How to neutralise a charge on an object that cannot be groundedFollow article
We have learnt in a previous post that within an ESD Protected Area (EPA) all surfaces, objects, people and ESD Sensitive Devices (ESDs) are kept at the same electrical potential. We achieve this by using only ‘groundable’ materials or. But what do you do if you absolutely need an item in your EPA and it cannot be grounded? Don’t sweat, not all hope is lost! There are a couple of options which will allow you to use the item in question. Let us explain…
Conductors and Insulators
In ESD Control, we differentiate conductors and insulators. Materials that easily transfer electrons are called conductors. Some examples of conductors are metals, carbon and the human body’s sweat layer.
A charged conductor can transfer electrons which allows it to be grounded
Materials that do not easily transfer electrons are called insulators and are by definition non-conductors. Some well-known insulators are common plastics and glass.
Insulators will hold the charge and cannot be grounded and “conduct” the charge away
Both conductors and insulators may become charged with static electricity and discharge. Electrostatic charges can effectively be removed from conductors by grounding them. However, the item grounded must be conductive or dissipative. An insulator, on the other hand, will hold the charge and cannot be grounded and “conduct” the charge away.
Conductors and Insulators in an EPA
The first two fundamental principles of ESD Control are:
- Ground all conductors including people.
- Remove all insulators.
To achieve #1, all surfaces, products and people are bonded to Ground. Bonding means linking, usually through a resistance of between 1 and 10 megohms. Wrist straps and work surface mats are some of the most common devices used to remove static charges. Wrist straps drain charges from operators and a properly grounded mat will provide path-to-ground for exposed ESD susceptible devices. Movable items (such as containers and tools) are bonded by virtue of standing on a bonded surface or being held by a bonded person.
However, what if the static charge in question is on something that cannot be grounded, i.e. an insulator? Then #2 of our ESD Control principles will kick in. Per the ESD Standard, “All non-essential insulators and items (plastics and paper), such as coffee cups, food wrappers and personal items shall be removed from the workstation or any operation where unprotected ESDS are handled. The ESD threat associated with process essential insulators or electrostatic field sources shall be evaluated to ensure that:
- the electrostatic field at the position where the ESDS are handled shall not exceed 5 000 V/m;
- if the electrostatic potential measured at the surface of the process required insulator exceeds 2 000 V, the item shall be kept a minimum of 30 cm from the ESDS; and
- if the electrostatic potential measured at the surface of the process required insulator exceeds 125 V, the item shall be kept a minimum of 2,5 cm from the ESDS.”
[IEC 61340-5-1:2016 clause 18.104.22.168 Insulators]
Always keep insulators a minimum of 31cm from ESDS items
Well, we all know that nothing in life is black and white. It would be easy to just follow the above ‘rules’ and Bob’s your uncle – but unfortunately, that’s not always possible. There are situations where said insulator is an item used at the workstation such as a hand tool. They are essential – you cannot just throw them out of the EPA. If you do, the job won’t get done.
So, the question is – how do you ‘remove’ these vital insulators without actually ‘removing’ them from your EPA? There are 2 options you should try first:
1. Replace regular insulative items with an ESD protective version There are numerous tools and accessories available that are ESD safe – from document handling to cups & dispensers and brushes and waste bins. They are either conductive or dissipative and replace the standard insulative varieties that are generally used at a workbench. For more information on using ESD safe tools and accessories, check this post.
2. Periodically apply a coat of Topical Antistat The Reztore® Topical Antistat (or similar solution) is for use on non-ESD surfaces. After it has been applied and the surface dries, an antistatic and protective static dissipative coating is left behind. The static dissipative coating will allow charges to drain off when grounded. The antistatic properties will reduce the triboelectric voltage to under 200 volts. It, therefore, gives non-ESD surfaces electrical properties until the hard coat is worn away.
If these two options are not feasible for your application, the insulator is termed “process-essential” and therefore neutralisation using an ioniser should become a necessary part of your ESD control programme.
Most ESD workstations will have some insulators or isolated conductors that cannot be removed or replaced. These should be addressed with ionisation. Examples of some common process essential insulators are a PC board substrate, insulative test fixtures and product plastic housings.
Electronic enclosures are process-essential insulators
An example of isolated conductors can be conductive traces or components loaded on a PC board that is not in contact with the ESD worksurface.
An ioniser creates great numbers of positively and negatively charged ions. Fans help the ions flow over the work area. Ionisation can neutralise static charges on an insulator in a matter of seconds, thereby reducing their potential to cause ESD damage. The charged ions created by an ioniser will:
- neutralise charges on process required insulators
- neutralise charges on non- essential insulators
- neutralise isolated conductors
- minimise triboelectric charging
Insulators and isolated conductors are common in ESD Sensitive (ESDS) Devices – Ionisers can help
Insulators, by definition, are non-conductors and therefore cannot be grounded. Insulators can be controlled by doing the following within an EPA:
- Keep insulators a minimum of 31cm from ESDS items at all times or
- Replace regular insulative items with an ESD protective version or
- Periodically apply a coat of Topical Antistat
When none of the above is possible, the insulator is termed “process-essential” and therefore neutralisation using an ioniser should become a necessary part of your ESD control programme.