Past Posted Questions

                                                 By: Harlan Snyder                                                                                    

Which ESD instruments would be most useful to our situation? 

This is very frequently asked, particularly in a new or renovated ESD sensitive manufacturing environment.  Although each situation is somewhat unique there are general guidelines that can be followed to minimize cost yet cover the major exposures.  One of the most used is the field meter, helpful in locating stray or unwanted fields or verifying that no fields exist in a given sensitive area.  These come in two sizes and price ranges.  The smaller (and cheaper) sell in the $300 to $400 range.  They are battery operated, hand held, and may have either an analog (d'Arsonval movement) or digital display.  The more expensive units are in the $1,000 range, are also battery operated and hand held, and are sensitive to surface potentials in tens of volts with good accuracy.  Both types have ranging capability which allows for measurements in the kilovolt range.  The better units will have a chopper stabilized amplifier and with proper grounding and operation will allow measurements in an ionized area.  I recommend at least one of the smaller units for each common work space which may include 30 to 50 work stations, and one of the larger units as a "gold standard" or calibration reference for the other smaller units.  Only one of the larger meters is generally required for the entire facility,  As both types are subject to the wear and tear of manufacturing, they need to have at least an annual calibration.  Also batteries will have to be replaced depending on usage.  Be sure to use the battery test frequently.

 The other important meter is the combination surface resistance and resistance meter.  These are actually high voltage ohmmeters with special electrode geometries for measuring surface or volume resistance.  Most of these units are battery operated (although more expensive ac operated laboratory types are also available).  The battery operated units  have an internal current limited (safe) high voltage supply that extends the range of a normal ohmmeter by three or more orders of magnitude, allowing fairly accurate readings in the giga-ohm to tera-ohm range (109 to 1012 ohms), which are common in static control materials and facility measurements.  These meters sell in the range $500 to $1,000 depending on the features and electrode options.  Direct reading of surface resistance (dimensionless "ohms per square") is very useful in verifying process containers, mats, conductive floors, etc.  The ohmmeter option is used in verifying grounds and individual items on an audit basis such as wrist straps, or the ground return from mats and floors using the technique of ASTM D-257.  As above, one unit per common work area is usually sufficient and the same caveats of calibration and battery replacement apply. 

If ionizers are installed you should also consider purchasing a charge plate monitor.  These are used to verify ionizer activity and balance by use of a standardized capacitor plate of 20pf with appropriate charging, timing, and sensing circuits.  Most commercial units are in the $2,000 to $4,000 price range and are ac operated.  Because of the relatively high cost, an instrument of this type will be used to service all of the ionizers in a given facility or establishment.  Smaller, battery operated, hand held units that are not as expensive are also available.  These work on a different principle, such as the discharge of a known volume of plastic.  Since the standard plate is not used they are considered by some to be less accurate, however they are smaller and portable.    

Other not so common instruments that may also be considered for your particular situation are the Faraday cup, and the ESD simulator.   

The commercial Faraday cups are small, ac operated units, with direct reading in units of  charge (nanocoulombs).  They cost less than $500 and are used to measure the actual charge on devices or product (that will fit within the cup).  These measurements are taken during process and packaging steps, particularly with products such as "dip" units, surface mount modules, etc.  With both charge and potential measurements, theoretical energy of discharge can be calculated to determine possible ESD risk to the product. 

The ESD simulator furnishes a standardized (waveform) ESD pulse of a given (settable) amplitude, and settable repetition rate.  It is used in determining the sensitivity of given equipment, such as a PC or printer to an ESD event causing possible soft or hard failures.  It is also used for the destructive testing of ESD sensitive products and components to determine the actual failure thresholds, both latent and  hard.  These units cost several thousands of dollars and the actual amount is determined by the features selected, such as max. peak voltage, varieties of waveforms, electrodes, etc.  Most are ac operated. 

In all of the above consider standardization of vendors as much as possible, particularly if you are going to be dealing with remote or off-shore locations as this will aid correlation of data.  Also consider reliability, calibration, and maintenance.  ESD instruments can usually be maintained and calibrated in your own instrument laboratory.  If not, service may be obtained from the vendor or an outside laboratory.

______________________________________________________________________________________________

              I am a new hire in our manufacturing facility.  I never heard of "ESD" until we had a team meeting on this subject.  I am in a wafer fab line and I can see that our  managers and supervisors are very serious about this.  I am female and there seems to be a lot of joking about my (our) undergarments such as body stockings, nylon, etc.  Am I creating a problem(ESD) because of my clothing ?  We have ESD garments and we are required to wear special heel and wrist straps.  We also wear gloves.

 First of all do not take any of these or related comments personally.  Clothing, whether female or male, may be contributors to strong electrostatic fields, which in turn may cause failure to the product or process.  However with proper ESD garmenting and grounding this can be minimized.  Your ESD coordinator should be able to measure each member of your team (body/garment stray field with a field meter) to determine the risk from any individual (possibly causing electrostatic discharge).  First, with all of the corrective procedures in place, such as you indicate with wrist and heel straps, and garment.  Then a measurement can also be taken (with the field meter) to determine the effectiveness of the wrist strap and heel strap  by disconnecting each.

 With regard to specific fabrics it is well known that the synthetics (Nylon, Orlon, acetates, etc.) are more susceptible to static charging than natural vegetable fiber materials such as cottons and linens.  Silk, although a "natural" (non-fiber) product, behaves like the man-made synthetics.  So do the various wools.  The synthetics tend to charge easily as they are chemically related to plastics which have the same property.  Another reason that the synthetics are more static prone is that they do not absorb as much moisture as the natural fibers do.  Unfortunately the natural fiber fabrics are not suitable for clean room use as they are major sources of fibrous particles.  This is the reason that the synthetics are used for garments but must be specially woven as indicated below.  

 I assume your ESD garment is a "bunny suit" which is common in the wafer fab industry.  These are required first of all for cleanliness reasons (you are probably working in what the industry calls a "Class 100" or "Class 10" clean room production facility).  Your garment is designed to prevent particle shedding from clothing, skin, cosmetics, etc. which would contaminate the product.  In addition, to prevent static charge build-up on the garment you will probably notice that it is woven with a pattern of darker thread.  These threads are conductive to static charge (they are usually made of carbon derivatives) and are connectable to static ground through your wrist and heel strap system.  Clean room garments, unless they are of the "throw-away" type must also be launderable and re-usable.  (Note:  There are non-woven clean room garments also.)

 In general, clothing including undergarments, can be isolated from product or process ESD exposures by the items you have indicated:  wrist and heel straps to an  ESD protective garment, ( I assume since you are in a "fab" area you have a groundable "bunny suit").  You may also have local or room ionization, which will also help to neutralize any ESD effects from body garments.

 Relative humidity can also be an ESD consideration in the fab area.  As indicated above most synthetics do not absorb moisture to a significant degree.  At low RH (below 20 per cent) most plastics and synthetics are easily static charged (tribocharged).  Above 50 per cent RH the effect of moisture becomes apparent, and static charging becomes much more difficult as surface conductivities increase.   

Your supervisor or ESD coordinator can explain how your particular RH is determined.  It is usually a compromise between process requirements, operator comfort, and cost.

 You also mention gloves.  These are required for product or process cleanliness as is the "bunny suit" or jump suit.  If ESD is a concern where the gloves are worn then a conductive glove is specified.  The glove, ESD garment sleeve, and wrist strap must all be properly connected for protection against static build-up.  Your ESD coordinator will demonstrate proper gloving and wrist strap attachment. 

 You will probably notice that there are other ESD protective items in your work area.  These include groundable chairs, conductive floors, conductive carts and totes, etc.  Your ESD coordinator can demonstrate how each of these contribute to a static free process.

 Comments and questions are invited.  Please address letters to the editor or author of this column.  Questions may be paraphrased for clarity.