Static Consultants Notebook

Benchmarking ESD best practice across industries

2011-06-03T12:30:00.000+01:00

I work across a range of industries and topics covering ESD in electronics manufacture, electrostatic ignition hazards in industrial processes and occasionally explosives handling. I think that static control has developed further in the electronics industry than in the other areas, partly because the sensitivity of components to ESD is often much greater than the sensitivity of (for example) flammable gases or dusts to ignition by ESD. The 100pF capacitor in a 100V HBM test contains only 0.5 uJ of energy, and most of that energy is not delivered to the device under test. In contrast 20uJ is required to ignite hydrogen-air mixture (which is quite sensitive) and 200uJ to ignite most hydrocarbon-air mixtures. So, for handling modern electronic components we have to control static electricity in some ways more carefully than in industrial processes.

One similarity that runs between the 3 areas is that human body ESD (from charged personnel) is very important in manual processes. So, grounding of personnel is a key ESD prevention measure.

One big difference is in the degree of standardisation of ESD control. In the electronics industry we have the option of compliance with one of the ESD control standards such as ESD S20:20 or IEC 61340-5-1. Standards like this do not exist for static control in industrial flammable atmosphere areas. This is deliberate on the part of the experts who write standards as industrial processes can be very different and what is a high risk in one process can be acceptable in another depending on the exact circumstances. It would be very difficult to write standards that do not burdon industry with inappropriate control measures. In contrast, in explosives handling there are well defined key control measures that are enshrined in documents such as the UK Manufacturing and storage of explosives regulations. These requirements depend on the sensitivity of the explosives handled.

In the electronics industry, we have highly automated manufacture of systems in which charged device ESD is a significant source of ESD damage. This occurs when a device itself charges to a high voltage and is the source of ESD. This type of risk is absent from industrial flammable atmosphere areas and explosives handling. However isolated conductors such as metal parts can become charged and be a significant risk in all three fields.

The basis of static control is similar in all three fields. In manual operations, the voltage built up on personnel is controlled by grounding them. In all 3 industry areas footwear and flooring are used extensively for this. Wrist straps are used especially for grounding seated personnel in the electronics industry.

The use of insulating materials is carefully controlled in all three industry areas. In electronics this is mainly due to their ability to build up charge and give high electrostatic fields which can induce high voltages on nearby conductors. In the flammable atmosphere areas, and in explosives handling, it is also because brush discharges from insulators can ignite sensitive materials.

In all three industry areas it is very important to avoid having isolated conductors which can charge to high voltages and provide a source of damaging or incendive ESD. So, all conductors (especially metal items) are normally grounded where possible unless risk evaluation shows this to be unnecessary.

Modern electronics ESD standards have a requirement that the user write an ESD Control Program Plan, and ESD Training Plan and a Compliance Verification Plan documenting what the organisation is doing for ESD control. This documentation imposes a rigour on the ESD Control Program which is very beneficial. Sadly this approach is often lacking in the other industry areas, although less so in explosives handling.

So in conclusion, I believe the electronics industry leads the way in ESD control compared to other industries. This is partly through necessity due to the particular sensitivity of the devices handled, and partly because the industry lends itself to standardised ESD control measures and approaches.

Will conformal coating help prevent ESD?

2011-06-03T12:16:00.000+01:00

Yes and no. It will help prevent direct ESD to the components which are protected by the coating. However the coating itself can become charged and can induce high voltages on the pcb. This can give ESD when a connector pin or other exposed conductive part makes contact with another conductor. The energy in this type of ESD is quite high and is a risk to any component through which the ESD current may flow on the pcb. Modules in plastic enclosures can also suffer this risk. Also, the conformal coating gives no protection against external electrostatic fields which can have the same effect as charge on the coating.

Of course the overall risk of damage occurring is difficult to predict.

Why use an ESD seat?

2010-09-28T11:54:00.000+01:00

I have read the IEC 61340-5-1/2 and we are building a new production area. If everyone who works with ESDS, uses wrist straps while seated, do they need to sit on an ESD chair? (The floor, the tables, the garments are ESD protected)
If they connected to ground via wrist strap, why should I connect them to ground via chair?

Firstly, the ESD chair is not for grounding personnel. The seat may have quite high resistance to ground, up to 10^10 ohms. For grounding personnel, we need a much lower resistance <35 Mohms and this cannot be achieved by contact with many seats. Contact between the seat and body is unreliable because there are layers of clothing in between!

The purpose of having an ESD seat is so that the material of the seat does not become charged and cause an electrostatic field and ESD risk.

What ESD packaging should I use?

2010-06-15T12:32:00.000+01:00

We need to move pcbs with ESD sensitive components on from an EPA to another EPA passing through a non-EPA area. What is the recommend method for tranporting these devices to minimise ESD damage? We don't want to use individual shielding bags.

There is no single recommended method or packaging type. However I reccommend the packaging used has the following characteristics:

1) no part of the packaging should be an exposed insulating materila which could charge up and cause ESD risk in an EPA.
2) the material in contact with the ESD sensitive parts should be dissipative
3) there should be a surrounding conductive material to form an electrostatic field shield
4) an air gap, dissipative material or other barrier should prevent ESD currents being conducted from the outside of the packaging to the ESD sensitive parts.

A shielding bag has all these characteristics, but it is quite possible to design a larger packaging system to contain many items that achieves the same using different packaging materials.

Why would a capacitor be ESD sensitive?

2009-10-08T15:31:00.002+01:00

Why do some capacitors have a dependency on capacitor size vs ESD?

There is good reason for a capacitor to be ESD sensitive. If you push enough charge into it, you will eventually exceed the dielectric strength and breakdown voltage, and the insulation will break down. So the ESD susceptibility is dependent on the capacitance and breakdown voltage. A high capacitance high breakdown voltage device will have low ESD susceptibility, but a low capacitance low voltage capacitor could be easily damaged by ESD.

2009-03-09T16:57:00.000Z

If I was within 2 inches of a printed circuit board with no ESD protection would I discharge any voltage to the pcb causing any partial damage? The reason I ask is our design engineer tells us 2 inches is a safe distance, but according to an ESD Trainer on a course I have recently done damage can occur from as far as 12 inches away.

This is a tricky question and the answer depends on various factors, but I will try to answer simply. I will only consider the risks due to your body being possibly at high voltage because it is not grounded. These are usually the most important and damaging ESD risks in manual handling of PCBs, and are completely removed if your body is grounded via wrist strap or ESD footwear and flooring. So,it is most important for all personnel handling PCBs to be grounded at all times. There are other ESD risks which I will not go into, if the PCB itself is at high voltage.

There are two types of ESD risk in this situation. Firstly, there could be a direct ESD from your body to the PCB if you get sufficiently close so that a spark jumps from your body to the PCB. At normal body voltages this can only happen if you get within a few mm of the PCB, as it takes a few thousand volts to jump each mm of air gap. If you are not getting closer than 50 mm (2 inches) then this is unlikely to happen.

The second risk happens because if your body is at high voltage it is surrounded by an invisible electrostatic field. Any isolated (non-grounded) conductor, including PCB tracks or components, which come within this field have a voltage induced on them. If the conductor becomes grounded at this point, ESD will occur and could if great enough, be quite damaging. (There is also another damage mechanism which could happen which would not require the grounding of the PCB, but it is unusual and I won't go into it here.)

The voltage that is induced on the conductor increases as the conductor gets closer to the high voltage source. Above a certain level, it gets to a point where any ESD arising could be damaging to the PCB. However it is very difficult to predict at what level the damage threshold would be passed. This would depend on the voltage on your body and other factors, as well as the closeness of your body to the PCB and the sensitivity of the components you handle.

So, we could say that the "safe distance" is a matter of guesswork and also influenced by your level of concern over possible damage and tolerance of the risk of ESD damage. If your component ESD susceptibility is low and you aren't too worried by the consequences of a possible ESD, you might judge that a closer distance is safe. If the component susceptibility is high and you have an expensive high reliability product you might judge that a greater separation is necessary for safety. In either case it is just based on guesswork unless backed by a considerable research program involving subjecting your PCBs to field induced ESD.

So, you could consider that both your Engineer or your ESD course Trainer could be right, we just don't know. The Trainer is being more careful and risk averse than the Engineer. But neither of them know for sure, and I can't advise you either without a considerable research program involving subjecting your PCBs to field induced ESD.

One thing I can say is that the risk is easily removed completely if you ground your body through a wrist strap or ESD footwear and flooring. So why not just ground yourself and remove the concern?

General guidelines for high voltage area?

2009-01-19T14:49:00.000Z

What is general guide lines for EPA area, where live AC and DC equipment is used?

If personnel are handling ESD sensitive devices, the general guidelines are the same as for any other ESD Protected Area. See my ESD Guide.

However where high voltages are present there may be a safety concern about earthing the body either through a wrist strap or through conductive footwear and flooring. In this case the risks must be evaluated and if necessary, the ESD precautions modified to reduce safety risks to an acceptable level.

Usually, wrist straps and footwear for ESD use have a minimum of 750k ohm resistance, which is designed to give some protction for up to 250Vac systems. For higher voltages higher resistance (pro rata) can be used if desired. If the risk of shock is unnacceptabe it may be necessary to avoid grounding personnel and use other ESD protection methods. There is no general advice on this as far as I am aware.

ESD damage to motherboard?

2009-01-19T12:22:00.000Z

I have a computer motherboard and I plugged it in a week ago and plugged it back in 2 days ago and now I got power but no video, no keyboard/ mouse, or hard drive activity and I'm told it is do to static build up how do I get rid of it?

If you have already damaged a board through electrostatic discharge (ESD) the damage can be permanent. This is why it is important to prevent static building up on your body while you are hndling the components in the first place.

The usual method is to wear an ESD wrist strap. When replacing a board, disconnect the computer from the mains and connect your wrist strap cord to the computer chassis. Don't take the board out of its packaging until you are ready to plug it in, and don't place it on any ordinary surface. Whilst attached to the computer via the wrist band, take the board out of its packaging and plug it in. Do not touch any of the ESD sensitive parts of the computer unless you are "grounded" to it via the wrist strap.

Is a foot strap adequate?

2008-04-02T11:59:00.000+01:00

When my floor is dissipative do I need to utilize a foot strap and a wrist strap, or is a foot strap adequate?

If you are handling ESD sensitive electronic components within an ESD Protected Area , then you body needs to be grounded with a resistance to ground less than 35 M ohms. If seated, this must be achieved using a wrist strap because you may take your feet off the floor, and grounding contact would then be broken.

If handling ESD sensitive components while standing, grounding can be achieved through footwear and flooring if the floor and footwear resistance is low enough. To achieve this, you may need a floor which has resistance-to-ground less than 35 Mohm as well as footwear which is less than 35 M ohm. You should wear two foot straps - one on each foot. If the resistance from body to ground when grounded by footwear/flooring is not < 35 Mohm, it may be advisable to wear a wrist strap to achieve < 35 Mohm.

ESD packaging

2008-04-02T09:36:00.000+01:00

We are receiving Diodes, Resistors, etc. from our warehouse in non-ESD bags. I was taught that these should be in ESD packaging. Can you send me information on this?

Diodes are semiconductor devices and are normally packaged in ESD protective packaging. Some diodes are quite robust and others may be very ESD sensitive, and so the ESD risk depends on the type of diode.

Usually the main problem with putting resistors and other non-ESD sensitive components in non-ESD packaging is that the non-ESD packaging is then taken into an ESD Protected Area. The non-ESD packaging is an ESD generator and can become an ESD risk to any ESD sensitive components that are present. So, if packaging is required for any components taken into the EPA, it should normally be ESD packaging.

ESD susceptibility of PCB (PWB)

2007-12-20T11:21:00.000Z

Does ESD effect components on PCBs (PWBs)? If so, what is the sensitivity of the PCB?

The first part is easy - yes, ESD sensitive components on a PWB can be damaged by ESD.

The second part is not easy. The answer can depend on the ESD sensitivity of the devices on the board, and the board design. The components on the board may be less susceptible or even more susceptible to ESD damage, and it is impossible to predict. Many experts say we should consider the ESD susceptibility of the board is the same as the most sensitive component on the board.

Is it necessary to wear ESD foot straps on both feet?

2007-11-09T15:07:00.000Z

Is it necessary to wear ESD foot straps on both feet? Where is this defined in the standards?

Most standards do not explicitly tell you to wear foot straps on both feet. However in order to control the body voltage reliably and prevent ESD risk, the body must be continuously grounded and the resistance from the body to ground must not be greater than 35 Mohm.

If you wear only one footstrap, your body is not grounded when that strap loses contact with the floor and the body voltage can quickly rise to hundreds of volts, giving ESD risk. So, it is not good practice to wear only one footstrap.

Why is a high resistance of Megohms suitable for grounding static electricity?

2007-10-26T14:27:00.000+01:00

Why is a high resistance of Megohms suitable for grounding static electricity?

Engineers are often surprised by the high levels of resistance that give an adequate ground in static electricity work. The reason is simple - static electricity charge generation is effectively a small current generator in the microamp or nanoamp range. We are usually happy to achieve limitation of voltages to a few volts. Simple consideration of Ohms law shows that for, say, 1 microA current generated (which is average level) a 1 M ohm resistance will only show 1V buildup. Increase that to 10⁹ ohms and it might start to get more problematic, showing 1kV!

You can now start to easily see why with modern insulating materials static charge build-up is common - with a 10¹²ohm material (which are quite common) only 10nA charge generation rate would give 10kV. Modern polymers can be well over 10¹³ ohm.

Of course life is not so simple in reality but it gives a good first approximation. The second important parameter is charge storage (capacitance) which with resistance forms a characteristic RC charge decay time. If this gets above about a second or so, static voltages stay around long enough for use to notice them.

So for a static dissipative floor in an ESD Protected area, 10⁹ ohms resistance or below is all that is needed to keep static voltages on chairs, trolleys and other items to a low level. Humans are more problematic as they move around and generate a higher current - someone found that 35 Mohms resistance from body to ground would keep body voltage below 100V in most cases with some margin of safety.

People start feeling shocks if their body voltage goes above about 3-4 kV. This can start to happen if the floor resistance goes much above about 10¹⁰ ohms. An average body capacitance might be of the order 100pF, so the time constant is around 1 sec. (People like me who have big feet have higher capacitance). Many modern laminates, glass, plastics, synthetic stone etc have resistance well over 10¹² ohms. So the decay time can be hundreds of seconds, and voltage reach tens of kilovolts. Any high voltages generated sty on the body for several minutes under these conditions.

There's some more information on this in my on-line article on Why static charge builds up on people

Is cardboard a problem in my EPA?

2007-10-11T14:47:00.000+01:00

We have various types of cardboards (dissipative & insulating types) in our ESD protected manufacturing area. Is the cardboard static-generating material? Is it ok to have them in the EPA?

Any insulating material is likely to be static-generating. One problem with cardboard and paper is that they are very variable materials and their electrical properties vary with air humidity by several orders of magnitude. Under humid air conditions they could be dissipative but under dry air conditions they could be insulating and cause a problem. So, unless you have specific ESD grades, it is better to keep them out of the ESD Protected Area.

2007-10-01T09:06:00.000+01:00

While vaccuuming sand with a 6m long and 5cm dia. pvc pipe connected to a flexible pvc hose attached to the vaccuum truck, a considerable amount of static is produced, shocking the worker holding the apparatus. How can I reduce the shocks?

It's difficult to say for sure without examining the situation, But there are two main possibilities. The main one is that the operator is getting charged up and eventually discharges to some nearby object, feeling a shock. If the operator is standing on a concrete floor, or other conductive material, and they wear "antistatic", static dissipative or conductive shoes, then their body should not charge up and the shocks can often be avoided.

However the root cause of the charging is the dust in the pipe. Dust particles impact the pipe walls and create static electricity on the pipe walls. If there are any isolated (not earthed) metal parts these will charge up and can be the source of shocks. The best way to get rid of these risks may be to replace the pipe with conductive or static dissipative pipe, and earth it. Any metal parts in the pipe system should also be earthed (grounded).

Can we use ESD chairs instead of wrist straps?

2007-09-06T09:11:00.000+01:00

In one area in our factory they want to buy ESD chairs so that they can eliminate their wrist straps.

Using ESD chairs is not a substitute for personal grounding through wrist straps. Personal grounding requires a reliable electrical connection between the body and ground, achieving a resistance less than 35 M ohm. Generally this cannot be achieved by a person sitting on a chair for two reasons. Firstly the resistance of the chair is usually too high, and secondly the contact between the body and chair cannot be guaranteed due to the clothing worn.

In my view the use of an ESD chair is to prevent the chair itself becoming a source of electrostatic fields which could cause ESD risks, and could increase the charging on personnel sitting on the chair. If you have chairs in your EPA they should be ESD chairs in any case.

Using ESD gloves

2007-08-09T09:25:00.000+01:00

Why should I have to use ESD gloves if we are using wrist bands? How can the ESD gloves help us? How can I explain to the people the importance of using esd gloves?

There may be various reasons for wearing ESD gloves. The wrist band grounds your body and prevents it acting as an ESD source. If you are not wearing gloves, anything you hold (e.g.tools or boards) are grounded through your body. If you wear non-ESD gloves this grounding is prevented - the tool or other item held in the hand could become charged and be an ESD source. So if you need to wear gloves when handling ESD sensitive components in an EPA, the gloves should be ESD gloves.

Why don´t I get static shock when I touch somethings like a wall or a tree or door?

2007-04-20T12:31:00.000+01:00

Shocks are only felt if your body is charged to over about 4000V, and you touch something conductive.

If the wall or door is made of wood, concrete or some other material that has low or intermediate conductivity, any static charge on your body escapes slowly and does not cause a shock. In contrast if you touch metal, water, or another person when your body is highly charged, the charge is discharged quickly as the material is highly conductive. In this case you may feel a shock.

How does a vacuum cleaner cause static electricity?

2007-04-03T17:24:00.000+01:00

How does using a vacuum cleaner cause static electricity?

When dust travels in the air sucked through a vacuum cleaner it impacts on the pipe walls and other internal parts. These impacts generate static charges on the particles and on the pipe walls. If these parts are made from plastics or other insulating materials they can charge up and give static shocks. Rotating parts such as carpet beaters can also charge up through rubbing action. If the suction pipe has a metal coil and is not earthed, this can charge up and give quite an energetic spark.

If there are flammable vapours (for example solvent fumes) present, these sparks could cause a fire or explosion risk. In larger vacuum cleaners (above about 1 m3) if the dust can give a flammable atmosphere, there may be a risk of fire or explosion in the dust collector.

What is high voltage for the purposes of ESD?

2007-03-14T15:19:00.000Z

I have read that personnel should not be grounded when working on or around high voltages. Can you tell me what is considered high voltages for the purposes of ESD?

That may depend on your local Health and Safety regulations. However the IEC 61340-5-1 ESD prevention standard regard over 250 V.a.c. and 500 V.d.c. as high voltage. Conventional wrist straps and footwear usually have some level of protection up to those voltages afforded by the resistance built into the wrist band cord or footwear. 61340-5-1 recommends that above these voltages the minimum resistance-to-ground from the person's body should be increased, with a minimum of 750 kohm per 250 V.a.c. (500 V.d.c.). Whether you are happy to do that may depend on your safety analysis and regulations. The manufacturers of your ESD equipment may give some further information on their partcular products.

Can a supermarket shopping trolley really build up a static charge?

2007-01-26T10:58:00.000Z

Can a supermarket shopping trolley really build up a static charge? I can see the insulated wheels having chance, but would have thought the very large surface area of metal would discharge this to the atmophere quickly enough so that a shock from the trolley would be very unlikely. I would have thought any shock from a trolley would have been a discharge of static built up on the person.

Yes, a trolley can charge up. Air is a very good insulator and does not allow charge on the trolley to escape easily unless quite high voltages (thousands of volts) are reached. The main paths that charge can leak away are through the tyres and floor, both of which can often be highly insulating, or through the person touching the trolley, through their shoes and the floor.

There are at least 3 ways in which a person could get a shock when they touch a trolley - either the person is charged, or the trolley is charged, or both are charged.

You may be interested to read my on-line articles

Static shocks and how to avoid them
Why static builds up on people

How to prevent ESD damage

2007-01-25T12:14:00.000Z

When you work with solid-state devices,what may help prevent ESD damage?

A device outside and ESD Protected Area (EPA) must be protected by ESD shielding packaging. Devices should only be taken out of their ESD protective packaging when inside an ESD Protected Area in which electrostatic risks are controlled to an insignificant level. In manual handling and assembly, it is most important to ground the body of the person who is working with the ESD susceptible devices. There are many other precautions that might be needed. Please look at our ESD guide

Can electrostatic discharges harm people?

2007-01-25T11:56:00.000Z

Can static electricity cause any physical damage to people?

Static electricity discharges do have significant current flow, which can be several amps or tens of amps for a few hundred nanoseconds. The stored energy which is released in a discharge is also important.

Small static discharges do not do damage to a person and may not even be felt. At the other extreme lightning is a static electricity discharge and can certainly kill. So between the two extremes we can expect there to be a range over which a person could be injured in some ways. Where that range lies, and what the effects are, is not well documented as far as I know.

There are standards which may be helpful, PD 6519-2:1988 probably being the most relevant in this case:

PD 6519-3:1999 (IEC 60479-3:1998).Guide to effects of current on human beings and livestock. Effects of currents passing through the body of livestock.

PD IEC/TR 60479-4:2004. Effects of current on human beings and livestock. Effects of lightning strokes on human beings and livestock.

DD IEC/TS 60479-1:2005. Effects of current on human beings and livestock. General aspects.

PD 6519-2:1988, (IEC 60479-2:1987). Guide to effects of current on human beings and livestock. Special aspects relating to human beings. (Under review)

Wanted - antistatic or conductive road surface material or coating

2006-12-12T09:44:00.000Z

I have a number of client who's customers suffer shocks when they touch a car park ticket machine. In most cases this is due to the electrostatic characteristics of the road surface materials. The solution is far from easy.

I am looking for a conductive coating material that can be applied to, or instead of it, asphalt or epoxy coatings on car park approach road surfaces.

Another possibility might be to fit conductive speed bumps before the barrier - I am also looking for any manufacturer who may be interested in supplying such things.

If you can help, please get in touch via my web site.

Kelvin electrostatic generator generates static from water

2006-11-30T09:23:00.000Z

I have been told of a simple apparatus that can generate an electrical potential out of drops of water. Can you give me any guidance as to where I can find details of the construction and the underlying science. I am told that it is called a storm in a teacup.

You may be thinking of the Kelvin electrostatic generator. This takes water from one container and drops it inoto two others. There is a system of loops and wires connected to the "output" cans which feeds back voltages to the input, resulting in the output cans charging to a high positive and negative voltages respectively

http://physicsnt.clemson.edu/physdemo/cat/elecstat/kelvinel.htm

http://www.physics.umd.edu/lecdem/services/demos/demosj2/j2-02.htm

If you search on google you will find other references.

Can rubbing the contacts damage a DIMM module?

2006-11-30T09:11:00.000Z

A friend told me to clean the finger contacs of a DIMM memory module with an eraser :))).. I told him don't do it that way because you will get esd damage.... am I wrong????

To be honest, I don't know the answer to this. I think the main thing is that you should wear a wrist strap to ground your body while handling the module. If you are not doing this the risk is far greater that ESD from your body will damage the module.

If you are wearing a grounded wrist strap while handling the board, and working on a grounded work surface (if you are using a work surface), then I think the ESD risk from static generation as you rub the contacts is probably small.

There must be a better way to clean the contacts, but I don't know it.

Did I damage my RAM stick?

2006-06-14T16:18:00.000+01:00

I'm recently going to upgrade my computer with a 200GB HD and 1GB RAM. I've been "messing" for months with the inside of my computer, but something happened yesterday. I opened it and took out both RAM sticks (256 + 128). Did some other stuff also, like cleaning and moving some cables, in order to make room for the HDD. When I closed my computer, opened it, it went directly from the POST screen to a black one. Then windows couldn\'t start because of a file missing. Then I couldn't start the Repair console or the Linux Live CD. It turned out to be the 128MB RAM stick. I took it out and it worked. I have some questions though.
1. Certainly it was damaged by static right?
2. Is there a chance that I ruined something else, including the RAM slot? I ran a 2h stress test and no errors came by.

We can rarely say for definite that ESD is the cause of damage unless expensive failure analysis confirms it to be so. However it certainly sounds as if it is possible, even likely. ESD can weaken semiconductor components and give later failures - so it's possible that another component could be damaged. You'll only know if it fails.

3. I\'m now buying a wrist strap from amazon now... to avoid more problems. Will this prevent any damage from ESD? I have to clip the wire to the metal chasis of the pc, unplugged?

Connect the wrist strap to the PC chassis and wear it in good contact with the skin of your wrist. For safety, the pc should not be plugged in. Don't touch and electronic parts until you are connected in this way.

I may have to work on a carpeted room... but if I absolutely have to, I'll work in some other room.

If you use the wrist strap it will remove most of the risk of damage, carpets or not. The biggest cause of ESD damage is from a charged person to a component at a different voltage. By connecting to the pc via the wrist strap you are "equipotential bonding" yourself to the pc and there will be no voltage difference between you and it - no voltage difference means no ESD.

Try to keep other sources of static such as polythene or foam packaging well away from the pc while the covers are off. Once the covers are back on, it is well protected.

Shocks in winter

2006-03-07T08:32:00.000Z

As a baby i would give my mother shocks when she picked me up, and if i touched a car and someone was leaning against it they would receive a small shock too. I am now 40yrs old and have had many many static shock free years but at xmas it returned with a vengeance. The shocks i receive closing my car door cause a heavy sensation in the muscle of my arm, sometimes elbow down, sometimes into the upper arm and the pain can last for up to 8hrs... this now extends to receiving shocks from payment machines in car parks, from my allow wheels when checking tyre pressure, from light switches in my cottage, from my lamp. It sounds over dramatic and daft, i know, but it is really getting me down now and i have a certain amount of trepidation about touching some items that cannot be avoided - let alone the shocks i give from time to time to fellow colleagues. Brushing my hair is a theatrical act in itself.

Have you read my on-line article ? Most shocks of this type are due to the person becoming charged up with static electricity because of the materials of their shoes, floor material and furnishing materials. This often becomes worse in winter due to dry air conditions.

It's hard to say what can help in any particular case without investigation of the particular case. Sometimes choosing different types of shoes can help. Unfortunately the real cure may be in changing floor or furniture materials which is often not practical or expensive.

These problems often become less or disappear in spring when the weather becomes warmer and damper.

Why does static electricity increase in cold weather?

2006-02-21T15:09:00.000Z

I'm hoping you could give me some information about why static electricity increases in the cold weather?

The reason static is more problematic in cold weather is because of low humidity of the air. "Relative humidity" is the percentage of moisture held in the air compared to the maximum it could hold at that temperature. So 50% rh means the air has only half the amount of moisture it could hold. It turns out that static is promoted if rh drops below about 30%.

Cold air can hold less moisture than warm air. So cold outside air at say 0oC and 100% rh is taken into a building and heated up to make it comfortable. The relative humidity drops by one half for every 10oC rise in temperature - so if no moisture is added, the air will be 50%rh at 10oC and 25%rh if heated to 20oC. 25% is certainly dry enough to promote static electricity!.

The main factors conributing to static electricity indoors are floor covering and shoe sole materials, and furniture covers materials, and dry air conditions.

Grounding through ESD footwear and flooring

2006-02-02T09:18:00.000Z

We have a very peculiar problem while using the ESD ( Static Dissipative ) Shoes. The the pass band - lower limit on the tester is 0.75M Ohm and the upper limit is 35M Ohm) some shoes pass and some fail. Are the limits set on the footwear tester ok or should we increase the upper limit?

The limits on your tester are correct for measuring resistance from the person's body to ground via the footwear and flooring, for a person who is handling ESD susceptible parts. This is recommended by IEC 61340-5-1 to be between 750 k ohm and 35 M ohm.

However the limits might not be correct for testing footwear as worn, to a metal plate electrode. Under the IEC 61340-5-1 standard the range limit for this is between 100k ohm per shoe, or 50 k ohm testing both shoes together, and 10^8 ohms. Footwear can be within this range (and so compliant with the standard) and yet fail your test.

For primary grounding of personel (in other words, the main means of grounding people who are handling ESD susceptible parts) the resistance for shoes needs to be lower than the maximum required resistance for grounding through footwear and flooring. There must be some margin for the resistance of the floor. Also, the floor resistance needs to be low enough to achieve a resistance to ground less than 35 M ohm - this will require floor resistance considerably less than 10^9 ohm allowed by the standard. In this case the best procedure in my view is to measure the resistance from person's body to ground while they stand on the floor - this is recommended to be between 35 M ohm and 750 k ohm. This measurement, and compliance with this resistance range, is not specifically required by the standard.

In summary:
1) 61340-5-1 allows footwear and floors to have higher resistance than would be needed to achieve 35 M ohm resistance from a person's body to ground
2) In order to achieve 35 M ohm resistance from body to ground you would have to choose footwear and flooring types that would reliably give low enough resistance
3) It is recommended, but not required, by 61340-5-1 that the resistance from a persons body to ground should be between 750 k ohm and 35 M ohm when grounded by footwear and flooring - if they are relying on this as the primary means of grounding while they are handling ESD susceptible components.

2006-01-23T09:09:00.000Z

What is the average static shock (in volts) that you get from touching a doorknob?

I cannot answer this question directly but I can say that body voltages can vary from zero up to 35000 V and above. In general you don't feel shocks unless your body voltage is above about 3000-4000 V. Getting out of a car, about 8000-10000 V is not uncommon. In general the likelihood of achieving a certain body voltage decreases as the voltage increases, so voltages of a few hundred volts are "normal", and a few thousand volts not uncommon, but tens of thousands of volts are more unusual.

2006-01-05T10:10:00.000Z

How much of a charge can a human body build up from static electricity?

This question is a bit like "How long is a piece of string?" The answer depends on various factors and circumstances. We tend to think in terms of voltage rather than charge, because as the saying goes, "It's volts that jolts" - in other words voltage, rather than charge, causes us to feel a shock. About 3000-4000 Volts on the body will cause us to feel a shock when we touch some object. A shock felt when getting out of the car may be caused by a body voltage of about 10000 V. Body voltages up to 35000 V have been reported, but I see no reason that higher voltages might not be possible in exceptional circumstances. Body voltages up to 10 kV (10000 V) are commonly responsible for the shocks we feel in daily life, and are largely caused by insulating flooring, footwear and furnishings. For more information on this see my article on Static shocks and how to avoid them.

2005-11-16T11:17:00.000Z

Recently I complained about a low charging bag. The manufacturer came to me and he wanted to prove that the bags are suitable. He used a charged plate monitor. He charged it to 1000V. He connectted his wist strap to EPA ground, than he touched the plate with the bag. He measured the time taken for the voltage to decay to 100V. I think it's not an adequate method.

Whether on not it is an adequate method it is not a standard test method used in 61340-5-1 for demonstrating compliance of packaging. Actually it works by electrical conductivity and so is not even a test of "low charging" but is a sort of "charge decay" test of conduction properties. In the case of "low charging" there is no test method in 61340-5-1, neither is there any pass/fail criteria. But I would not use "low charging" bags for protection of ESDS, I would only use them to package documents or non-ESDS components to prevent them causing electrostatic fields.

For use to protect ESDS within the EPA under the 61340-5-1 standard the bags must be low charging AND either dissipative or conductive. They should therefore pass the criteria using a surface resistance measurement. If the bag does not pass this then it is not compliant with 61340-5-1.

To protect ESDS outside the EPA the total packaging solution (which could have many packaging types e.g. conductive box and dissipative foam) should also have shielding properties.

Documents innside ESD packaging

2005-11-02T14:11:00.000Z

As a producer of electronic printed board assemblies I work within manufacturing. I've recently been tasked with overseeing the effects of static during board production. As part of the storage of product I have found printed computer paper and photocopier paper placed directly on top of the boards in carriers without shielding. As far as I was aware with BS 61340-5-1:2001 this was found to create possible damage by static from the paper. However, I am locked into debate with the person concerned as I've always believed this to be unacceptable. Please could you advise.

I have not found any specific mention in 61340-5-1 or 61340-5-2 of inclusion of paper within ESD packaging. However I agree with you, I do not think that it is good practice. The characteristics of paper are extremely variable and this is introducing an unknown ESD risk into what is supposed to be a protected environment with controlled ESD risk. If you value your product, as I'm sure you do, why take the risk? A good way to put costs into perspective is to think about the potential cost of even one ESD failure in the customer's site. Usually this is sufficiently high to convince one that a small saving or doubtful practice in pursuit of convenience, is not worth while.

I have found computer paper in particular to often be at the insulating end of the paper spectrum and therefore possibly at higher risk of causing ESD problems. Usually in such cases it is much easier to remove the doubtful practice than it is to prove one way or another whether there is significant ESD risk.

If you can prove that the paper full fills the requirements for "intimate" packaging over the full range of environmental conditons (especially low humidity) then I concede you may have a technical argument for allowing the paper in with the boards. The requirements for "intimate" packaging are given in Table 2 of 61340-5-1 - the material should be low charging and at least static dissipative (< 10¹¹ ohm surface resistance measured at 100V with a concentric ring electrode. I recommend testng at 12 % rh). My guess is that you do not want to perform this characterisation test for the paper, including periodic verification of any paper you may use for the purpose in the future. Special ESD paper may be available if you really want to use this practice. Note that the resistivity of paper typically increases by several orders of magnitude with reducing humidity and so measurements at higher humidity will not give worst case results.

How to seal ESD shielding bags?

2005-09-19T08:30:00.000+01:00

How important is it to fully close a static shielding bag that contains static-sensitive components? Is it good enough to fold and tape the bag (with what kind of tape?), should a double-fold be used, or are the zipper-style bags better?

A shielding bag gives best protection when closed and fully surrounding the components or board within. It has two functions - 1) to shield against electrostatic field, and 2) to protect against direct ESD to the ESD susceptible parts inside. A typical bag is a laminated material having some insulating layers to stop ESD curent flow through the material, and a metallisation layer to provide electrostatic field shielding. Inner and outer surfaces should be static dissipative (or in some cases may be conductive) to prevent charge build-up.

If you fold the bag do not crease it as this may break the metallisation layer and impair the protection. A loose single fold, held by a label or tape is fine. However, remember you don't want to be using ordinary high charging tapes within an ESD Protected Area. Avoid having component leads puncture the bag.

Never staple a bag as this gives a route for ESD to pass through and make contact with components inside.

A zipper style bag is fine.

A very useful article on ESD bags is given here

What level of ESD will damage parts?

2005-09-07T08:07:00.000+01:00

>> I have currently been tasked with trying to improve the ESD standards
>> within our manufacturing facility.
>> One question which has arisen which I cannot find an answer to is this. Is
>> there a level of ESD at which it is generally accepted that on or above
>> this level, significant damage will occur to electronic parts?
>> We are shortly going to be going through the process in question, and
>> measuring the charges and voltages which are created whilst working. It
>> would help us greatly to know if there is a "safe" level of ESD which can
>> be allowed to occur.

The ESD susceptibility of devices depends on the particular component. Each has an "ESD withstand voltage" determined during QA tests. Virtually all semiconductor components have been tested using "Human Body Model" (HBM), some have also been tested using "Machine Model" (MM) or "Charged Device Model" (CDM). Unfortunately many manufacturers do not make this information readily available to users. As a result we end up relying on guesswork for the susceptibility of components. A rough guide is given in our ESD Guide

The ESD standards such as 61340-5-1 are designed to protect devices down to 100 V HBM. For many processes this will be adequate. Some types of components are more susceptible than this (< 100V HBM) and special measures and care are then required.

It is not easy to assess ESD risk in a process. The usual way is to measure electrostatic fields and potentials in the region of the ESD susceptible parts. The 61340-5-1 standard recommends that electrostatic fields should not exceed 10,000 V/m and potentials (voltages) should not exceed 100 V. Note that a 1,000 V/m field could be a 10,000 V source at 1m, or a 100 V source at 1 cm distance etc. In practice I regard the field criterion as the most useful.

The best approach is to remove all non-essential insulators and electrostatic field sources in the ESD Protected Area (EPA). Any essential insulators or other sources may then be assessed for ESD risk, and appropriate ameliorating action (e.g. use of ionisers) taken.

2005-08-02T15:41:00.000+01:00

>> My company would like to implement an ESD program because we handle
>> electronic repair parts for our semiconductor capital equipment.
>> 1. Would you recommend having different levels of protection for
>> components in different HBM classes?
>> 2. We do not manufacture the parts and do not receive any technical spec.
>> sheets to determine which parts are ESD sensitive. How would you
>> recommend we go about determining which type of parts, in our list of
>> thousands of repair parts, are susceptible to ESD damage?
>> Creating a comprehensive list of parts or even types of parts seems
>> daunting, especially without convenient access to the technical
>> specification sheets.

Normally I would not recommend having diferent levels of ESD program unless there were highly different requirements in some areas, for example < 100V HBM parts handled in a specific area.

It can be very difficult to get ESD data. There is some generic information in our on-line ESD Guide .

You can assume that almost any semiconductor device, or pcb containing such a device, is ESD susceptible. The usual approach is to implement an 100V HBM ESD control program which would cover almost everything. However if you handle more sensitive parts, you need to identify these and may need very stringent handling measures.

Can static electricity cause skin rash resembling insect bites?

2005-08-01T17:22:00.001+01:00

There are occasional reports of skin rashes resembling insect bites attributed to static electricity (often associated with dry air conditions). I am are not fully convinced that static electricity is the cause. However I understand that some professionals in the pest control industry believe that this can be the case.

Chester County Council stated in an on-line article that :
""The environmental problems, however, have gone on from there as our buildings and their furnishings become increasingly modernised and synthetic. Sharp paper dust particles cause skin irritations, and in Chester the culprit has usually been pig-hair carpeting. In addition, certain combinations of temperature and relative humidity seem to set the stage for itches. The skin becomes hypersensitive and then contaminants irritate it.
Whenever someone walks across a carpet – or other floor covering – the friction between their soles and the flooring generates an electrostatic charge. This passes onto their skin and accumulates with each step. The charge drains slowly from the body back to the floor, but, when walking quickly or for a long distance, people accumulate static electricity on their bodies faster than it can drain away. Problems arise, if someone takes 20 or 30 paces across the floor and then touches or passes very near another object.
Although the discharge occurs unnoticed, it is often sufficient to cause localised skin irritations and leave a tiny red rash similar to an insect bite. Temperature and relative humidity influence the magnitude of the discharge, while sweat, oils and other materials on the skin improve the electrical conductivity of the body surface and aggravate the situation. ""
(See: Trafford Council Cable bug article )

I do not necessarily agree with the Trafford Council analysis but if static is involved it is likely that dry air conditions, and the floor covering material, are both major factors. We would expect the inhabitants would also receive shocks from static discharge, they would feel, hear, and possibly see the spark that accompanies the discharge. The voltage required for humans to feel these effects ( above about 3000 volts ) would easily be generated by walking on a nylon, or other man made fibre, carpet and other insulating materials.

For further interest, see also http://citybugs.tamu.edu/FastSheets/Ent-1012.html

ESD susceptibility of components on PCBs

2005-07-19T19:09:00.000+01:00

>> There are some journal stating the differences in ESD sensitivity before
>> and after the parts were mounted on the PWBl. Could anybody explain the
>> actual difference for each ESD class?

ESD susceptibility may be increased or decreased by mounting a component on a PCB. It is not generally possible to tell what will happen to ESD susceptibility without doing an in-depth investigation - clearly most people will not want to do that because it would take an extensive research program. The general guidance is that a PCB should be considered as ESD susceptible as the most sensitive part on the PCB.

The only reference I have on this is Warren Boxleitner's paper " ESD stress on PCB mounted ICs caused by charged boards and personnel" Proc EOS/ESD Symp. EOS-12 1990 pg 54-60.

Are capacitors ESD suceptible?

2005-06-13T19:49:00.000+01:00

>> What is the likely device sensitivity of a thru-hole ceramic capacitor?
>> My general understanding is that passive devices are not ESD sensitive.

Many types of component can be damaged by ESD - including passives. Whether damage is likely to occur depends on the ESD withstand of the component and the strength of the ESD. (Taking an extreme example, lightning is a form of ESD!)

In the case of capacitors, damage can occur if the capacitor is charged up enough to exceed the dielectric breakdown voltage. This can occur if sufficient charge is dumped into the component in an ESD event. Low value low voltage types will require less charge to exceed the breakdown voltage and so it is normally these that might be susceptible to damage. Low value MOS capacitors may be particularly prone to damage.

Many higher value capacitors are reasonably rugged from an ESD view and can largely be considered not susceptible to ESD under ordinary circumstances - especially if the y have high breakdown voltage.

Some types of resistor, such as some film resistors, my also be susceptible to ESD damage.

ESD bags

2005-06-07T11:51:00.000+01:00

ESD packaging and bags must be one of the most frequently misunderstood aspects of an ESD protection program. Today I have been writing a workshop on the subject for the Electrostatics 2005 Conference.

Three main types of bag are commonly used - pink polythene, black polythene, and metalised shielding bags. Pink polythene is cheap - but gives little or no ESD protection to a susceptible electronic component or board. It is best used for other purposes within an ESD Protected Area (EPA), for example enclosing non-ESD sensitive parts or documentation. Black polythene ESD bags are quite conductive and can give good shielding against electrostatic fields. However, because of their high conductivity, a direct discharge to the bag can be conducted through the bag to damage a component inside.

Shielding bags are a multilayer structure, with low charging and dissipative outer layers. They include a conductive metalisation layer that acts as a shield to electrostatic fields. They also include an insulating layer to stop direct ESD current flow through to the bag's contents. The 61340-5-1 standard requires this shielding performance to protect any ESD susceptible parts outside an EPA. Bags are tested by subjecting them to a simulated Human Body Model ESD event - a sensor inside the bag picks up the residual ESD signal, and the energy is measured. Shielding bags pass this test if the residual energy detected from a 1 kV HBM event is reduced to less than 50 nJ.

Surfing the web while writing my presentation I came across quite a useful article on the subject of Choosing the right ESD bag.

2005-05-20T15:51:00.000+01:00

>> I have one query, regarding the compliance of acceptable electro-static field
>> strengths to 61340-5-1. I am unsure as what the standard states is an acceptable
>> field potential for items such as vdu screens, printers, fans, walkmans
>> etc. I have a figure of < 10Kv/m but am unsure where this came from.

< 10kV/m is the recommendation of 61340-5-2.

I interpret this as 10kV/m electrostatic field measured at the site of the ESDS parts. Another way of looking at it is that an object that has a surface voltage of 10kV must not come within 1 m of an ESDS, or 1 kV within 10 cm, or 100 V within 1 cm ......This rule of thumb applies to charged insulators.

Older CRT VDUs are certainly an issue, especially direct after switch-on or off. Newer VDUs - especially LCD types - may not be a problem. Laser printers are an issue - papers come out hot (and insulating) and highly charged. I prefer not to have them in the EPA especially as it usually means documentation is around and uncontrolled.

Other items such as radios, walkmans, computer keyboards etc - I don't believe they are generally an issue - but if in doubt measure the fields associated with them. If necessary keep them well away from ESDS. These items are often so contaminated with salts, grease and moisture from people's skin that you would be hard pushed to charge them up. Radios and walkmans can often be kept on a top shelf away from ESDS for example.

Charged conductors are a different matter - an isolated conductor of any significant size charged to 100V within 1 cm of an ESDS could be a risk because of machine model ESD if it touched the part. Conductors should be grounded (and therefore have no significant voltage).

ESD garments and compliance with 61340-5-1

2005-05-20T09:15:00.000+01:00

An enquirer asked today:

>> A supplier has said that they are not going to use ESD coats but use a
>> standard poly cotton coats and this was acceptable. I believe this is
>> not correct please can you confirm that they should be using esd type
>> coats

This is an interesting question and really subject to personal judgement of the facility ESD Coordinator.

61340-5-1 includes garments under "ESD Protective items" and says "Specific ESD protective items when used within an EPA shall have the characteristics described......measured in accordance with the test methods.... at the highest and lowest expected or rated humidity values".

In other words you don't have to use ESD protective garments, but if you do use them they must comply with the specifications given. The specifications include:
"Coats, jackets, smocks and overalls shall completely cover all clothing in the area of the arms and torso"
"There shall be electrical continuity between all parts of the garment."
Garments shall characteristics on the outward facing surface in accordance with table 1"
"Garments complying with ESD requirements shall be clearly marked" (with the appropriate ESD symbol recommended)

Table 1 merely says that the point-to-point resistance (Rp) must be less than 10^12 ohms. In addition, a charge decay test is mandatory where Rp > 10^10 ohms or the "material is of non-homogenous woven or other construction containing insulating areas"

So, providing any coat supplied for use in the EPA has Rp < 10^10 ohms over there range of expected or rated operating humidities, it is compliant with the standard. If it has 10^10 ohm < Rp < 10^12 ohm, it should probably also have to pass the charge decay test. In practice many perfectly good ESD coats will fail the charge decay test, and it is planned to be omitted from the next version of the standard! Pragmatically I normally advise users if necessary to waive this last requirement, making note of the technical reasons for doing so.

So in short, if your suppliers coats meet the Rp < 10^12 ohm criterion (especially at low humidities) I would probably accept this as compliance with 61340-5-1.

Shock horror probe – the world of a static electricity consultant

2005-02-13T09:55:00.000Z

Mention static electricity and most people think of dusty physics school books with odd experiments – the Van de Graff generator, long sparks and hair standing on end….. rubbing things with cats fur…gold leaf electroscope and electrophorus…..I ask you, what relevance has it to industry and every day life in the real world?

A great deal, actually. All materials and objects are made of electrical charges, and electrical charges are separated whenever two materials in contact are separated. This is what leads to static charge build-up. Charges are being separated all the time in many different situations around us. We probably didn’t notice this much until we started using highly insulating materials, such as plastics and rubber, in our homes, offices and workplaces, preventing charges from dissipating harmlessly and encouraging them to build up as electrostatic voltages. Who hasn’t experienced a static electric shock? Nowadays we put insulators on our feet and lay highly insulating floor materials, use man-made fiber clothes and furnishings. We put plastic wheels on carts and make our machines out of engineering plastics.

A shock often shows we had charged to about 4kV or more before we released that charge in an electrostatic discharge (ESD). In the electronics industry most semiconductor devices are susceptible to ESD damage. A human body charged to 100 V or less can destroy some sensitive components. Circuit boards have to be handled and assembled in ESD Protected Areas where electrostatic fields and voltages are kept to a low level. A voltage susceptible device may suffer breakdown of a thin insulating layer (e.g. gate oxide of a mos transistor) – it takes only a small amount of charge to charge up the small gate capacitance ( a few picoFarads) to the breakdown voltage (a few volts). An energy susceptible device may fail by a high ESD current of a few amps passing through a micron size device junction or interconnect metallization, bringing it to melting point. Outside the EPA susceptible circuit boards and devices must be protected against electrostatic fields and ESD currents by shielding packaging. A whole industry has grown up supplying ESD protective equipment and packaging to the electronics industry – for my part I research ESD topics, provide consultancy services and advise on best practice, run ESD training seminars and help write standards such as IEC 61340-5-1 to provide guidance to industry.

Small consultancy jobs can be extremely varied. Often the solutions are simple in principle, and in practice with a little knowledge the situation could have been avoided, but remedial action may be difficult or expensive. In one case, the client reported drivers in their car park experienced severe shocks when pulling parking tickets from a dispenser. A site visit showed that a new epoxy floor covering had been fitted up a long ramp that brought cars to the barrier –and all the charge stored on the car (say 800 pF capacitance, charged to several kV?) would be discharged through the driver’s arm. The cure – fit conductive floor next to the ticket barrier so that the car could dissipate its charge through its tyres.

An embassy in a northern climate complained that the ambassador was getting shocks when she ascended stairs and touched her office doorknob. The stairs were made of glass, a good insulator and at the other end of the triboelectric series from most shoe sole materials. Ideally, I would recommend not fitting a highly insulating glass floor. The remedial action – a surface treatment – is probably unreliable and requires regular refreshment. Similarly, in a new prestige UK site severe shocks were experienced on the stair wells. The architect had specified beautiful but highly insulating tread tiles that charged a typical person to over 5 kV within a few steps. A well earthed stainless steel banister ensured that they would get a good shock. It is difficult to specify low cost and reliable remedial action in the face of such built-in electrostatic problems.

In a high street retail site staff complained that “the lift was giving (them) shocks”. It had a metal fascia that had been shown to be well earthed. The Saturday lad had taken to wearing rubber washing up gloves, he suffered so much. The lad’s duties included loading a large mobile metal rack (on insulating wheels of course) with highly charged garments in polythene covers from a lorry, and then wheeling them along a highly insulating carpet to the lift, both lad and trolley reaching over 8 kV.

In the USA, many petrol fueling facilities have latching nozzles that allow the user to go away while the vehicle is filling. Many people return to wait in their car when the weather is cold. When they get out, their body voltage may rise to around 10 kV. When they pick up the fuelling nozzle to remove it from the tank aperture, the resulting spark can ignite the emerging petrol vapour causing fire and injury.

Many fine dusts can be ignited by electrostatic sparks. Factories have been destroyed, and avoiding fires and explosions is a real issue in chemical industries. With solvents and flammable vapours the risk of ignition is even greater. It may only take 0.2 mJ energy to ignite a vapour - the equivalent of about 10 kV on a drinks can, or a few kV on a charged person. Vapours can also be ignited by brush discharges from charged insulating surfaces. Insulating solvents charge to high levels when running through pipes, splashing or during filtering. Conductive objects (including people!) must reliably earthed, and plastic surfaces are typically limited to less than 100 cm2 area. Insulating powders charge highly during transport. The charge builds up when they are deposited in a silo, creating a highly charged pile with potentially flammable fine dust cloud above. The CENELEC TR50404 standard has been written to give guidance covering a wide range of industrial circumstances.

Static electricity can also be used for our benefit. Electrostatic separation can be used to separate some materials that are difficult to separate by other means. A mixture of plastics chips from recycled wasted can be separated on the basis of their triboelectric charging properties. An insulating material such as rubber or plastic can be separated from a conductor such as wire fragments, paper or card.

The architect with me on the car park visit, faced with several kV measured on a car, commented incredulously “How could we have predicted this?”. Actually, even a little knowledge of static electricity applied during the design stage would avoid many electrostatic problems. Unfortunately it is rarely considered until problems show, by which time it is too late.

Far from being irrelevant, static electricity is highly important in a very wide range of modern home and industrial environments and processes and is an everyday fact of modern life.

ESD garments - do we need them?

2005-01-21T14:42:00.000Z

In EN 61340-5-1:2001 the user guide suggests that parts of the standard need not be adopted if they are not relevant or needed for the processes carried out. In practice this is open to interpretation. In an electronics facility, is the use of ESD garments really necessary?

I interpret the standard as clearly saying that if you use ESD garments then they must conform to the standard. The more difficult question is, are they necessary and appropriate in your particular case? This is a very hard one to argue and in the end it often boils down to a personal judgement based on your view of your product and processes.

At one extreme if you were making a cheap "throw away" consumer product with cheap components that were not particularly ESD susceptible then I think you might not worry about using ESD garments. At the other extreme if you have a high reliability product with high cost of failure (e.g. satellite) using high susceptibility components then you would clearly take more care and use ESD garments. Most facilities operate somewhere in between.

As another consideration, if your operators all wear cotton tee shirts in a tropical climate there is likely to be little ESD risk. If they may wear long sleeved fleeces or woolly jumpers there may well be some risk from charged clothing and use of ESD garments may be advisable. Another consideration is that use of ESD garments helps to enforce ESD discipline and keep people in the right frame of mind that they should be ESD aware while wearing the coat.

Why do we get static shocks?

2005-01-10T17:37:00.000Z

I get a steady trickle of emails from all over the world from people suffering from static shocks. For some it can seemingly become a real problem. For example, getting a shock every time you touch someone can be a problem for personal relationships! I used to get so many of these emails that I wrote my on-line article "Static shocks, and how to avoid them ". This seems to help many people and attracts many visitors to our web site.

There are many possible factors and so it is not possible to say for sure what causes shocks in any particular situation without doing a first-hand analysis. However two factors are very commonly important - shoes and floor materials. Over the last 50 years or so big changes have happened in these materials - nowadays floor materials and the soles of shoes are often made from insulating materials - often plastics.

The strange thing is that not all shoes and floors give the same charge build-up problems even if they are equally highly insulating. Why should this be? The answer lies in the charge generation characteristics of the particular combination of materials. Some materials generate much more charge when rubbed together than others do. The charge build-up is the balance of charge generation rate and charge dissipation rate - if the charge is generated much more quickly than it can escape, static charge build-up occurs. If it is generated less quickly than it can escape, no build-up of static electricity is found.
A dissipative materials is one which allows the charge to escape slowly, within a few seconds. If manufacturers of shoes and flooring always made their products out of dissipative, rather than insulating materials, we would suffer far less static shocks than we do now.

Minimum voltage for electrostatic discharge?

2005-01-10T17:24:00.000Z

Someone asked me today if there is a minimum voltage for required an electrostatic discharge (ESD) to happen. He was probably thinking of the Paschen minimum - this is a minimum voltage to break down an air gap between two electrical conductors. When two electrodes are brought closer together, the breakdown voltage between then reduces nearly linearly with distance. However a minimum occurs at about 0.1 mm - at this distance an air discharge requires about 300-350 V across the gap. If you bring the conductors closer, the breakdown voltage theoretically increases! In practice it may not, because a different type of discharge occurs at small gaps. So really there is no minimum - at the limit an ESD will occur when the conductors touch.

Architects and others who design buildings should know something about static electricity

2005-01-09T22:42:00.000Z

Often I get calls about this time of year (January) from people who moved into a new building sometime last year. Around this time they start to really suffer static shocks to the extent they contact me to try to do something about it.

There can be many factors and many variations - but one really common one is the floor material. If the floor material is highly insulating, this is a recipe for trouble. Static charges are built up by people walking on the floor (or by cars or fork trucks driving on the floor) , especially under dry air conditions. Charge build-up quickly leads to high voltages - and ZAP!!!!

For much of the year in the UK the air is damp, and this helps to prevent excessive charge build-up. As soon as winter comes, the air can be cold outside and warm and dry inside. This is ideal for static charge build-up. The user suddenly starts to experience shocks. What can they do about it? Unfortunately many treatments are only temporarily effective. If the real problem is the floor material the real solution is to replace it - but this can be very expensive. Not the sort of thing you want to be told if you've moved into your nice new building in the last year or so....
I've visited several car parks now where the floor was covered with epoxy. A nice hard wearing coating, I'm sure, and looks good. Unfortunately a car running on epoxy charges like a Van de Graaf generator and can be at several kV voltage by the time the driver reaches to pull a ticket from the machine at the barrier..........ZAP!!! A highly charged car can pack a lot of stored energy and the shock can be very painful!

Are air temperature and humidity important to static electricity?

2005-01-09T21:49:00.000Z

Are temperature and humidity important from a static electricity view?

Normal ambient temperatures are not usually an issue but from an electrostatic view but humidity is - electrostatic charge build-up problems increase at low humidities, especially < 30%rh. There is a link between temperature and humidity in that for a given air misture content, humidity approximately halves for a 10 degree rise in temperature. This is why ESD is often worst in winter (Jan-Mar) when the air is cold and dry outside and is brought in and warmed by about 15-20 degrees. Even high humidity cold air (e.g. 80%) can become low humidity (20%) when heated by 20 degrees if no moisture is added..

It is often under such dry air conditions that people feel static shocks, and in the electronics industry component damage may be exacerbated. I get many emails from people who are suffering shocks during dry air conditions, whether it be during winter (especially in northern climates) or in desert areas where the air is naturally warm and dry.