esd-protective clothing for a new european...

ESD-protective clothing forelectronics industry - A new European

research project ESTAT-Garments

VTT TECHNICAL RESEARCH CENTRE OF FINLAND

6th Dresden Textile Conference, 19-20 June, 2002, Jaakko Paasi 2

Authors

• Jaakko Paasi, VTT (FIN)• Gianfranco Coletti, University of Genova (I)• Lars Fast, SP (S)• Philippe Lemaire, Centexbel (B)• Christian Vogel, STFI (D)• Terttu Peltoniemi, Nokia (FIN)• Giuseppe Reina, Celestica (I)• Arne Börjesson, agb-konsult (S)• Jeremy Smallwood, Electrostatic Solutions (UK)



Content

• Function of ESD (ElectroStatic Discharge) protectiveclothing

• ESD sensitive electronic components• Modern ESD-textiles• How garments could damage electronic components?• Standard test methods for ESD-garments and -textiles• ESTAT-Garments project• Conclusions



Function of ESD-protective clothing

• About 30-50 % of all failures in electronic productsdetected during manufacturing can be attributed tosome kind of electrical overstress, of which electrostaticdischarge (ESD) is one type.

• The main function of ESD-protective clothing (ESD-garments) is

to protect sensitive electronics from failures ofelectrostatic origin.



ESD sensitive electronic components

Example of an ESD damage to an IC

Photos courtesy of RohmElectronics (UK) Ltd



ESD sensitive electronic components

• Progress in semiconductor industry has lead, as a side-effect, todevices which are more sensitive to electrical disturbances thanever before.

• ESD sensitivity of many components is in kV-range (HBM) butthe number of ultrasensitive devices with ESD sensitivity evenbelow 100 V is rapidly increasing.

• There are several device technologies which are very ESDsensitive, such as MOS-devices, RF-transistors, RF-diodes, RF-ICs, MR-heads, flat panel displays, etc.

• Human being cannot feel an ESD below 2-4 kV.• An operator can easily charge up to several kV if correct ESD

protective measures are not taken.



Modern ESD-textiles

• Requirements of electronics industry have resulted inthat modern ESD-garments are made of heterogenouscomposite fabrics where a grid or stripes of conductivethreads are present inside an insulating matrix ofcotton, polyester or mixtures of these materials.

• The conductive threads are more and more frequentlymade by composites, i.e. by a mixture of conductiveand insulating fibres.

• There are several variations in both fabric and threadstructures.



Modern ESD-textiles

(a) Structures ofhomogeneous andheterogeneous textiles

(b) Structures ofcommonly usedconductive fibres



Modern ESD-textiles

Cross-sectional SEM-images of commonly used conductive fibres



How garments could damage electroniccomponents?

• An ESD failure caused by charged operator or charged clothingcan happen, at least, in three different ways:

1 . by a direct discharge from the body of the operator, fromunearthed conducting threads of the garment, or from insulatingsurfaces of the garment fabric.

2 . by discharge from the charged device. A device becomes, atfirst, charged by induction due to electric field from chargedclothing or by triboelectrification and, after that, gets groundcontact giving rise to an ESD.

3 . by radiation (damage by electromagnetic pulse due to nearbyESD) but strong ESD is required for a failure.

• A damage will happen in an ESD if the withstand level of thevictim component is exceeded.



How garments could damage electroniccomponents? - Direct discharge

Directdischargefrom thebody of theoperator to adevice



How garments could damage electroniccomponents? - Direct discharge

• A potential difference (voltage) is required for an ESD.• Many components are not damaged by the overvoltage

due to the ESD but by the discharge current (or actuallydischarge energy or discharge power).

• The risetime, duration and peak value of the dischargecurrent have also an influence on whether a componentcan withstand the ESD or not.

• Discharge current and its waveform are determined by• the discharge voltage• the conductivity and capacitance of the ESD source• the impedance of discharge circuit.



How garments could damage electroniccomponents? - Discharge from charged device

Dischargefrom achargeddevice toground



How garments could damage electroniccomponents? - Discharge from charged device

• A device may become charged in an electrostatic field arisingfrom the garment.

• Electrostatic fields external to the garment may be dependent on• rate of charging or charge dissipation of the garment

materials• ability of the garment material to shield the electrostatic field

of underlying garments• suppression of fields by coupling to the body of the person

wearing the garment.• A device may become charged also by accidental rubbing of the

package against the garment material.



Standard test methods for ESD-garmentsand ESD-textiles

Measurement of surface resistivity and point-to-point resistanceaccording to the standard IEC 61340-5-1.




Measurement of point-to-point resistances of ESD garments accordingto the standards ESD STM2.1 and IEC 61340-5-1.




• The standard test methods for ESD-protective fabricsand garments used in electronics industry are resistive.

• They were originally developed for homogeneousmaterials.

• They do not fully satisfactory qualify the effectiveness ofnovel ESD-protective garments made of compositefabrics.

• Too often the current practice methods measure onlythe electrical performance of the net formed by theconductive threads.




• Current practice resistive methods do ignore potentiallyimportant factors such as

• the effect on performance of grounding the conductivegarment elements,

• the risks introduced by unearthed conductive fibres as apossible source of ESD,

• possible charge storage and ESD risk arising frominsulating areas of a heterogeneous fabric,

• the possible penetration of electrostatic fields fromundergarments through the fabric,

• triboelectric propensity.




• Resistive methods do not correctly qualify the performance ofcore conductive fibre fabrics.

• Lack of good electrical contact between the electrode and theconductive fibre can happen also for other types of compositethreads.



ESTAT-Garments project

• As it is not certain that the present standard testmethods for garments used in electronics industryindicate how much the garments will protect theelectronics from ESD, the European Commissionissued a call for a research about a new test method forESD-garments.

• As a response to the call, a European research project“Protective clothing for use in the manufacturing ofelectrostatic sensitive devices (ESTAT-Garments)” wasstarted in early 2002.




• The project partners –

VTT Technical Research Centre of Finland (FIN), University of Genova (I),

SP Swedish National Testing and Research Institute (S),Centexbel (B), STFI Sächsisches Textilforschungsinstitut (D), Nokia (FIN), Celestica (I) - consist of experts of electrostatics, electrostaticmeasurements, textile technology and electronicsmanufacturers (end-users of the garments).




• The main goal of the three-years project is to supply thestandards body IEC TC101

1. with a basis to qualify the effectiveness of clothingused for the ESD-safe handling of ESD sensitive(ESDS) devices and

2. to develop appropriate test methods for thecharacterisation of such ESD protective garments.

• The development of test method(s) is backed byfundamental research and studies.




• During the first half of the project we will1 ) identify and quantify different types of electrostatics threats

caused by clothing to sensitive electronics,2 ) elucidate the behaviour of charges on composite textile

surface,3 ) study the electrostatic interrelations between the

components of the system composed of operator - ordinaryclothing - protective garment - sensitive electronics,

4 ) characterize the material and construction of different kindsof ESD-garments and

5) characterize the electrostatic properties of ESD-garments.




• After the mid-term the findings will be collected andtransformed to a new test method(s) or an existingmethod will be modified.

• Verification of the methods will be done byinterlaboratory tests.

• Final results should be available in spring 2005.



Conclusion

• An European research project ”ESTAT-Garments” isrunning with an aim to give a basis to qualify theeffectiveness of clothing used for the ESD-safehandling of ESD sensitive (ESDS) devices and todevelop appropriate test methods for thecharacterisation of such ESD protective garments.

• Further information is available at

http://estat.vtt.fi

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