FAQ Miscellaneous

 

 

Q:    Simply, how can I get rid of static charges?

A:    Static charges may be eliminated from a charged body, if it is of the conductive type, by just connecting the body to a good ESD earthing point. Should the body be charged positive electrons will flow from the earth into the body neutralising the charges, if is charged negative then electrons will flow from the body into the earth, hence neutralising the body. However, should the charged body be of the insulating type then we may have to use a balanced ioniser, use a topical antistat spray or simply paint the surface of the body with some antistatic compound as to facilitate neutralisation. 

 

Q:    How can I measure static charges on a conductive surface?

A:    This is equivalent to measure the potential of the electric field. You can use an instrument based in the principle of attraction of the forces of two metallic tongues, where one is fixed and the other is allowed to move.

 

Q:    Static Discharge only occurs as some form of a spark, right?

A:    No. Actually the most known is the spark discharge which for a very short period of time joints or bridges two bodies. However also exists as the corona discharge where a “spark” jumps a sharp edge shape of the charged body by breaking the surrounding air or gas and actually never reaches the other body and produces a luminescent effect, this occurs in a non uniform static field and the discharge is in the microamps range.  

 

Q:    Are “Touch Me”  metal grounded antistatic points for people effective?

A:    No. And they are now obsolete. A person may touch a point and discharge quickly, however as soon as this person starts walking again the body gets quickly recharged.  

 

Q:     What is an electrostatic precipitator?

A:     An electrostatic precipitator is a device/electrode designed to remove charged particles from a  solution or residual airborne ashes leftover as a result of an industrial process, they are configured in various ways to satisfy different applications, the most common being the plate-wire precipitator. Sometimes randomly charged particles are forced to pass through a section where they are forced to acquire a defined charged, then at some later stage proceed to collect them, this require pulsating high voltages up to some 100KV. Precipitators achieve this by manipulating particle charges by using electrical forces, eg. if the residual particles are charged positively collecting plate electrotrodes charged negatively can be used to collect them or vice-versa. These are also very useful to capture poluting particles being produced by a fabrication process or to use as air purifiers on coal burning based processes. Some physics problems have been identified in the use of precipitators that makes them less efficient like sneakage/rapping reentrainment and back corona effect (complex papers about these latter issues can be found elsewhere). 

Q:    Which is the minimum distance for placing a ESD sensitive equipment from a charged body or one which is susceptible to accumulate or generate static charges?

A:    If the charged body has an electrostatic potential of some 2,000V, then the equipment should be placed no closer than 30cm away, unless a antistatic spray or ioniser is used to reduce the possible ESD events.

 

Q:    We talk about ESD damage to electronic devices and we understand the reasoning behind. However there is another term used, the is EOS which sometimes is used for similar problems, could you please clarify?

A:    The acronym EOS stands for Electrical OverStressed and is used to refer to any failure or fault to an electronic device, for example due to a mains power surge or ESD discharges, so ESD is a subset of EOS.

 

Q:    Do I need to use ESD protections when handling test, extender, check PBA cards using passive components only?

A:    Yes. There is no difference as to how you ESD protect a normal PBA as you should treat a passive component streaker card. If they get charged then is likely that will ESD damage your dynamic PBAs when coupling them together. 

 

Q:    How do we control ESD prone personnel's apparel?

 A:    One way is for the company to give away some type of bonus to staff when using clothing indicated to be antistatic friendly. Of course the best way is to provide staff with a smock (lab jacket which is shorter and reaches down to about hip) or lab coat (is longer and reaches down to about a half of the thigh), for clarity we refer to both as simply apparel. The apparel is to be manufactured with at least  50% cotton and complemented with polyester, it is however recommended to use a 100% cotton material (cotton is a natural fibre with the inherent quality of absorbing humidity from the ambient and hence becoming more  conductive), creating the effect of a Faraday cage. The latter facilitates the controlled passing of static charges from the clothing underneath, through the apparel and into the air. For better results an apparel should make contact with the skin of the person eg at the wrist. Some manufacturers produce apparel with metallic fibres woven into the fabric and compounded with a blend of conductive carbon.

The problem with this type of material is that they have to be connected to ground to work best, hence restricting movements and creating the need to apply a maintenance routine as to ensure that its conductive qualities are preserved. Unfortunately they are considered to be dangerous to users in locations where mains, or voltages in excess of +/-50V are used. A cotton apparel can be grounded while working at a bench as it can be connected to a wrist strap or can be grounded by use of a coil cord. A method of grounding the apparel if we are wearing ESD dissipative footwear on dissipating flooring is to make sure the sleeves make sound contact with the skin of the wrist, then the smock collected charges will neutralise through the footwear to/from the  ground. The apparel can be measured like a simple dissipative surface by using a megohmmeter and a special cylinder type probes and proceeding to check them against the manufacturer's specification.  

 

Q:    How an ioniser neutralises static charges?

A:    There are two types of generic ionisers, the common one mostly found in some homes and produces only one type of ions eg negative charged, as it is folks' believe that these are good for the wellbeing of people, however production of only one type of ions is bad for ESD as accumulation of charges will be encouraged. The ESD inhibiting ioniser is known as "balanced" as it produces great amounts of ions of the two types, negative and positive this is also known as the AC Faraday cage system. The latter one is usually used to control accumulation of charges on ESD troubled spots of a production line or a technician's work bench. Units can be mounted in front of a work bench surface or on a wall, they come in many shapes and effectiveness depending of the application where needs to be used. Due to the corona principle used by ionisers are not to be used in flammable or explosive environments.

There is an optimum distance where the ioniser will work most effectively, this is called the neutralisation efficiency mechanism and has got to do with the reduction of potential of a charged plate, 6"x6", from 1000V to under 100V in a given time and at a distance of 1ft. This test is defined by the ESD Association and is also used to calibrate ionisers and it is concerned with the decay time  to a target, follow instructions of the manufacturer closely as it depends of the type of unit. The fan provided can be adjusted, the more the air flow de faster the neutralisation of the charges, also the emitter electrodes must be cleaned continuously as air contaminant particles circulating will be attracted to them (again follow closely manufacturer instructions). Some manufacturers provide ions balancing calibration facilities (auto/manual) to allow for finetuning in situ.

 

Q:    What are the elements comprising an ESD/ESC workstation?

A:    A ESD/ESC workstation is a place set apart for a person to do work on ESD sensitive equipment which possesses elements designed to reduce the danger of ESD discharges occurring and also aimed to inhibit the build up of electrostatic charges (ESC process).  The intention of the workstation is to have all elements interacting together to a very similar potential value. Typically the elements found to be part of a ESD/ESC workstation are:

*    A dissipative work surface, resistance to ground in the range of 1MΩ to 1GΩ, grounded to the common point ground (CPG) by use of a highly flexible multiple strand copper cable which may not have a 1MΩ resistor imbedded in the 10mm mat's snap stud.

*    A bench mount connection facility box  made of an angled aluminium anodised bracket providing insertion of two or four banana plugs for wrist straps or other grounding, this is connected to a CPG, it may have a 1MΩ resistor or not and they are also provided with a pair of wrist straps parking slots.

*    A ESD/ESC protective mat on the floor which must be connected through a cable wire to the CPG.

*    A CGP which is connected the local (green wire) ground.

*    Appropriate labelling as recommended in current standards produced by the ESD Association.

Pending of the ESD sensitivity of the equipment being worked on, some other ESD protective mechanisms may have to added to the above list eg an ioniser, grounding to CGP of any metallic part of the bench's structure. Replace for metallic any plastic insulating glides found in drawers, etc...

 

Q:    Can we directly connect to ground an insulating body?

A:    You can but it will be ESD ineffective. However if the body is covered with a conductive or antistatic material, like a sprayed metallic paint, then a state of antistatic conducting resistance is obtained and the body can then be connected to an earthing point. Even if we do not connect the body to earth there in time will be a dissipation of the charges into the ambient air. 

 

Q:    How an ESD dissipating glove works?

A:    A glove like this is not much different to the way a wrist strap works, it must be connected to an earthing point to be effective. Gloves of this type are normally used in clean-room applications (they are commonly made of latex and are very low in particulates) where electronics research material needs to handled.

 

Q:    It is said that by strapping only one foot whilst on a conductive or antistatic floor is not enough, could you please elaborate?

 A:    When a person walks only one foot will discharge to the floor and the other will not. However the two shoes will be collecting triboelectricity charges. Hence, it needs the two shoes foot-strapped to ground to be effective as ESD protection, otherwise, due to the capacitance relation to the human body model the body will build up charges (ESC) and may in turn charge an object in the hand.

 

Q:    Once a conductive or antistatic floor is treated, typically how long is expected the effect to last?

 A:    According to known products, if the flooring is maintained properly, an effective life expectancy of some 8 to 15 years is reasonable to expect.

 

Q:    Having ESD in mind. A metallic object is better left as is or we better paint it, perhaps with a metallic spray? 

 A:    From an ESD point of view is better to spray the metallic object with metallic spray as it will be achieved an antistatic range of resistance somewhere in between 1MΩ a 1GΩ from the surface to ground and hence allowing for non destructive and controlled charges' dissipation to earth.   

 

Q:    Much literature on ESD and particularly on surfaces mention the "square" as a measure, what is this?

A:    The "square" is a common measure of surface resistivity (ohms per square) and has no dimension. Typically a resistance in between to sides of a square of say 10cm per side is the same if the square had 10m per side, you see as the distance is increased in between the two sides to measure from 10cm to 10m, its resistance will be increased proportionally. However the width also was increased proportionally so roughly a similar amount of resistance is expected to the passing of current in both squares. However the term "square" is disliked by technologists as many people gets confused. The naming of ohms per square was introduced by researchers doing work on three and two dimensional surfaces. 

 

Q:    How important is the use of an ESD protecting kit while working out in the field?

A:    This is most important. You will be handling electronic PBA off some equipment in a customer premises of which you do not know anything about their ESD local control, ambient parameters, etc... You can be exposed to high static generating materials (ESC) in furniture or carpets, your little working environment must be ESD preventive and reliable, this is provided by a proper ESD  field kit.

 

Q:    What are the basic elements to be found in an ESD  field kit?

A:    Typically will be around an area of  50cm x 50cm of vinyl antistatic dissipative material with a crease in the middle so it can be folded, some provide a carry handle and heat stamped pocket for holding tools. The surface resistivity of this mat is somewhere between 100MΩ/sq to 1GΩ/sq and the volume resistivity around 1GΩ/cm. It has a couple of female ground snaps so a ground cord with appropriate connections can be grounded to a local ground point. The kit will also include an elastic type wrist strap with the required grounding cord kit. Also will have some appropriate ESD warning label.              

 

Q:    What labels are to be used to identify ESD sensitive equipment?

A:    Labels are intended for using during the transportation process in the exterior of different types of containers. The Internationally accepted ESD related symbols are used in many different types of signs and posters for different applications. There are also products like aisle tapes which are designed for usage at the sides of an aisle which may be sided by ESD susceptible equipment and therefore clearly demarking for staff the walking boundaries, the tape is self sticking once a protecting layer is peeled-off, a continuous repeat message runs along the tape hence warning personnel.  

 

Q:    What a dessicant is used for?

A:    This is a product that absorbs humidity off the surrounding air, hence drying it. Is made of some humidity absorbing material which is provided within a sac. These sacs are usually placed inside moisture sealed antistatic bags as to eliminate any humidity that may have remained at the time of closing the bag, also will continue absorbing humidity should any leaks into the bag occur whilst on transit or elsewhere, these units can be baked as to revitalise them. You will find dessicant sacs with any electronics' equipment you purchase these days.  Some standards provide an empiric formulae to help calculate the number of dessicant units needed to cover a space in a bag. Usually humidity indicator cards are used in conjunction with the diseccants as to see, through the bags, whether de inside of the bags have a humidity value within limits.

 

Q:    What is a layered antistatic mat?

A:    These mats have more than one layer of different materials sandwiched together. Usually one of the internal layers is conductive, a top may be dissipative and a bottom layer could be insulating. By bolting through this mat to a ground point an  electrical contact is made with the conductive layer, there is some discussion regarding the disrupting effect that this alternate parallel ground path may have on persons safety which are already grounded through their wrist or footwear straps to ground.

 

Q:    How do we use conductive foams and what benefits are there for ESD prevention?

A:    These foams are used to protect ESDS devices and are typically made up of polyurethane and polyethylene. By inserting an integrated circuit into the foam we nsure that all pins are short circuited to the same potential value. Also these foams provide a mechanical cushioning effect which helps to add protection for the pins so they do not get bent or bumped during transport or handling. They can be purchased in sheets of different sizes and thicknesses, their surface and volume resistivity are in the order of 1KΩ to 100KΩ.

 

Q:    How do we attach grounding snap sockets and studs to dissipative or conductive surfaces?

A:    There is a variety of special tools in the market designed to snap studs and sockets as well as clinch snaps. Usually the tool looks similar to a pair of pliers. The sockets and studs are provided with eyelets which are to secured under the surface material by the tool. The eyelet is pressed to puncture through the material from under nit.  The tool is sold as a kit with a number of snap studs, sockets and eyelets.

 

Q:    What is polarization?

A:    The standards call the net charge transferred because of polarization induced charge. When a neutral charged device experiences attraction or repulsion due to a charged object in the vicinity the electrons in the neutral device will move away from the object (if the object is negatively charged) or towards the object if otherwise. This phenomena creates in our device two concentrations of charges, closest and farthest from the object in the vicinity. Should then an earthed body (a person with wrist strap or a heel/toe strap) touch one of these charges' concentration spots then the electrons will move into or out of the device to ground and hence creating a charge imbalance in our previously neutral device, so now it will be charged. Anytime now the device may experience an ESD event. 

 

Q:    What is and where we use a vacuum sealer?

A:    A vacuum sealer is designed to extract the air and its humidity off from a moisture barrier bag and then heat sealing it. These are built on different sizes and their vacuum pressure is no more than 1.5cubic ft/m, they are mainly used in dry packaging methods.

 

Q:    I am a bit confused on which standards we actually have to test our ESD workbench mats to, S4.1 or S11.11?

A:    The ANSI ESD S4.1-1997 is the standard recommended for measuring the electrical resistance of work surface materials of workstations, say ESD dissipative surface mats. The ESD STM 11.11-2001 is recommended for measuring dissipative planar materials used for packagings of ESD sensitive electronic components eg antistatic bags, ESD protective containers, etc...  The application of STM 11.11 needs a special concentric probe and the result of the measurement which is in Ohms/sq need to be multiplied by a factor of 10.

 

Q:    What is the channel length en an IC and why this is relevant to ESD?

A:    The channel length is a term used in the IC manufacturing industry and is the distance between a power source and its drain of a transistor unit. As miniaturisation of ICs continues the channel length is made shorter, hence the ESD susceptibility of an IC increases.

 

Q:    What is the meaning of the acronym EPA?

A:    It stands for Electrostatic Protected Area/Workstation. This is an area designated for safely handling  ESDS devices (devices with ESD Sensitive components). The area would have only the most essential insulating materials or it will use ionisers to neutralise their charges. The area which must be clearly visualised is designed to maintain low electrostatic potentials by "soft" earthing all antistatic/conductive bodies, also, "hard" earthing is allowed where careful considerations regarding mains' hazards are considered by the ESD co-ordinator. People moving into this area will wear earthed wrist straps and/or foot straps where there is a conductive/semi-conductive flooring. Those elements within the EPA area which are of insulating material need to be treated with a dissipative coat to change its condition.