Category Archives: Smocks and Shirts
We have mentioned the term “Faraday Cage” many times on this blog before – for example when talking about the transport and storage of ESD sensitive items or the role of ESD lab coats in ESD Protected Areas. When discussing ESD protection, the concept of the “Faraday Cage” will always come into play. But what exactly is it? Read on to find out…
A Faraday Cage or Faraday shield is an enclosure formed by conducting material or by a mesh of conductive material. Such an enclosure blocks external static and non-static electric fields. Faraday Cages are named after the English scientist Michael Faraday, who invented them in 1836.
An impressive demonstration of the Faraday Cage effect is that of an aircraft being struck by lightning. This happens frequently but does not harm the plane or passengers. The metal body of the aircraft protects the interior. For the same reason, a car may be a safe place during a thunderstorm.
Lightning striking an airplane
In ESD Protection, the Faraday Cage effect causes charges to be conducted around the outside surface of the conductor. Since similar charges repel, charges will rest on the exterior and ESD sensitive items on the inside will be ‘safe’.
ESD control products that provide a Faraday Cage or shielding include Statshield® Metal-In and Metal-Out Shielding Bags or Protektive Pak™ impregnated corrugated boxes with shielding layer when using a lid.
ESD shielding packaging is to be used particularly when transporting or storing ESD sensitive items outside an ESD Protected Area. Per Packaging Standard EN 61340-5-3 clause 5.3 Outside an EPA “Transportation of sensitive products outside of an EPA shall require packaging that provides both:
– dissipative or conductive materials for intimate contact;
– a structure that provides electrostatic discharge shielding.“
ESD Smocks create a Faraday Cage effect around the torso and arms of the operator and shields charges from the operator’s clothing from damaging ESD sensitive devices. (Technically, suppressing the electrical field from clothing worn underneath).
There are standard tests measuring the energy penetration of electrostatic discharges to the interior. The Shielding test method per Packaging standard EN 61340-5-3 is ANSI/ESD STM11.31 and the required limit is less than 50 nanoJoules of energy.
Definitions from the ESD Association Glossary ESD ADV1.0 include: Faraday Cage “A conductive enclosure that attenuates a stationary electrostatic field.”
Electrostatic discharge (ESD) shield “A barrier or enclosure that limits the passage of current and attenuates an electromagnetic field resulting from an electrostatic discharge.”
Electrostatic shield “A barrier or enclosure that limits the penetration of an electrostatic field.”
Most people tend to believe that if a person is wearing a wrist strap, an ESD lab coat (also known as smocks) is redundant. This is due to the belief that any charge on the person or their clothes would find its way to ground via the wrist strap. This is a very common misconception and today’s blog post will explain in more detail why you should be considering the use of ESD lab coats in your ESD Protected Area (EPA).
Purpose of ESD lab coats
Although the ESD Standard does not require ESD lab coats, they are a very practical. Some even believe, ESD lab coats represent the single most important step to demonstrate commitment to an ESD control programme.
As we have learnt previously, all process essential insulators should be kept at a minimum distance of 31cm from ESD susceptible items. Clothing, particularly when made from synthetic fibres, are significant charge generators. Worse for ESD control, the fabric is an insulator so the result can be very threatening: an isolated charged insulator which cannot be grounded.
An insulator will not let charges flow and will therefore hold the charge until either neutralised over time (naturally over hours or days) or with an air ioniser (artificially under a few seconds).
In the meantime, your sleeves, waist, etc. may have several thousand volts (a very significant electric field to expose nearby conductors) that may induce charges on nearby isolated conductors. This is the main reason people wear ESD lab coats: so they can shield the insulative clothing and minimise the electric fields generated from their clothing.
Examples of lab coats – for more details click here
“The ESD risk provided by everyday clothing cannot be easily assessed. The current general view of experts is that the main source of ESD risk may occur where ESDS [ESD sensitive items] can reach high induced voltage due to external fields from the clothing, and subsequently experience a field induced CDM [Charged Device Model] type discharge. So ESD control garments may be of particular benefit where larger ESDS having low CDM withstand voltage are handled, and operators habitually wear everyday clothing that could generate electrostatic high fields.” [CLC TR 61340-5-2 2008 User guide Garments clause 188.8.131.52 Introductory remarks]
ESD lab coat properties
Most lab coats are constructed of a dissipative material which incorporates texturised polyester and carbon nylon fibres. The conductive nylon fibres are woven in a chain-link design throughout the material, providing continuous and consistent charge dissipation.
ESD lab coats are an ESD protective product that should possess the following ESD control characteristics:
- Antistatic low-charging so they minimise the generation of electrostatic charges;
- Dissipative so when grounded they will remove charges to ground;
- Shielding creating a “Faraday Cage” effect so they will restrict charges generated on the user’s clothing to the inside of the ESD lab coat and
- Groundable so the user can easily and reliably connect them to ground.
Installation and grounding of ESD lab coats
Follow the directions below for proper installation and grounding of the ESD lab coat:
- Put on the lab coat and fasten all of the snaps on the front of the lab coat, making sure that clothing is not exposed outside of the lab coat.
- Throughout use, it is essential that the conductive cuff is in intimate contact with the wrist skin. The conductive cuff should never be allowed to be pulled up and over the shirt sleeve.
- Ground the ESD lab coat. A popular way to ground an ESD lab coat is with a coiled cord either attached to a snap on the waist area of the lab coator via a wrist strap snapped to the inside cuff of an ESD lab coat. If none of these methods are suitable, the lab coat should be grounded via the person’s wrist removing charges via ESD footwear to ESD protected flooring.
Wearing your ESD lab coat correctly
“Garments on which high levels of static electricity can be generated are one of the causes of ESD damage. It is important that such charged garments do not come into contact with ESDS. The covering garments need to be grounded, either through direct contact with the wearer’s skin, or by alternative means such as being electrically connected to a wrist strap. It is important that the ESD protective garment sleeves cover the end of the inner garment sleeves.” [EN 61340-5-2 paragraph 5.2.5.]
Grounding a lab coat using the snap at the waist
ESD lab coats are a conductor and therefore should be grounded. If not grounded, the ESD garment can be a potentially threatening isolated charged conductor. If an operator is wearing a lab coat but is not electrically connecting the lab coat to either their body’s skin or ground, then charges on the lab coat may have nowhere to go or discharge to.
Testing of ESD lab coats
Panel-to-panel conductivity is essential to ensure portions of the lab coat are not left as isolated charged conductors. A Resistance Test Kit can quickly measure resistance of the fabric and ensure panel-to-panel conductivity by placing five pound electrodes on different fabric panels.
Testing panel-to-panel conductivity using 222635
To ensure that the fabric is low tribocharging, a Static Field Meter can be used to measure charges generated by causing contact and separation with other materials. In addition, the Static Field Meter can demonstrate shielding by measuring a charged object and then covering the charged item with the ESD lab coat. Being shielded the measured charge should be greatly reduced.
Cleaning of ESD lab coats
The proper method to clean a lab coat is to wash the garment in cool or warm water, tumble dry with low heat or hang dry. Do not bleach your ESD lab coats! Make sure you only use non-ionic softeners and detergents when laundering.
Please also note that lab coats should not be altered in any way. The lab coats effectiveness is in fully covering the human body and street clothes – especially at the wrists and front of the body. Altering the lab coat in any way will nullify its effectiveness.
The typical useful and effective life of a lab coat under normal wearing and recommended washing conditions is a minimum of 75 washings.
Questions for you: Do you use lab coats? If so, what’s the reason you started using them?