We’ve previously published a post that explains when you need ionisation. However, following this post, we got a number of questions that prompted us to dive a bit deeper into the whole subject of ionisers. Basically with this post we’re starting right at the beginning so stay tuned…
Before talking about ionisers in more detail, we need to have a little chat about the types of materials that can be found in an EPA – conductors and insulators:
• Electrical current flows easily
• Can be grounded
Materials that easily transfer electrons (or charge) are called conductors and are said to have “free” electrons. Some examples of conductors are metals, carbon and the human body’s sweat layer. Grounding works effectively to remove electrostatic charges from conductors to ground. However, the item grounded must be conductive.
The other term often used in ESD control is dissipative which is 1 x 104 to less than 1 x 1011 ohms and is sufficiently conductive to remove electrostatic charges when grounded.
When a conductor is charged, the ability to transfer electrons gives it the ability to be grounded.
• Electrical current does not flow easily
• Cannot 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. An insulator 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. Grounding is a very effective ESD control tool; however, only conductors (conductive or dissipative) can be grounded.
Insulators like this plastic cup will hold the charge and cannot be grounded and “conduct” the charge away.
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
“Process essential” Insulators
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 the ESD control programme.
Examples of some common process essential insulators are a PC board substrate, insulative test fixtures and product plastic housings.
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.
Reduction of charges on insulators does occur naturally by a process called neutralisation. Ions are charged particles that are normally present in the air and as opposite charges attract, charges will be neutralised over time.
A common example is a balloon rubbed against clothing and “stuck” on a wall by static charge. The balloon will eventually drop. After a day or so natural ions of the opposite charge that are in the air will be attracted to the balloon and will eventually neutralise the charge. An ioniser greatly speeds up this process.
A balloon “stuck” on a wall by static charge.
What is an ioniser?
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.
An ioniser creates positively and negatively charged ions.
Note: Ionisers require periodic cleaning of emitter pins and the offset voltage must be kept in balance. Otherwise, instead of neutralising charges, if it is producing primarily positive or negative ions, the ioniser will place an electrostatic charge on items that are not grounded.
This citation from the ESD handbook provides an excellent summary:
“The primary method of static charge control is direct connection to ground for conductors, static dissipative materials, and personnel. A complete static control program must also deal with isolated conductors that cannot be grounded, insulating materials (e.g., most common plastics), and moving personnel who cannot use wrist or heel straps or ESD control flooring and footwear. Air ionization is not a replacement for grounding methods. It is one component of a complete static control program.
Ionizers are used when it is not possible to properly ground everything and as backup to other static control methods. In clean rooms, air ionization may be one of the few methods of static control available.” (ESD Handbook ESD TR20.20 Ionization, section 220.127.116.11 Introduction and Purpose / General Information)
Now that you know what conductors and insulators are, how to treat them in an EPA and when to use ionisation, the next step is to learn about the different types of ionisers available. However, as this post is already quite long, we will save that part for next week so stay tuned…. Click here to read the follow-up post.
Factory ESD control is expected to play an ever-increasing critical role as the industry is flooded with even more HBM (Human Body Model) and CDM (Charged Device Model) sensitive
ElectroStatic Discharge (ESD) is the hidden enemy within your factory. You cannot feel or see most ESD events but they can cause electronic components to fail or cause mysterious and
annoying problems. There are two types of ESD damage: 1) catastrophic failures, and 2) latent defects. By definition, normal quality control inspections are able to identify catastrophic failures, but are not able to detect latent defects.
In general, the ESD susceptibility of modern electronics are more sensitive to ElectroStatic Discharge; that is the withstand voltages are lower. This is due to the drive for miniaturization and with electronic devices operating faster. Thus the semiconductor circuitry is getting smaller. What’s happening currently? The width of electronic device structures continues to get smaller. Intel began selling its 32nm processors in 2010 that would be 0.032 micrometer equal to 0.000032 millimeter or 0.00000128 inch.
See www.ESDA.org, the ESD Association’s latest White Paper “Electrostatic Discharge (ESD) Technology Roadmap” Revised April 2010 forecasts increased ESD sensitivities continuing the recent “trend, the ICs became even more sensitive to ESD events in the years between 2005 and 2009. Therefore, the prevailing trend is circuit performance at the expense of ESD protection levels.” The White Paper’s conclusions include:
- With devices becoming more sensitive through 2010-2015 and beyond, it is imperative that companies begin to scrutinize the ESD capabilities of their handling processes. Factory ESD control is expected to play an ever-increasing critical role as the industry is flooded with even more HBM (Human Body Model) and CDM (Charged Device Model) sensitive designs. For people handling ESD sensitive devices, personnel grounding systems must be designed to limit body voltages to less than 100 volts.
- To protect against metal-to-device discharges, all conductive elements that contact ESD sensitive devices must be grounded.
- To limit the possibilities of a field induced CDM ESD event, users of ESD sensitive devices should ensure that the maximum voltage induced on their devices is kept below 50 volts.
- To limit CDM ESD events, device pins should be contacted with static-dissipative material instead of metal wherever possible.
See May 2010 article by Dr. Terry L. Welsher The “Real” Cost of ESD Damage which includes “Recent data and experience reported by several companies and laboratories now suggest that many failures previously classified as EOS [Electrical Overstress] may instead be the result of ESD failures due to Charged Board Events (CBE). Some companies have estimated that about 50% of failures originally designated as EOS were actually CBE or CDE [Charged Device Events].”
ANSI/ESD S20.20, the ESD Association document covering the development of an ESD control program, lists numerous ESD Protected Area (EPA) ESD control items. Each company can pick and choose which ones are appropriate for their program. The selection of specific ESD control procedures or materials is at the option of the ESD Control Program Plan preparer and should be based on risk assessment and the established electrostatic discharge sensitivities of parts, assemblies, and equipment. [ANSI/ESD S20.20-2007 Annex B] “An EPA [ESD protected area] shall be established wherever ESDS [ESD Sensitive] products are handled. However, there are many different ways to establish ESD controls within an EPA. Table 3 lists some optional ESD control items which can be used to control static electricity.” [ANSI/ESD S20.20-2007 section 8.3 ESD Protected Areas (EPAs)]
There are companies with good ESD control programs who are pleased with their quality and reliability results. But to maintain that level, they would be wise to consider ESD control program improvements. Now might be a good time to do that.
From published article “Now is the Time for ESD Control Programs to be Improved” by Fred Tenzer and Gene Felder. See full article at InCompliance Magazine– September 2012
Seeing ElectroStatic Discharge (ESD) damage is basically impossible. Damage to semiconductor device structure is NOT visible at ordinary magnifications of an optical microscope. If the microscope is capable of 1000X-1500X magnifications, you just might be able to “see” something. The method used, only occasionally as there is considerable expense, is by delayering and etch enhancement producing high magnification photographs using a scanning electron micrograph (SEM). See Images of ESD Damage, photos of Human Body Model (HBM) ESD damage provided by Hi-Rel Laboratories, Inc. at 6116 N Freya, Spokane, Washington 99217 (509-325-5800 or www.hrlabs.com). Used with their permission.
School of Electronic Engineering Science,
University College of North Wales,
Dean Street, Bangor, Gwynedd LL57 1UT
It is now well established that electronic devices and systems can be damaged by exposure to high electric fields as well as by direct electrostatic discharges to the pins. While good circuit layout and on-board protection may reduce the risk of damage by such events, the only safe action at present is to ensure that devices are not exposed to levels of static electricity above a critical threshold. In BS 5783: 1984 a safe level is deemed to be a potential of 50V adjacent to the device. This low level of static can be achieved by instituting a Static Control Programme which usually involves setting up a Special Handling Area (SHA) in which personnel are correctly earthed and all materials e. g. flooring, bench tops etc. meet the specifications laid down for conductive and static-dissipative materials. However, setting up.a SHA does not of itself guarantee a low static environment. Production procedures may change, new materials may be introduced, the performance of older materials may degrade and so on.
To ensure the effectiveness of any static control programme it is important that regular measurements should be carried out:
(a) to determine the sensitivity to ESD of the devices being produced or handled.
(b) to confirm that static levels are lower than the critical level, and that new or modified work practices have not introduced high static levels.
(c) to ensure that both new and existing materials in the SHA meet the necessary specifications for conductive and static dissipative materials. Only after an ‘operational baseline’ has been established by regular auditing will it become possible to identify the origin of unexpected problems arising from the presence of static.
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School of Electronic Engineering Science,
University College of North Wales,
Dean Street, Bangor, Gwynedd LL57 1UT
It is now widely accepted that Electrostatic Discharge (ESD) events are a significant cause of device failure and that instituting static control measures is not only desirable but essential. The exact cost of ESD induced failures to the Electronics Industry is difficult to calculate since many of the costs cannot be quantified, e. g. loss of customer confidence as a result of early product failures in the field. However, it has been shown¹ that while the cost of static control measures can be high, nevertheless if correctly applied, the return on investment does justify the implementation of such measures.
MOS devices are generally regarded as the most prone to ESD damage but, in fact, all devices and technologies are susceptible, differing only in the degree of sensitivity. Furthermore, it is important to remember that ESD damage can occur at any stage from device production through system assembly, testing and packaging to final use in the field.
2. THE ORIGIN OF THE PROBLEM
ESD problems have arisen in the last decade because of two major developments.
(b) As complexity increases, it is necessary to fabricate integrated circuits from smaller and smaller device elements in order to achieve higher operating speeds and improved production
The intrinsic electrical properties of man-made fibre and plastic materials are such as to render them very good insulators, their bulk resistivities exceeding 1014Ωm. When brought into contact with other insulating, or even conducting materials they will become electrically charged by a process known as triboelectrification. Such charging cannot be prevented since it is a natural consequence of electron (or possibly ion) transfer between two contacting surfaces2, a process which brings the surfaces into thermodynamic equilibrium. As a result of the charge transfer, one surface acquires a positive charge and the other a negative charge. The degree of charging depends on (i) the intimacy of the contact, (ii) whether any rubbing occurs during contact and (iii) the manner in which the surfaces are separated. Generally, the highest charge levels are generated when surfaces are rubbed rapidly together under high contact pressure and then separated quickly from each other so as to minimise the opportunity for the transferred charges to recombine. Once generated, static charge can remain on the surface of good insulators for minutes, hours and even days unless steps are taken to neutralise it. Electrically isolated metal surfaces can also retain static charge thereby posing an ESD threat.
To continue reading ESD: The Problem It Causes In Electronics Click Here