REDUCING ESD (ELECTROSTATIC DISCHARGE)THROUGH CLEANROOM GARMENT DESIGN

Norman Hurst, Director
Michelle Barker, Research and Development Manager
Micronclean, Skegness, UK

Static electricity is a contaminant causing possible problems in all areas of cleanroom activity. Polyester cleanroom garments produce static electrical charge as fabric rubs together or rubs against garments worn under the cleanroom garments. Electrical charges of many thousands of volts may be present on the garments during use. The charge may be discharged at any time to the cleanroom structure or articles or products in the cleanroom. Discharges of static electricity may cause problems in the following ways:

  1. Micro-electronics The discharge of static electricity may damage sensitive products such as microchips and disc drives. Discharges of static electricity occur so rapidly that current flow for a short duration may be high enough to damage products. For example magnetoresistive (MR) read heads used in hard disc drives are susceptible to ESD damage. ESD derived voltages of 100 volts and lower will produce currents of 25mA for 1ns which will damage MR heads. This type of ESD event would be undetectable during manufacture but products may fail at an early stage in use.
  2. Pharmaceutical Manufacturing These cleanrooms are not normally associated with products that are damaged by electrostatic discharge, however many pharmaceutical cleanrooms require protection:
    1. Protection against explosion caused by spark discharges of static electricity.
    2. Protection against damage to sensitive microelectronic measuring equipment.
    3. Protection against attraction or repulsion of micro-organisms by electrically charged cleanroom garment fabrics. In order for the cleanroom garment fabric to be validated to the original specification the fabric should carry no electrical charge.

Cleanroom Fabrics and Electrostatic Charge

Almost all modem cleanroom fabrics have a 'grid' of conductive yarns woven into the fabric. These yams contain carbon or other electrically conductive material so that the fabric becomes electrically conductive. It was thought that garments manufactured from such fabric would discharge to 'earth' if connected to a grounding point in the work area. The methods commonly used are by a wrist strap or by use of an electrically conductive floor and electrically conductive soled footwear. Garments manufactured by traditional methods prove difficult to electrically connect to ground and this approach, at best reduces electrostatic charge from several thousand volts to several hundred volts. In the past this degree of control was adequate but as products have become sensitive to voltages as low as 25 volts more effective control is required.

The Challenge

Some four years ago our company was approached by a major electronic component manufacturer requiring a cleanroom garment system that, when grounded at one point on the sleeve of the body garment, would produce ESD voltages of no more that 20 volts. In addition the garments were required to be capable of undergoing 50-70 commercial laundry process cycles with no degradation of original performance. It would also he necessary for the garments to meet both British and American Standards for static dissipative cleanroom garments.

Summary Of Standards Used To Design The Static Dissipative Garments

Garments shall be capable of being bonded directly to operators skin. 'Mere shall he electrical continuity of between 7.5 x 105 and 1 x 1012 ohm/sq. between both sleeves and the body of the garment.

The material of the garment shall have a surface resistivity on both the outward facing and inward facing sides of between 7.5 x 105 and 1 x 1012 ohm/sq. and be capable of being grounded.

The resistance from a contact with the hand to a metal plate on which both feet are standing shall be less than 3.5 x 107 ohm/sq.

Static Decay - 1000 volts to 50 volts in less than 2 seconds.

The Project

To produce a static dissipative garment system that:

  1. Is capable of being grounded from a single point on the garment
  2. Has a residual charge after grounding of less than 20 volts
  3. Has static decay times of less than two seconds when grounded so that charging the garment system to voltages greater than 20 volts in use is almost impossible
  4. Must be capable of commercial laundering with no loss of electrical performance
  5. Will continue to be improved to achieve zero volts residual charge.

Problems Associated With Connecting The Operator And/Or Garment To Ground using traditional garment systems

  1. If an operator is connected to ground by a wrist strap in contact with the skin the operator’s body voltage will be reduced to almost zero but charges of hundreds of volts may remain on the garments.
  2. If the garment does not electrically bond between panels or the fabric has high electrostatic decay times electrostatic charge may increase at a faster rate than dissipation to ground.
  3. The wrist strap may be connected to the garment in addition to the skin of the operators wrist but the earthing of the garment may be difficult as most garments of traditional manufacture are not electrically connected across the seams. Thus connecting the garment to earth at the wrist of the garment will only guarantee that charge is conducted to ground from the sleeve panel of the garment leaving charge on the remainder of the garment
  4. Although boots may have conductive soles they do not necessarily electrically bond to the body garment at all times.

Electrostatic Dissipation From Cleanroom Fabrics

The conductive yarns incorporated into cleanroom fabrics may have the conductive material exposed, partially encapsulated or totally encapsulated in a non conductive polyester. In order for our suggested system to function the conductive yarns must be partially or totally exposed in order that the panels of the garment may be electrically bonded and bonded grounding points may be established on the garment.

The Solutions

  1. Design garments that reduce the number of seams.
  2. Manufacture a garment system with electrically bonded panels and total electrical bonding between the body garment, headwear and footwear
  3. Trial to prove that system withstands commercial laundering (50 - 70 cycles).
  4. Use only fabrics that do not retain an electrostatic charge when grounded.

Meeting The Challenge

Using existing data on the electrical performance of cleanroom fabrics, and testing fabrics in-house a cleanroom fabric was chosen for the garments.

Totally new garment designs were produced and a number of systems of electrically bonding panels were developed. Methods of electrically interconnecting the footwear and headwear to the body garment were developed.

Garment systems were tested in-house and at an independent test house.

Static Dissipative Garment

"Normal " garment in similar fabric.

Further tests

The garment system may be continuously charged while the garment is permanently connected to ground.

It should not be possible to charge the system.

The garment system may be connected to the hood or boot and these garments may be connected to ground to verify electrical continuity of system.

Relevance Of Static Decay

The "normal" garment displays long decay times and a much higher residual charge than the "new" static dissipative garment. However, will the garments behave in a similar manner when worn by an operator? The answer is that cleanroom garments on the operator in use are influenced by many factors such as the method of grounding, the fabrics worn under the cleanroom garments and the conductivity of the cleanroom flooring.

While testing is difficult in cleanroom conditions it is possible to measure body voltage.

Testing Electrostatic Voltage On The Body

  1. Person wears the test garment seated on a plastic covered chair. A probe connects an electrostatic voltmeter to the hand.
  2. The wearer moves to create an electrostatic charge
  3. (tribocharge by rubbing garment on chair).
  4. The wearer stands. At the moment the chair and garment separate an electrostatic charge may be recorded by the electrostatic voltmeter.
  5. Garments may be tested in various ways

Results

Clean room garments- Traditional construction

With the test person grounded at the wrist body voltages generated decayed rapidly. However residual charge often remains on the garment (typically 50 to 300 volts)

Static dissipative cleanroom garments

With the conductive cuff of the garment grounded and in contact with the wrist of the test person body voltages voltages decayed rapidly (typically in less than 0.2 seconds) and residual charge on the garment was below measurable levels.

Conclusions

Electrostatic testing must be carried out both on fabric and on the complete garment system.

It is possible to produce an electrostatic dissipative garment system that prevents an electrostatic charge of more than 20 volts being retained.

Provided the cleanroom floor is conductive (surface resistivity of between 1 x 104 and 1 x 1012 ohm/sq.) or the new garment system is grounded to a bonded connection on the body garment electrostatic charge will not reside on the garment.

It will be possible to produce systems that retain voltages less than 20 volts by the further development of the system.

It is not possible to measure accurately electrostatic charges that are produced by cleanroom operators in the production environment. The use of a proven static dissipative cleanroom garment system ensures that the operator is removed as a source of damaging electrostatic discharge (provided that the system is at all times bonded to ground).

 

References

1.British Standard BS EN 100015-1 1992 Basic specification: Protection of electrostatic sensitive devices.

2.ESD association standard for the Protection of Electrostatic Discharge Susceptible Items.