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.
Wednesday, November 16, 2005
Wednesday, November 02, 2005
Documents innside ESD packaging
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.
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.
Monday, September 19, 2005
How to seal ESD shielding bags?
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
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
Wednesday, September 07, 2005
What level of ESD will damage parts?
>> 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.
>> 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.
Tuesday, August 02, 2005
>> 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.
>> 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.
Monday, August 01, 2005
Can static electricity cause skin rash resembling insect bites?
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
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
Tuesday, July 19, 2005
ESD susceptibility of components on PCBs
>> 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.
>> 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.
Monday, June 13, 2005
Are capacitors ESD suceptible?
>> 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.
>> 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.
Tuesday, June 07, 2005
ESD bags
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.
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.
Friday, May 20, 2005
>> 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).
>> 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
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.
>> 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.
Sunday, February 13, 2005
Shock horror probe – the world of a static electricity consultant
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.
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.
Friday, January 21, 2005
ESD garments - do we need them?
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.
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.
Monday, January 10, 2005
Why do we get static shocks?
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.
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?
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.
Sunday, January 09, 2005
Architects and others who design buildings should know something about static electricity
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!
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?
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.
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.
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