CORDIS Archive

Protective clothing for use in the manufacturing of electrostatic sensitive electronics

1. CONFORMITY WITH THE WORK PROGRAMME

This topic falls under the Competitive and Sustainable Growth Programme, generic activity Measurement and Testing. Specifically, it is related to Objective GROW-2000-6.2.1 Methodologies to Support Standardisation and Community Policies for which expressions of interest have been called

2. KEYWORDS

Composite materials, protective clothing, garment, electrostatic, ESD, discharges, electronic, test, method, measurement.

3. SUMMARY OF OBJECTIVES AND JUSTIFICATION

Operators handling unprotected electronics (ICs, PCB, components, equipment etc.) have to wear protective clothing (garments) to avoid electrostatic discharges (ESD), which could cause failures to the electronics. Modern garments are made by composite (conductive threads and insulating matrix) fabrics. The physical phenomena (injection, motion, retention/release of charges within a composite material) underlying the ESD protection function of such garments (in combination with normal clothing) are, as yet, not fully understood. This fact hinders the definition of the properties/processes providing the garments protective function, vs. electronics and vs. operator's safety, thus making the standardisation of requirements and test methods a very difficult task, as evidenced by the recently formed WG5/Team 2 of IEC TC 101 (Electrostatics). It is proposed to implement a research work to solve such a problem.

4. BACKGROUND

General

The problems arising from static electricity are caused by electrically insulating materials. The problems extend from the failure of electronic devices and systems to life-threatening situations (fires and explosions). One common factor is people. Charge can be acquired on the body or clothing during normal activity and the consequent electrostatic discharges (ESD) can initiate the hazardous event/s. An obvious solution is to electrically ground all objects, including people, in the hazard area, but this cannot be practically achieved as some objects have insulating nature. New, complex, composite materials have been developed to counter these problems, both as textiles for protective clothing and as less flexible materials are concerned. The characterisation of these materials, however, poses a difficult problem for the development of quality control and functional effectiveness tests. Indeed, several international standardisation bodies, including IEC (namely TC 101 "Electrostatics"), CENELEC (e.g. TC 162) and ISO TC122/SC2, had or have on-going projects or activities to develop standard test methods in this area. For instance, the past EU project SMT-CT96-2079 established requirements and evaluation methods of protective clothing for use in flammable atmospheres, supplying CEN TC162/WG1/PG2 with a basis for a relevant standard. While those and similar studies on composite materials [1] are clearly to be considered here, a key difference deserves attention: such problems are characterised by critical ESD voltages/energies much higher (even orders of magnitude) than the limits typical of the electronic industry.

In one sentence: the characteristics of static control materials can be very different for the several hazardous circumstances.

As ESD hazards to electronic devices/systems are due to their sensitivity to low or to very low levels of ESD voltage/energy, specific and careful protection schemes are usually enforced.

Practically, damage (direct or latent) to sensitive electronic devices/systems during manufacturing or service, can originate from static charge on the body, or on the clothing of the operator. Today's lower sensitivity limit to an ESD event is about 10-30 V (VMOS IC technology, magnetoresistive heads for hard disks, et al.). Tomorrow's expected limits are more critical: for instance, the 0.050 microns ICs semiconductors technology (indicated by the International Roadmap for Semiconductors 1998 Upgrade (SIA), as a CMOS generation for years 2003-2006 [2], compared to the present 0.18 microns technology, is likely to cause much tougher ESD protection problems.

The present solutions are the direct grounding of personnel (e.g. conductive bracelet) and the provision of protective clothing (or garments): over his/her normal clothing the operator is required to wear a garment. Typically, this garment comprises an electrically-conductive, fibre matrix within an, essentially, insulating fabric.

Apart from the tribo-electric effects, the electrostatic behaviour of homogeneous, isotropic materials is commonly (see IEC 61340 series of Publications) and basically described by two properties: (surface or bulk) resistivity (or resistance) and charge decay. However, the above garments material (composite fabric) static performance cannot be described in that way, as demonstrated in [3, 4].

Resistance and charge decay measurements in composite fabrics

In this case, measurement of resistance between two electrodes contacting the material can yield a very low value if the electrodes contact only the conductive matrix, but extremely high values if contact is made only to the substrate electrical insulator. Similarly, measurement of macroscopic charge dissipation (or charge-decay) can show very fast charge loss via the conductive matrix but quasi-infinite charge retention on the insulator. As the contact between electrodes and material cannot be dominated, any measurement, performed in agreement with the current standards, of the latter properties is not repeatable.

Therefore it is not a measurement.

Evaluating garments ESD protective performances.

As the failure of initial approaches, based on the adoption of properties as resistance and charge decay, to measure the performances of the above protective clothing (garments) called for (1) a medium term research effort to identify new properties and/or testing procedures suitable for a reliable characterisation and/or evaluation of such items, the electronic industry adopted a "pro tempore" pragmatic approach (2) to the problem.

While an operator, grounded through a conductive (1 Mohm) bracelet and wearing, over his/her normal clothing, a garment, replicates his/her "usual" movements, the electrostatic voltage around him/her is measured at specified distance/s. This functional test approach is currently adopted in industry: it can be quite effective to select the protective clothing most suitable for a very specific job (related to specific working ambients, people, normal clothing, habits, special shoes, process requirements, etc.). However this approach has limitations, from the point of view of standardisation..

For instance, the first and basic issue is the lack of modelling: it is debated wheter protection is provided by shielding of the charges residing on normal clothes or by dissipation of such charges to ground (via operator body and bracelet).

A secondary issue is represented both by the influence, on the output of this functional approach, of "non-controllable" variables (e.g. type of normal clothing, people habits, etc.) and by other factors [effects of repeated washing of the garments, release of fibers (particularly critical when operating in clean room), insulating capabilities of the garment outer surface (required to avoid dangerous short-circuits: e.g. a sleeve on a PCB), et al.[4] ].

Actually, a successful path to achieve a Standard for evaluating the garments performance requires medium-term research work to overcome even the latter problems.

In summary:
focused medium-term research work is needed to arrive at a Standard to evaluate the performances of ESD protective garments, taking in account point (1) and point (2).

This is exactly the position of IEC TC 101 (Electrostatics), which, in July 1999 [docs. IEC 101/58/NP, IEC 101/75/RVN, IEC 101/77/RM], acknowledging the above situation:

- assigned to WG5/Team 2 the task of preparing a Standard on this subject (Project IEC 61340-4-2; Title: Requirements for ESD-protective garments )
- allowed WG5/Team 2 a 3-4 years working schedule [doc. IEC 101/85/PW and hereby enclosed letter by the IEC TC 101 Chairman]

5. ECONOMIC AND SOCIAL BENEFITS

European dimension

For the proposed topic to succeed a high degree of competence is required in very specialised skills that cannot be found in any single nation of the European Union. The required expertise is related both to the end-user side and to the scientific side.

As to the end-user side, three main elements are in evidence:

1) the European dimension of the proposed research is reflected into the benefits brought, by the relevant results, to the electronics industry, which is widely active in all EU countries, even at the SME level
2) in this field, the strength of european industry lags well behind the north american industry, as demonstrated by the numerous US "Standards" issued by the ESDA (Electrostatic Discharge Association, formerly EOS/ESD association) and by a comparison of the number of related scientific publications originated in Europe and in the US. The proposed research activities would help filling this gap.
3) direct cooperation from electronic industries affected by these electrostatic protection problems is essential to the success of the proposed research: the osmosis between the every-day practical experience/s and the scientific investigation activity would create precious synergies both during the project development and at its validation stage.

As to the scientific side, which includes access to specialised facilities, sectorial expertise has emerged in some EU countries (e.g. UK, France, Germany, Sweden, Italy, et al.). For a project addressing this topic to achieve success in the required 3 years time-frame (see below), it would be very important that recognised centres of expertise are brought together into a collaboration that synergistically builds on the existing physical knowledge, techniques and know-how.

Furthermore, the suggested cooperation between industries and research centres (this denomination includes, here, also university laboratories) would bring, besides technical results, important advantages from the point of view of :

- increasing the exchanges between industry and research centers in Europe
- establishing, between laboratories from different European countries, new links which could give rise to future collaborations
- strengthening the cultural exchanges among citizens from different European countries

Safety Impact

Besides the above outlined principal issues related to this proposed research topic, a further safety issue deserves attention.
A modern composite garment, should satisfy also a simple requirement:
"it should avoid any short-circuit which, through an inadvertent movement of an operator (e.g. a contact through a sleeve) could cause damage to the electronics or even cause harm to operator (electric shock)". So, the proposed research should show the route to fulfil such operator's safety protection requirement and the test/s to check its implementation.

Improved metrology. Impact on Trade.

The proposed research topic is focused on creating the basis for a new Standard which does not exist yet. The immediate consequences of the relevant results, therefore, will be obvious: the existing trade of garments would be improved, an appropriate competition between garments manufacturers would be fostered, the latter manufacturers would be supplied with "incentives" to improve their products, etc.

However, achieving the goals of the proposed topic could bring further consequences.
That is it could bring an advancement of the present physical knowledge about injection, motion and retention/release of electrostatic charge within inhomogeneous materials, both at macro and at micro scales (that is in a resolution range which could span from nm to mm ).

Detailing here the possible "fall-out", within the realm of Standardisation problems, would lay out of the presently proposed topic boundaries. Nevertheless if a "panoramic view" of similar physical mechanisms underlying static problems in composite materials was to be outlined, it should include also "standardisation works" which are linked to safety issues and are under way both within IEC 101 (WG7: "Electrostatic properties of flexible intermediate bulk containers (FIBC) - Test methods and requirements" : in connection with ISO TC 122) and within CEN TC 162/WG1/PT2.

Economical Benefits and Social Impact

Spanning all the spectrum of the electronics industry activities, from manufacturing to service, general estimates of the economic losses due to ESD related failures in electronic ICs/components indicate [5] average losses ranging from 8 % to 33 % . In absolute terms, the actual annual cost (loss of money due to ESD related failures detected before shipping a device/system) of ESD damage to the electronics industry world-wide can run into the billions of US dollars. In fact, while the cost of damaged devices themselves range from only a few cents for a simple diode to several hundred US dollars for complex hybrids, when associated costs of repair and rework, shipping, labour, and overhead are included, the direct economic losses can become considerable.
Therefore, achieving the aim of the proposed research topic (obtaining a firm ground to build a standard concerning ESD protection clothing) would bring the technical consequences related to garments as well as noticeable advantages in terms of improved electronics manufacturing statistical yield (decrease of detectable defects) and in terms of increased reliability of electronics in the field (see problems of latent defects).

But the above losses, though quite important, are not the only key point.
Latent defects in electronic devices/systems can be originated by ESD events.
Latent defects, by definition, go undetected past the in-house tests and can display their potential as "source of early failures" in the field, during normal operation. In case such unfortunate "early failures due to latent defects" happened, the manufacturer's image would suffer a huge negative impact.
Although the latter highly undesired events cannot be "measured" simply in terms of money, nevertheless they are widely acknowledged as "catastrophic" events.
So another focal point is what would happen if the products were shipped to the customer while possessing undetected defects or latent defects (in this case, caused by ESD events).
Both the above illustrated impacts of ESD damages on electronic devices/systems are quite undesired.
The counter-measures are the same: a specific and detailed ESD control program. Within such a program, especially when production and handling of highly ESD sensitive objects is concerned, a key role is played by ESD protective clothing (i.e. garments).

6. SCIENTIFIC AND TECHNOLOGICAL OBJECTIVES

The research work required to develop the proposed topic regards composite materials, made by a network of conductors (different types: from metallic threads to carbon filaments, et al) buried in an insulating matrix (a fabric) and it would imply two main and different, though complementary, objectives:

7. TIME SCALE

The time scale of the research work required by the proposed topic should approximately match the working schedule of IEC TC101/WG5/Team 2, that is three years.

8. IMPORTANT ADDITIONAL INFORMATION

The proposal must demonstrate that there will be an effective contribution to European standardisation in this area. In particular, it is here recalled that IEC TC 101/WG5/Team 2 should be kept aware, about the results of the proposed research activity. Besides the output of the relevant studies should be disseminated though all potentially interested standardisation bodies .

References