Thursday, December 20, 2007

ESD susceptibility of PCB (PWB)

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.

Friday, November 09, 2007

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

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.

Friday, October 26, 2007

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

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

Thursday, October 11, 2007

Is cardboard a problem in my EPA?

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.

Monday, October 01, 2007

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).

Thursday, September 06, 2007

Can we use ESD chairs instead of wrist straps?

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.

Thursday, August 09, 2007

Using ESD gloves

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 ( 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.

Friday, April 20, 2007

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

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.

Tuesday, April 03, 2007

How does a vacuum cleaner cause static electricity?

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.

Wednesday, March 14, 2007

What is high voltage for the purposes of ESD?

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.

Friday, January 26, 2007

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

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

Thursday, January 25, 2007

How to prevent ESD damage

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?

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)