Static Consultants Notebook

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.