ESD – steps against electrostatic discharge – prevention of electronic
devices and assemblies
Dipl.-Ing. Hartmut Berndt
B.E.STAT ESD competence centre
Electronic devices become more and more smaller. Electronic assemblies are sensitive against electrostatic
discharge just as much as its smallest and most sensitive element. So protection systems are necessary in all
areas, where these devices and assemblies are handled and used. Sometimes it seems to us, that steps are not any
more necessary. But the last time showed us, that every time new sources for electrostatic charge can be
developed, which have not been imagined until now. A person can be equipped ESD - required very good, but
the whole environs of the proceeding is a danger, which become bigger and bigger. Especially the activities,
which are made automatically, cause big damages at the devices and assemblies. In the first part necessary steps
are described, which are needed to equip persons and working places. The second part deals with machines and
assemblies. Especially there the most problems come out. In the last part an overview is given about the
necessary test methods.
Introduction damaged through electrostatic discharge
Nobody, neither the manufacturer nor the user nor proceedings. First tests already showed, that these
the service people of electronic devices, and of electronic devices can also be damaged.
course the manufacturer of the devices can work
without ESD – protection systems during the whole The structure of electronic devices become more
manufacturing. All of the active electronic devices, and more smaller. 5 V electrostatic charges are
it doesn’t matter at all whether a simple diode, a enough to cause changes in the structure of those
transistor, a power transistor or complex integrated small devices. The structures achieved high
assemblies need external protection systems during dimensions, so that already these small charges last
their handling. In the next years we’ll have got, that out to have permanent damages.
SMD – resistances and – capacitors can also be
Figure 1 Development of the sizes of IC
In the meantime there are many regulations and protection of electronic devices and assemblies
worldwide standards for the ESD – required against electrostatic discharge. In common with the
handling of ESD – sensitive devices and American standard ANSI/ESD-S.20.20 the user can
assemblies. The basic principle of all steps is find a whole complex of regulations for the ESD –
always to discharge safety the electrostatic charge protection systems. The international standard IPC-
or to prevent the development of electrostatic A-610C refers to these standards. All standards are
charge. These basic principles are decisive and are constantly improved worldwide. Thereby the use of
contained in all regulations and standards. the ESD – protection systems are changed and
improved, not the basic principles.
The international standards IEC 61340-5-1 and IEC
61340-5-2 contain the basic requirements for the
For the understanding of the requirements are General protection systems – requirements to
describe any definitions in the following part: ESD – protection systems
The person is the greatest danger for electronic
Electrostatic Discharge (ESD) devices. The person always become charged
transfer of charge between bodies at different electrostatic.
electrostatic potentials caused by direct contact or
induced by electrostatic field /3/. The main source of electrostatic charge: the human.
Because of this the general protection principles
Electrostatic Discharge Sensitive Device (ESDS) can be explained: the person is not allowed to
discrete device, integrated circuit or assembly that produce electrostatic charge, the person have to be
may be damaged by electrostatic fields or connected with the potential balance, so that all
electrostatic discharge encountered in routine charges can be discharge safety.
handling, testing or transit /3/.
Two mechanism of influence are shown on the
ESD protected area (EPA) following pictures. First a person approaches the
area in which ESDS can be handled with accepted electronic devices directly. The person is charged
risk of damage as a result of electrostatic discharge electrostatic. The electrostatic field, which is arisen
or fields /3/. by the person, is able to be influence the electronic
devices just before the real touch. The electrostatic
EPA ground bonding point (EBP) field can provoke an electrostatic charge on the
dedicated point to which an EPA equipment may be devices.
Similarly the facts, when a person, who is charged
electrostatic, approaches a PCB. Electrostatic field
can be inserted over different copper lines.
Electrostatic charges will be produced through this
and will be saved on the PCB. At a contact with a
grounded piece, electrostatic charges flow off and
can cause damages.
Figure 2 Influence of an electronic device through
an electrostatic discharge
Figure 3 Insertion of an Electrostatic Field into a PCB
The complete working places, where you work with those ESDS, have to be constructed to prevent electrostatic
charge and to discharge safety still existing charge.
Basic principle 1 A general ESD – protection system is the only, which guarantees a sufficient
protection for the sensitive devices and assemblies.
Basic principle 2 The grounding is the best opportunity to discharge a person.
Requirements to the person: have walk around permanently because of his/her
The person have to be equipped with ESD – activity, the discharge can be done over the shoes.
required shoes and a conductive garment. The most Naturally the floor have to be conductive. The
important step is the wristband. shoes have to have a defined resistance to ground
(see table 1). An additional step can be a conductive
The wristband guarantees a permanently discharge garment. The garment prevents the covering of all
of personal charge to a grounding point. Persons are electrostatic charges from normal daily clothes. If
connected through their wristband and no these steps are realized the person can be equipped
electrostatic charge can be developed. If the person ESD – required.
Table 1 Requirements to personal equipment
Wristband < 3.5 ∗ 107 Ω
Shoes < 1.0 ∗ 108 Ω (3.5 ∗ 107 Ω)1
Working clothes < 1.0 ∗ 109 Ω 2
Note 1 The higher requirements are necessary, when the grounding happens exclusively over the floor.
Note 2 One can only determinate the surface resistance.
The primary means of grounding personal shall floor and the footwear shall be the primary means
normally be by a wrist strap connected to an EBP. of ESD control /3/.
When the use of a wrist strap is impractical, the
Requirements to working places: safety. Moreover the working place serves as
The working places have to be construct in this central grounding point. Grounding points like
way, that no electrostatic charge can be developed. earth bonding points and earth bonding boxes serve
Further the surface of the place have to guarantee for that. The picture 4 shows an optimal ESD –
that electrostatic charge of ESDS can be eliminated single working place.
Figure 4 ESD – required working place with a central grounding point
The resistance to ground of the working surface has of the upper resistance will be defined in addiction
a limited area. The resistance should not be to to the fast and controlled, but still safety, discharge
small. If it is to small a hard discharge can happen of electrostatic charge. At the same time the decay
suddenly, which will damage the ESDS. The limit time is determinant.
Table 2 Requirements to working place surfaces
Working place surface > 7.5 ∗ 105 Ω and < 1 ∗ 109 Ω
Decay time < 2 s (from 1000 V to 100 V)
All working surfaces and storage racks on which unprotected ESDS may be placed shell be capable of being
connected to EPA ground and shall have a point-to-point resistance and resistance to EPA ground in accordance
with table 2 /3/.
Requirements to floors: material of the probe maybe not correct? Some
The floor is an important part of the ESD – area. interested parties have the opinion that the contact
It’s necessary for persons, if those wear conductive material on the probe do not agree with the reality.
shoes and whose discharge happens mostly over the Other assume that probes are not the reflection of
floor. The electrical characteristics are in table 3. the contact person – shoes – floor. A further
There are many experiences with conductive floors. question cause quite a stir: Can a person be
Basically conductive coverings are suitable because standardized? Additional tests have been realized
hard coatings (Epoxy) have additional problems and will be realized in the future /1/.
with the contact behaviour. Some materials are not
suitable. Previous tests arise many questions as like The basic requirements to conductive floors are not
as: Do the measurement probes and sample are influenced through the tests. Electrostatic charges
suitable at all? Do the probes really establish the should be discharged over a conductive floor.
contact person – shoes – floor? Is the contact
Table 3 Requirements to floors
Floor < 1 ∗ 109 Ω
Higher requirements < 3.5 ∗ 107 Ω at a maximum electrostatic charge of 100 V
Decay time <2s (from 1000 V to 100 V)
Extensive attempts arose, that higher requirements shoes – floor is decisive. The reason for it is the
have to be fulfilled at working places, where people basic principle of the discharge of charged persons.
have to work in the stand. A higher resistance Tests with different floor materials showed, that
would give rise to electrostatic charge higher than only a few of them are suitable /3/. The additional
100 V. There are different types of floors: floor measurements of the decay time are urgent
coverings, floor coatings, which can be thin or necessary to qualify the material.
thick. But the contact resistance between person –
Requirements on the EPA: developed, which are caused by unsuitable
For having an optimal protection of ESDS, ESD – packaging materials, one can discharge them
required working places and working areas are without any danger. Picture 5 shows a typical ESD
necessary. The basic equipment: an ESD – required – area.
working place, which contains a conductive surface
covering, a wristband and a grounding system. All A typical EPA may be many forms:
equipments have to be connected with a grounding
point. That grounding point guarantees the same - a single bench
potential at all points of the working place. - a store
- an area of work
The installation of ESD – areas (EPA) are wiser. - a field work area
Because of the design of all materials and - a work station or equipment (for example: a solder
equipments electrostatic potential over 100 V can’t flow machine)
be developed. If nevertheless some should be
1 groundable wheels
2 groundable surface
3 wrist-strap tester
4 footwear tester
5 footwear tester foot plate
6 wrist cord and wrist band
7 EPA ground cord
8 EPA ground
9 Earth bonding point
10 groundable point for trolley
11 ESD protective footwear
13 working surface
17 shelving with grounded surfaces
18 groundable racking
19 EPA sign
Figure 5 Typical ESD – protected area /3/
After having equipped, according to the ESD – Persons and working places have to be handled like
requirements, all persons, working places a. s. o., ESD – required. The charges can be controlled.
new sources of electrostatic charge will be seen.
Requirements to ESD packaging material: provide shielding the ESDS and PCB’s against
For the packaging material and the transport of electrostatic fields and discharges.
ESDS and PCB’s with ESDS are necessary special In the EPA the material are not produce
requirements. The special properties are defined in electrostatic charge. It’s better, the packaging shell
the standard IEC 61340-5-1. The electrical be capable of providing charge drainage to EPA
properties in table 4 shall be maintained during use, ground when brought into this. Protective
storage, transport, distribution or application. packaging shell be marked with ESD signs.
An other point is, shell are use the packaging in the Packaging material shell be controlled in
EPA or outside of the EPA. The primary functions determined intervals. Packaging material have the
of the protective packaging outside the EPA are characteristics described in table 4 at the highest
reduce or limit the tribo-electric charging and and lowest expected or defined relative humidity.
Table 4 Requirements on ESD – packaging materials
Low charging reducing of a minimum of charge generation
Conductive > 1 ∗ 102 … < 1 ∗ 105 Ω
Dissipative > 1 ∗ 105 … < 1 ∗ 1011 Ω
Shielding max. energy: 50 nJ at a HBM-discharge of 1000 V
ESD and machines, measurement methods, potential for ESDS, which are independent of the
protection systems human. Already the motion of ESDS in automatic
Extensive tests in manufacturing proceedings pick-and-place machines is enough to produce high
showed, that machines and device as well as the electrostatic charges. Newest discoveries prove
simple PCB are sources of electrostatic charges. that.
These charges cause the development of big danger
All tools and machinery (electrical, mechanical or
pneumatic) shall be so constructed that any non-
insulating part of the tool or machine which may
touch an ESDS shall be at EPA ground potential.
/3/ It’s enough?
Figure 6 Charge of an IC on an transportation system
Figure 7 Charge and Discharge of an IC in a “Pick and
The ESDS can already be charged electrostatic,
when these are taken out of tray’s or reel’s. They Past measurements proved electrostatic charges as
will be transported with their electrostatic charge to high as 700 V. As the proceedings are very fast, the
the PCB, because the principle of “pick and place” measurements is very difficult. The discharge
don’t discharge them. The PCB, which is made of proceeding can also be called as CDM. Especially
an isolated material, has still electrostatic charges very small electrostatic discharge proceedings are
with another potential. The ESDS is set down and able to arise big damages. Until now the only
will be discharged. This charge current is enough protection step is the isolation. Specific discharges
for damaging the ESDS. The PCB charges itself with ionisation of electrostatic charges, which
electrostatic at the transportation form one working already exist, can reduce such damages.
station to another or during the manufacturing
Test methods stations and test processes are used. The ESD –
For the guaranty of an extensive ESD – protection working places, ESD – floors and ESD – chairs
it is urgent necessary to check the ESD – protection have to be checked regularly, for example in a
steps regularly. Tests showed, that a missing monthly, half yearly or yearly intervals. You will
control can cause an endangering for the ESDS. So find some notes for this part in the mentioned
it is necessary to have a daily check of the standards. The intervals are well-tried.
wristbands and the footwear. For this suitable test
Figure 8 Test of a wristband
Figure 9 Test of footwear
There are many methods for the test of conductive The resistance to ground is the resistance, which is
floors. For the present a resistance measurement is measured between the probe on the surface of a
enough, but in other cases, especially in the higher sample and for example with the ground connection
limit areas, the measurement of the personal charge or the copper band of a conductive floor covering.
is necessary. The system resistance person – shoes The resistance, which is measured between the
– floor is also a opportunity to do an assessment surface and the earth potential, could also called as
about the floor. earth resistance to ground.
Figure 10 Arrangement according to IEC 61340-4-1 for the measurement of resistance to ground
Figure 11 Arrangement according to IEC 61340-4-1 for the measurement of the surface resistance
Another method for the test of ESD – protection Moreover for further tests at working places and
systems can be the permanently observation of the especially at machines, it is necessary to measure
ESD – working place and the ESD – area. This and observe the field strange. Today the most
method guarantees a constantly control about all problems appear at the automatic pick-and-place
protection steps at the working place. Beside the machines, like already described. The registration
control about the resistances to ground of persons of the electric field strange can only be done with a
and working places, the grounding systems can be suitable sensor. Field meters are often not able to be
observed with the permanently observation used. Further test are necessary.
Conclusion It is decisive, that a general ESD – protection
The existing regulations/standards give a scope for system have to be installed and will be controlled
extensive ESD – protection systems. But it is not regularly. The observation of the ESD – protection
enough for it, because of the new sources of system have to documented as well as the regular
electrostatic charges, which are always developed audit of the complete ESD – manufacturing. The
and which are not recognized until now. The company management is responsible for the
conscious realization of the valid standards and of realization of the observation and the audits.
course the perfection of the knowledge about Usually an ESD – coordinator is employed, who
electrostatic charge and discharge proceeding are check the steps at place and who perfected its
the best. knowledge every time.
/1/ H. Berndt
A study of the Variables of Electrodes Used in the /4/ IEC 61340-5-2 Electrostatics - 02.99: Part 5:
Measurement of Table and Floor Materials and Specification for the protection of
How They Affect the Test Results electronic devices from electrostatic phenomena
23. EOS/ESD-Symposium 2001, Portland Section 2: User guide
/2/ H. Berndt /5/ IEC 61340-4-1 Electrostatics - 1998: Part 4-1:
VDE-Schriftenreihe - Normen verständlich - Band Standard test methods for specific applications –
71 Elektrostatik - Ursachen, Wirkungen, Electrostatic behaviour of floor coverings and
Schutzmaßnahmen, Messungen, installed floors
Prüfungen, Normung, VDE-Verlag, 1998 /6/ ANSI/ESD S20.20-1999 ESD Association
/3/ IEC 61340-5-1 Electrostatics - 12.98: Part 5: standards for the Development of an Electrostatic
Specification for the protection of electronic Discharge Control Program for – Protection of
devices from electrostatic phenomena Electrical and Electronic Parts, Assemblies and
Section 1: General requirements Equipments
/7/ IPC 610 C
/8/ IEC 61340-4-5 Electrostatics – xxxx: Part 4-5:
Standard test methods for specific applications –
Methods for characterising the electrostatic
protection of footwear and flooring in combination