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ESD prevention People grounding by Flooring system and - ESD Journal

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ESD prevention People grounding by Flooring system and - ESD Journal Powered By Docstoc
					          ESD prevention

        People grounding by

Flooring system and conductive shoes
                                        The institute of Physics…..
                                        Electrostatic Measurement
                                        13 -th November 2001……



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     The place of ESD at Workshop

• 1 - 60 to 70 % of electronic components failures are due
   to ElectroStatic Discharges (ESD).
• 2 – In 1991 HP said : 90 % of ESD failures are latent
   ones .
A latent failure does not give an immediate failure but
   entrain a failure later, a few days or some months
   later.
This kind of failure affect directly the reliability of
   electronic equipments.


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                              ESD Cost
• Many data are available from several sources.
• Hewlett Packard, at the end of 80 th, estimated the gain
  equivalent to 50 000 PC the first year following the setting up of
  an ESD control program.
• Estimated cost given for year 1993 at world level:
            8 % of electronics market (about FFR 45 milliards (GBP
  ~ 5 billions).
• My own experience (15 years with ESD protection): -
        About 10 ESD faults formally identified
        Cost from GBP 100 to GBP 100, 000
• One example: During qualification of new ASIC components
         20 components failed, cost ~ GBP 15,000.
• But ESD failures, are difficult to identified !….


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       Different ESD models (RD 1)
• 1 - Human body Model :               HBM,
• 2 - Machine Model :                  MM,
• 3 - Charged Devices Model :          CDM,
• 4 - Field Induce Model :             FIM ,
• 5 - Charged Chip Model :             CCM .
The ESD Model constraints depend on process used.
• A basic rule:

       No charge, no discharge.

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 Components class 0 thresholds
               Model                Class / Thresholds

                HBM                      0 < 250 V

                MM                      M 0 < 25 V

                CDM                     C 0 < 125 V


• In an Electro Static Discharge control plan, the
  sensitivity of components to be handled, must be well
  known.
• The rules and practices must take into account the
  sensitivity of components and the processes used.

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Handle Components/equipments class 0.
• Handling of class 0 components or equipments involve using every
  possible methods to fight against the presence of electrostatic
  charges:
• Grounding the person‘s body, (wrist-strap, conductive shoes on
  dissipative floor);
• Personal equipment (smocks, wrist-strap and its controller,
  conductive gloves,…
• Grounding materials or equipments (working surface , iron…),
• Shielding,
• Packaging (antistatic, conductive),
• Ionization,
• People training,
• Audit…..
 STATIC ELECTRICITY MUST BE UNDER CONTROL
            No charge, no discharge.
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   How to eliminate Electrostatic Charges?
• Conductors materials:
  Connect them to ground in respect of security rules, a
  person is a conductor too, he must be connected to
  ground.
• Insulating materials:
  Eliminate all insulating materials which are not necessary
  in the environment of the working surface.
Get the other conductors, with antistat for instance and connect
  them to ground.
• Elimination of parasitic electrostatic fields.
  Use, ionization systems, dissipative garnements, gloves…
            No charge, no discharge.
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         Standardization Procedure
• The rules, method and limits to eliminate the ESD risk
   are given in standard documents:
        IEC 61340 5 -1 (RD2),
        IEC 61340 -5-2 (RD3).
• 1 - Methods:
Elimination of electrostatic charges on person :
. Persons can be grounded by antistatic wrist-strap or by
   dissipative flooring system with conductive shoes, it
   depends of people activity.
          No charge, no discharge.

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        Standardization Procedure

• 2 - Limits
  Depend on components sensitivity,
  Most sensitive components are classified in class 0 in
   each model, and sensitivity can be less than 20 V.
"For components with an ESD sensitivity less than hundred
   volts the ESD coordinator have to define complementary
   rules“like indicated in standard documents ( 61 340-5).

          No charge, no discharge.

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                        The objective
• The objective was to handle, very sensitive and expensive electronic
  equipments by people in standing-up-position.
• ESDS (ESD sensitive components) are announced as very sensitive.
  The threshold ESD in human body model is announced at about 20 V.
• Smocks are antistatic/dissipative. The other classic equipments and
  rules are respected.
• The use of antistatic wrist strap entrains large difficulties.
• So we want to use the floor as primary way to eliminate static charges.
• In this configuration, people walking with conductive shoes must not
  develop a static charge > 10 V (10 V take into account a margin of
  6 db(1/2) versus ESD sensitivity of components).

              No charge, no discharge.

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   The performance of a flooring system can be
   characterised by the following specifications
• Body voltage generation (BVG) as low as possible:
                < 10 V in our case,
• Resistance to ground (Rtg):
Lowest value must take into account security of persons.
Highest value must guarantee the eliminations of electrostatic charges
   and a low charge accumulation.
• Surface Resistivity (Rs):
Same constraints as for resistance to ground (Rtg)
• Decay time (Td).
The value must be low enough to eliminate charge in time compatible
   with the human activity.

            No charge, no discharge.
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How does the static electricity is generated?
• The static electricity is mainly generated by tribo-electricity, due
  to the shuffle, rubbing, between sole shoe and flooring covering.
• If the resistance between sole shoe and ground cannot eliminate
  the charges, there is accumulation of charges at sole shoe level.
• Then by influence, charges present on sole shoe attract the
  opposite charge at the feet of person who is charged by influence.
• As body is a conductor, the equivalent charges are present at the
  level of his hands.
• The charges balance is got when charge generation is equal as
  charges elimination. The potential obtained is equal to BVG.
• The simplified equivalent electrical diagram is given next page.

             No charge, no discharge.

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Equivalent electrical diagram



                                Rtg    BVG
                  Cp
    Ig




         • BVG = I (g). Rtg 1*

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                                        BVG measurements
              Only few manufacturers give data about this parameter.
     These measurements are done following STM ESD 97-2 specification (RD 9).
      They are done with conductive shoes Transversal resistance < 10 M ohm

    Materials Specification (V) Measure (V) 2*                           Comments
              1*
                                                    *1 value available for HR > 25 %
       A             15                     ~0      *2 measure done for HR ~ 50 %
                                                    1*Value available for HR > 25 %
       B            < 10                    ~3      *2 measure done for HR ~ 50 %
                                                    1*Value available for HR >? %
       C          < 100 V                   ~5      *2 measure done for HR ~ 50 %
                                                    1*Value available for HR >? %
       D          < 400V                  ~ 160     *2 measure done for HR ~ 40 %
                                                    *2 measure done for HR ~ 50 %
       E      No specification              ~6
                                                    F :others manufacturers tested
       F      No specification           30/to140   *2 measure done for HR ~ 50 %
                                                    *1 Following IEC 1340 (insulative shoes)
       G         < 2000 V                   ~6      *2 measure done for HR ~ 50 %
                (answer to 61340-4-1)
                                                    *1 Following IEC 1340 (insulative shoes)
       H         < 2000 V                  ~6       *2 measure done for HR ~ 50 %
                (answer to 61340-4-1)




•    Materials A to F are resin, Materials G and H are PVC carbon loaded.
•    F line cover several resin materials tested.


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                        Rtg Measurements
         Materials Specification Measure                                           Comments
                   (M ohm)1* (M ohm) 2*
                    Consistent resistance                   *1 value available for HR > 25 %
            A      measurement at 10 to 500 <6              *2 measure done for HR ~ 50 %
                           V
                                                            1*Value available for HR > 25 %
             B          1/100                   4           *2 measure done for HR ~ 50 %
                                         (Few points <330)
                      <1 (61340-4-1)     <0.15 (61340-4-1) 1*Value available for HR >? %
             C      <35 (ESD STM97-1)   <35 (ESD STM97-1) *2 measure done for HR ~ 50 %
                                          (few points <330
                                                           1*Value available for HR > 25 %
             D        0.01 to 1         0.1 ~ to 20 K      *2 measure done for HR ~ 50 %
                                                           *2 measure done for HR ~ 50 %
             E            ?               0,1 to160
                                                             F :others manufacturers tested
             F        0.1 to10            1 to 1000          *2 measure done for HR ~ 50 %
                                                            *1 Following IEC 1340
             G           10               33/300*1          *2 measure done for HR ~ 50 %
                                                            *1 Following IEC 1340 (insulative shoes)
             H           0.1                  4             *2 measure done for HR ~ 50 %




•   Materials A to F are resin, Materials G and H are PVC carbon loaded.
•   F line cover several resin materials tested.


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                              Rs Measurements
    Only few manufacturers give informations about this parameter

    Materials Specification Measure                                           Comments
              (M ohm)1*              (M ohm) 2*
               Consistent resistance                    *1 value available for HR > 25 %
       A       measurement at 10 to
                                         <1             *2 measure done for HR ~ 50 %
                      500 V
                                                        1*Value available for HR > 25 %
        B            1/100                4             *2 measure done for HR ~ 50 %
                                   (Few points ? 330)
                                    <0.15 (61340-4-1)   1*Value available for HR >? %
        C               /         <35 (ESD STM97-1)     *2 measure done for HR ~ 50 %
                                   (few points <1000)
                                                        1*Value available for HR >? %
        D          0.01 to 1     0.1 ~ to 20 K          *2 measure done for HR ~ 50 %
                                                        *2 measure done for HR ~ 50 %
        E
                                                        *2 measure done for HR ~ 50 %
        F          0.1 to 10        1 to1000            Depends on probes used and materials
                                                        *1 Following IEC 1340
        G             10               600              *2 measure done for HR ~ 50 %
                                                        *1 Following IEC 1340 (insulative shoes)
        H             0.1               4               *2 measure done for HR ~ 50 %



•    Materials A to F are resin; Materials G and H are PVC carbon loaded. Materials
     with very low resistance < 10 K ohm are not include in these results and are not
     always good on BVG parameter.


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                            Theories….
• U= RI
• Q = CV = it
• S = 15*10–6 (r1 -  r2)
(Where, S is the surface charge density, r1 and r2 are permitivity of both
  materials in contact see RD 20)
• Q = s*S
(Where S is the surface in contact)
• V = Vo e t/RC
( t = 2.2 RC  V = 0,1 Vo; t = 5 RC  V = ~ 0 V)

              No charge, no discharge.


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                             Remarks (1)
• Measurement of resistances on some resin flooring system with
  ASTM probes (Ø 63.5 mm, weight 5 pounds) give very large
  variations.
• On same flooring system the use of IEC 61340-4-1 or 1081 probes
  give lower resistance values and better repeativity.
• But In case of difficulty with ASTM probes, very often BVG is
  larger than the lower resistance measured that we hope.
• Measurement on a good flooring system usually give very uniform
  resistance value, even with ASTM probes.
• Resistance values less than 100 M ohm is low enough.
• So the best solution is to use the real configuration following the
  documents ESD-STM 97-1-1999 with conductive shoes*(RD 8).

*Not all ESD shoes give the same results but these with Rt < 10 7 ohm usually give good
   results.

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                  Remarks (2)

• Measurements of Body Voltage Generation is defined
  by different standard documents (NF EN 1815, NF P
  62-001, ESD ASTM 97-2 1999 ).
We choose American norm ESD ASTM 97-2 1999 this
  document advocate real configuration ( conductive
  shoes, low displacement).

• For decay time measurements, normalisation document
  advocate to measure from 1000 V to 100 V; but an
  American flooring system manufacturer measure this
  parameter from 100 V to 10 V(it depends of needs).

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                      Apparatus (1)
• Resistance measurements
• Megohmeter
       Measure voltage 10 /100 V
       Resistance measure : 10 4 < R (ohm) ≧ 10 12.
       Normalised probes :
       ASTM (difficult to use on hard resin flooring system) ,
       IEC 61340-4-1,
        NF EN 1081.
*Repeteative measures at 100 or 10 Volts are a good omen to obtain à
  low BVG.
• Conductive Shoes
       Transversal resistance < 10 M ohm

            No charge, no discharge.
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                     Apparatus (2)
• Voltage measurements
• Charge analyser:
       Voltage > or = 1100 V
Adjustable thresholds.
       time measurements From 0.1 to ~ 60 seconds.
• Graphic recorder:
• Thermometer/ Hygrometer .

Many similar materials can be used.



            No charge, no discharge.

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                       Conclusions
• The single characterisation of flooring system by measurement of
  resistance to ground is not sufficient. The performance of a
  flooring system must be characterised by the following
  specifications with RH compatible with the environment.
• Body voltage generation (BVG) (following ESD-STM 97-2-1999) :
                      < 10 V (For ESDS 20 V)
• Resistance to ground (Rtg) :
• Rtg1* (following IEC 61340-4-1 / NF EN 10 85)
                      10 5 < Rtg 1 (ohm) < 10 8
• Rtg2 (following ESD-STM 97-1-1999)
                      10 5 < Rtg 2 (ohm) < 10 8
• Surface Resistivity (Rs*):
                        10 5 < Rs (ohm) < 10 8
• Decay time (Td)
                 Td < 0.3 s (1000 V to 100 V or less)

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                     Final Conclusion
   • The European standardization, the flooring
     system manufacturers, and the users should
         take into account these methods and
                      parameters.
             No charge, no discharge.
Do not forget :
A static dissipative flooring system with conductive shoes is an element
   of an ESD protection system. in workshop. It allow to completely
   eliminate ESD risk Human Body Model in standing working
   position.

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                Reference Documents (1)
•   RD 1: HDBK 263 Electrostatic Discharge Control Handbook for Protection of
    Electrical and Electronic Parts, Assemblies and Equipments.
•    RD 2: ICE 61340 -5-1 Protection of Electronic Devices from Electrostatic
    phenomena- general requirements.
•    RD 3: ICE 61340 -5-21 Protection of Electronic Devices from Electrostatic
    phenomena- user guide.
•    RD 4: ICE 61340 -4-1 Electrostatic behaviour of floor covering and installed floors.
•    RD 5: NF EN 1081 Revêtement de sol Résilient ; détermination de la résistance
    électrique.
•    RD 6: NF EN 1815 Revêtement de sol Résilient et Textiles; Evaluation de
    l’accumulation des charges électrostatiques.
•    RD 7: NF 61 002 Revêtement de sol, propriétés antistatiques.
•    RD 8: ESD STM 97-1 Floor Materials an Footwear- Resistance Measurement in
    combination with a Person.
•    RD 9: ESD STM 97-2 Floor Materials an Footwear Voltage Measurement in
    combination with a Person.
•   RD 10: ANSI ESD 20.20 Protection of an Electrical and Electronic Parts, assemblies
    and Equipment

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               Reference Documents (2)
•  RD 11: ANSI /ESD S 03.1 Ionization
•  RD 12: EOS /ESD Standard S 5, Human Body Model (HBM) ESD Sensitivity
   Testing
• RD 13: EOS /ESD DS 03/01/1996
• RD 14: Warmbier internet site address
• RD 15: ESD Systems Internet site address
• RD 16: ESD Association Internet web site address (www.esda.org)
• RD 17: NASA Internet site address
• RD 18: Grounding Personal via the Floor/ Footwear System Siemens AG document
   presented at the EOS/ESD symposium, Santa Clara USA 1997.
• RD 19: Electrostatic Damage in.Electronics Devices and Systems (William D.
   Greason)
• RD 20: Electrostatic discharge (Michel Mardigian)
Some other internet sites from manufacturers of resin or PVC flooring system, can
   be consulted.



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                         Knowledge
• Mister P PILLET the person who asked me to work on ESD risk
  about fifteen years ago. M. B. DEUTZ D'ARRAGON the person
  who asked for me to work again on this problem, and M
  P.MARSAL and M H. ZUGAJ for their listening during these last
  months, all these people belong to Alcatel Space Industries
  company.
• Warmbier, a German company for his presentation of the
  difficulties with resin flooring in 1999 and the solution proposed; I
  specially want to thank M. Meir GOLAN and M. PFEIFLE and
  their French representative M A.CHAPON.
• ESD Systems for its Internet site and the interesting answers from
  M. A.RYNE
• The persons of different flooring systems company who help me
  and send me samples to do my work.
• And my correctors madam M. BESSON and J.C. LEFRANC.

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