the following conditions:
q Low humidity. The electrostatic properties of materials are very dependent on atmospheric humidity. A controlled humidity of 50 %rh suits most circumstances. If humidity reduces below 20 %rh, ensure that selected materials and equipment will perform effectively under operational conditions.
q Clean room conditions. Some current materials and techniques in use for ESD protection are not suitable for use in clean rooms (class 100 or tighter).
q Where high voltages may be present. If high voltages greater than 250VAC or 500VDC are present then take particular care to consider safety requirements.
Definitions
Key elements in an ESD Program Principles of electrostatic safe handling There are two simple principles we can use to protect ESD sensitive components from ESD damage:
q Only handle sensitive components in an ESD Protected Area (EPA) under protected and controlled conditions
q Protect sensitive devices outside the EPA using ESD protective packaging
What is an EPA? An EPA is an area that is maintained safe for handling static sensitive components by keeping electrostatic fields and voltages to an insignificantly low level. An EPA should have well defined boundaries so that it is clear where the safe area is entered and left.
Insulating materials are strong ESD sources and so they are excluded from the EPA where possible. Where this is not possible special measures such as ionisers are used to neutralise electrostatic charges.
All non-insulating and conductive objects are grounded, so that electrostatic charge cannot build up on these. Most importantly, all personnel (people are conductive objects!) are grounded so that they do not charge to high voltages. There are two ways in which people are grounded:
q Using a grounded wrist strap (this is the preferred method)
q Using conductive footwear (conductive or dissipative shoes, or heel & toe straps on both feet) in conjunction with a grounded conductive floor.
Three key elements of an effective ESD program An effective ESD program requires three main parts:
q The ESD Program. This is a document that specifies the equipment used, and procedures followed in order to ensure electrostatic safe handling. q The Training program. This is required in order to make sure that everyone is aware of, and understands the ESD Program, use of equipment and correct procedures. q The Audit program. This is required in order to make sure that equipment remains functional, and that the ESD Program procedures are correctly followed.
If any of these is absent, it is likely that the ESD prevention measures will eventually fail. A fourth factor is also extremely important - Management commitment. If Management does not provide the resources and authoritative support required, it is unlikely that the ESD Program will succeed.
EPA design Design philosophy A primary objective of EPA design is that all personnel handling sensitive devices are grounded. The preferred means of grounding personnel is by using wrist straps. If this is not practical, use compliant footwear or heel and toe straps with compliant flooring. Seated personnel should be grounded via wrist straps - footwear grounding is not reliable in this case, and grounding via the seat should not be considered a reliable option.
Any surfaces on which ESDS are placed must be connected to EPA ground, and have a point-to-point resistance between 104 Wð and 1010Wð, and resistance-to-ground between 7.5x105Wð and 109 Wð. The ESD Coordinator can authorise “hard grounding” (<104Wð ) of surfaces when required.
General design An EPA (Electrostatic Protected Area) is a region within which no item or activity must be able to cause damage to a sensitive device. In the simplest case - a field work station - it may consist of a dissipative mat, a wrist strap and common grounding facility for both. At the other extreme it may consist a large manufacturing area where all the measures of 61340-5-1 are implemented. In some cases the EPA may consist of a single bench work area, which may not have an EPA floor.
The ESD Coordinator is responsible for determining EPA boundaries, construction, selection of equipment and general appropriate design of EPA for their particular application. Equipment used within the EPA must comply with the requirements of 61340-5-1.
If possible, a group of EPA work stations should be combined into one large EPA, including an EPA compliant floor. If this is not done, and each work station forms an individual EPA (which may not include an EPA floor). Sensitive devices must then be packaged in ESD protective packaging for transport between work stations.
Marking the EPA boundaries Personnel must be able to see signs complying with 61340-5-1 before they enter the EPA. Where appropriate (i.e. in a multistation EPA), compliant signs must also be visible within the EPA, and exit signs should mark the boundary of the EPA.
EPA ground system design The EPA ground provides a low resistance path (<2 Wð) to ground, usually to mains protective earth. A single EPA ground must be used within one EPA facility.
EPA ground bonding points (EBP) must be provided next to each working area or surface. These are points to which wrist straps are connected, and must be clearly marked in compliance with 61340-5-1. There must be sufficient EBPs to provide for operators and visitors, and they must be accessible. EBP are connected to EPA ground through a resistance of up to 106 Wð . Surface to ground resistance values may be achieved by including discrete resistors in the ground path, or by the resistance of the material which is directly grounded.
EPA ground cords are used to connect between groundable points and the EPA ground facility. One or more resistor may be included in the ground cord to give the required resistance to ground value. If a single resistor is used, it must be at the end of the cord at the groundable point. If more than one resistor is included, the resistor at the end connected to the groundable point must be at least half the total resistance in the cord. The ground cord connections, if accessible, must be shrouded with insulating material.
Figure 1. Typical EPA ground system
The EPA ground connection system must not be compatible with any connecting system used for any other purpose. The conducting parts of the connectors must be shrouded by insulating material when a cord is connected to the bonding point.
Wrist straps A wrist strap has a snugly fitting band worn around the wrist, and cord fitted with a quick release connection. The resistance between the band inner surface and its groundable point must be less than 105Wð. The conducting parts of the connecting system must be shrouded when connected. The strap ground cord must incorporate a resistor (7.5x105Wð to 5x106Wð) at the wrist end (see EPA ground cord B). The total resistance from hand to Earth Bonding Point must be between 7.5x105Wð and 3.5x107Wð.
Certification of conformance The ESD Coordinator must audit a new or modified EPA to ensure that compliance is achieved, and must issue a Certificate of Conformance before the EPA is used.
Departures from the guidance of the 61340-5-1 Technical Report should be recorded on the Certificate of Conformance, along with any extra equipment or procedures required to ensure ESD protection.
Responsibilities of personnel Responsibilities of all personnel All personnel handling sensitive devices has responsibility to be aware of the ESD threat to reliability of electronic products. They have prime responsibility to implement and maintain ESD prevention measures. Any failure to do so should be reported to the ESD Co-ordinator.
All personnel are expected to inform the ESD Co-ordinator of any unsatisfactory measures, and suggest improvements or corrective actions if appropriate.
General Management responsibilities 61340-5-1 places responsibility for implementation of the guidelines of the Technical Report in the hands of General Management. They are also ultimately responsible for making sure that all aspects of ESDS handling are carried out, including;
q Maintaining a register of trained personnel
q Providing appropriate training for all personnel
q Ensuring inspection, supervision and maintenance of EPA facilities are carried out
The General Management bear the responsibility for appointment of the site ESD Co-ordinator. A named ESD Co-ordinator is required for each site. They must be given total Management back-up, and adequate resources and authority.
ESD Co-ordinator responsibilities The ESD Co-ordinator is responsible to the Management for all site ESD matters. This includes;
q Ensuring that a copy of the ESD Program or 61340-5-1 standard is available to all relevant personnel
q Specifying and maintaining a list of ESD equipment
q Making sure ESD training is adequate and conforms to the requirements
q Maintaining a list of trained personnel
q Ensuring that all equipment and procedures conform to the standard
q Inspection, maintenance and verifying conformance of the EPA
q Making ESD program tailoring decisions when required
q Giving assistance when required
The ESD Co-ordinator has responsibility for making certain technical decisions in tailoring the implementation of the ESD Program:
q Defining the boundaries of Electrostatic Protected Areas (EPA)
q Defining and selecting equipment for use within the EPA
q Construction and certification of EPAs
q Deciding which (if any) of the recommended measures are not required
q Deciding the frequency of certain audit actions
The ESD Co-ordinator can appoint deputies if required.
EPA equipment, surfaces and furniture Equipment used in an EPA should conform to the requirements over the full range of operational atmospheric humidity conditions.
It is not necessary to use all the items specified – however if any item is used, it must conform to the specification given.
Working surfaces and storage racks, trolleys, carts Any surfaces on which ESDS are placed must be connected to EPA ground, and have a point-to-point resistance between 104W and 1010W, and resistance-to-ground between 7.5x105W and 109 W. It is permissible to “hard ground” (<104W) surfaces when approved by the ESD Co-ordinator.
Surface to ground resistance values may be achieved by including discrete resistors in the ground path, or by the resistance of the material which is directly grounded.
Floors & footwear Use of a compliant floor is strongly recommended in 61340-5-x, particularly for new installations. However an EPA does not have to include a compliant floor. If the EPA does not include a floor, ESD protective packaging must be used when moving sensitive devices between EPA workstations - as in any situation where ESDS are transported outside an EPA.
EPA floor surfaces must have a surface-to-EPA ground resistance of <109W. (N.B. A minimum resistance-to-ground may be required due to safety considerations). The personnel-to-ground resistance of a footwear-flooring combination (for grounding personnel) is recommended to be between 7.5x105W and 3.5x107W.
The resistance from the wearer’s body to EPA ground (metal plate) should be between 5x104W and 108W (and at least 105W for each shoe)
Seating If seats are used, the resistance to groundable point, from any area of the seat which may have contact with personnel, must be less than 1010W. At least two wheels or feet must provide a path to ground.
Garments Compliant garments must be clearly marked and must completely cover all clothing in the arms & torso region. There must be electrical continuity between all parts. The point-point resistance over the outer surface must be less than 1012W. Decay of charge on the outer surface to less than 10% of initial value within 2 sec. is required.
Gloves and finger cots Decay of charge to less than 10% of initial value within 2 sec. is required.
Ionisers Ionisers must bring the potential due to charge (of either polarity) on items to be brought from 1000V to less than 100V in less than 20 seconds.
Tools, machinery, dispensers and test equipment Tools should not have insulating handles. Soldering irons must have grounded tips. Any non-insulating part of a tool must be grounded (resistance-to-ground less than 1012W). Decay of charge to less than 10% of initial value within 2 sec. is required.
Trolleys and carts These must have a groundable point. Surfaces must have a point-to-point resistance between 104W and 1010W, and resistance-to-ground between 7.5x105W and 109W. It is permissible to “hard ground” (<104W) surfaces when approved by the ESD Co-ordinator. At least two wheels must provide a path to ground.
ESD Protective packaging Definition of the types of packaging 61340-5-1 defines three types of packaging - intimate, proximity, and secondary packaging:
Warning labels (compliant with 61340-5-1) must appear on all packages containing ESDS. If removed for access, they must be replaced.
When unwrapping a package containing sensitive devices, it is important to remove packaging as far as the ESD protective packaging layer that bears the ESD symbol. This layer must only be removed within an EPA.
Secondary packaging, such as polythene bags, cardboard boxes, jiffy bags etc. must not be taken into an EPA as they can be the source of electrostatic charge and ESD.
Packaging used in the EPA Packaging brought into the EPA must not have an insulating outer surface. It must allow charge to dissipate to EPA ground. Secondary packaging must not be brought into the EPA.
If a material has a surface resistance of >109 W, charge on the surface must decay, returning the surface potential from 1000V to 100V in less than 2 seconds. Surface resistivity less than 104 W (“hard ground”) is allowed at the discretion of the ESD Coordinator.
Packaging material resistances must be maintained to the highest and lowest humidities expected under operational conditions.
All components, tools or materials that are not made of EPA compliant material, should be packed in low charging and dissipative, or conductive, materials before transport into the EPA.
Packaging used outside the EPA An ESDS transported outside the EPA must be protected by a low charging and dissipative (or conductive, if non-powered) layer of intimate packaging around the ESDS. This must be further surrounded by shielding proximity packaging.
Some packaging, such as shielding bags, combine the intimate and proximity shielding packaging functions.
Definitions of packaging material types In 61340-5-1 & 2, the following definitions apply to packaging materials:
Training IEC61340-5-1 specifies that ESD training shall be provided for all personnel who specify, procure, design, mark or handle Electrostatic Sensitive Devices (ESDS), and those who manage or supervise them. The objective of the training is to effectively use the materials, equipment and procedures required by the Technical Report, and to understand why they are needed. Training should be provided as part of employee introduction courses, and should be appropriate to the ability and job function of the trainee. A register of trained personnel must be maintained.
The training course should include:
q Theory of electrostatic charging
q Basic knowledge of electrostatic discharge
q Special handling procedures
q Knowledge of ESD protective equipment and items
q Identification of sensitive devices (ESDS)
q The limits of ESD protection measures
q ESDS sensitivity
q Use of new techniques & processes before implementation
q Any conflicts with safety requirements
q High voltage precautions, if required
q Awareness of the 61340-5-1 Technical Report
Follow-on training should be provided at an appropriate (documented) interval specified by the ESD Co-ordinator.
A typical training scheme might include 3 levels:
q Level 1. Dos & Don’ts list for visitors, customers, temporary employees, and contractors etc. This might be handed to personnel at reception or on entry into the EPA.
q Level 2. A course for permanent employees, as part of an induction course or periodic training course. This consists of theory and practical aspects, as well as use of equipment and packaging. Typically a course may be 1-2 hours long, and may use a variety of formats. A formal test, or some feedback, of individual performance may be given if required by the ESD Co-ordinator.
q Level 3. This would typically be specialist training given to particular personnel according to their job requirements. This level would be taken after a level 2 training course.
Auditing program The 61340-5-1&2 documents are designed for use with quality systems such as ISO9000. The requirements of 61340-5-1 should be treated as mandatory, while 61340-5-2 can be considered recommendations.
Daily checks It is most important to check conformance of wrist straps and their ground cords (as worn), and heel & toe straps or ESD shoes before entering the EPA.
Visual check must be made of EPA and packaging, trolleys, excluded items, and ioniser location & orientation. Documentation of these checks is optional.
Monthly checks Check the continuity of equipment earth bonding on a sample basis, and the functionality of ionisation systems.
Six-month checks Check electrostatic fields <10kV/m, or alternatively potentials <100V. Check conformance of signs and labels, and ESD garments.
Periodic audit Periodic audit should be performed at not more than 12 month intervals. A follow-up audit should be scheduled if appropriate and an Audit Report circulated.
Check:
q Wrist strap discipline, and the conformance of wrist strap and footwear testers.
q EPA signs and labels
q Conformance of the resistance of floors, work surfaces, and seating.
q Conformance of tools.
q Conformance of packaging and disposal of discarded packaging and materials.
q Electrostatic fields <10kV/m, or alternatively potentials <100V.
q Conformance of ionisers.
q High voltage area procedures
q Humidity control operation.
q Records and selection procedures
Field Work Field work is perhaps the most risky situation of handling ESD sensitive devices because of the uncontrolled nature of the environment. It is also often the most neglected aspect of ESD damage prevention.
The situation is risky as there are usually many potential ESD sources in the modern environment. A good awareness of ESD issues and practice is required if the risk from these is to be reduced with the minimum of portable equipment.
All field work must maintain the same standards and requirements specified for permanent EPAs in terms of training, material quality, labelling, packaging and personal responsibilities.
Figure 2. A field service kit (photo courtesy of Static Safe Environments Ltd. )
Any secondary non-ESD packaging is removed and discarded a safe distance from the working area. The operator, any temporary EPA work surface used, and the equipment must be electrically bonded together. Where practical they are grounded, usually to mains earth. The operator is grounded via a wrist strap.
A new pc board should not be removed from its ESD protective packaging until ready for insertion in to the equipment.
Any faulty circuit boards required for failure analysis must be packaged in shielding packaging if further ESD damage is to be avoided.
ESD Sensitivity of electronic devices As the human body was originally the most common and damaging source of electrostatic discharge, the most common measurement of ESD sensitivity is by Human Body Model (HBM) electrostatic discharge. In this test a charged 100pF capacitor is discharged into the device via a 1500Wð resistor. The 100pF capacitor simulates charged stored on the average human body, and the resistor simulates the resistance of the human body and skin. 61340-5-1, if fully implemented, aims to protect devices down to 100V HBM sensitivity.
The ESD sensitivity of devices is given as an “ESD withstand voltage”, which is the maximum test voltage at which the device did not suffer damage. Typical HBM withstand voltages of various device technologies are given in the following table.
As component technology progresses, internal device sizes reduce and become more ESD sensitive. Many modern components are protected by on-chip protection circuits, without which they would be extremely sensitive. In most cases the design goal is to increase the device ESD withstand voltage to 2 kV. In some cases this goal cannot be met for various reasons - there is often a tradeoff between ESD protection and device performance.
The ESD Guide is written by Dr Jeremy Smallwood and maintained on-line by Electrostatic Solutions Ltd. to promote understanding of current ESD standards and best practice, and as an on-line reference for ESD professionals. The ESD Guide will be updated and developed as time goes on. Jeremy welcomes comments, questions and suggestions for development of the ESD Guide and can be contacted via the Electrostatic Solutions web site or email address below.
Biography: Jeremy Smallwood achieved a BSc in Electronics in 1979 and spent seven years in industrial electronics circuit design and development. From the mid 1980s he specialised in electrostatic discharge measurements research at the University of Southampton. He subsequently led the Electrostatics Section at ERA Technology Ltd before in 1998 forming his own R&D and consultancy company Electrostatic Solutions Ltd, working on ESD damage avoidance in electronics manufacture, electrostatic hazards and applications. Jeremy is Chairman of the International Electrotechnical Commission Technical Committee TC101 (Electrostatics) responsible for the 61340 series of standards.
Where to get BS EN 61340-5-1:2001 & BS EN 61340-5-2:2001 These are available from BSI and can be ordered online at: www.bsi-global.com
Links: The ESD Guide on-line: http://www.static-sol.com/61340-5-1/welcome.htm
Electrostatic Solutions Ltd.: http://www.static-sol.com/
Dr Smallwood’s research publications: http://www.static-sol.com/publications.htm
Dr Jeremy Smallwood: Email: jeremys@static-sol.com
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