First Published in EOS/ESD Technology
Testing Be Done Beyond The Lab?
control veterans share some thoughts on
setting up and running a test program.
Publisher/Editor, EOS/ESD Technology
After listening in on the salesmen's
presentation, this writer asked how garments are selected? "I
rely heavily on test data," said Melissa Feeney of TRW, "when
buying garments, or any kind of ESD equipment for material, for
Her comments underscore a commonly held view about the importance
of testing. Tests are done to select a material and then to periodically
evaluate its performance. Materials are judged and compared based
on the following measurable properties: tribocharging, surface or
volume resistivity, static decay time and voltage suppression.
Because test labs are important and expensive, EOS/ESD Technology
invited four ESD-control veterans to share some of their thoughts
on setting up a test lab.
The question of what types of equipment should you buy and why arises.
Each person in the forum offered a list of basic equipment and an
explanation on why each piece of equipment is important. In addition,
they added, here and there, some interesting user notes. Generally,
equipment is not identified by company name because some of the
participants were concerned about lawsuits. Also, the comments made
are personal opinions and should not be considered company policy.
The lists differ. Members of the panel interpreted the idea of "basic"
in different ways. But at a more significant level, they learned
that there are different ways to do their jobs and different ways
to do tests. (These ideas are explored at the end of the article.)
Their suggestions should provide a beginning for building a lab
or, for those with a lab, a way to compare your approach with four
John Kolyer of Rockwell International
made the following comments. "I approach testing and a test
lab from the material-science approach, rather than the electronics
point of view.
"First of all, I recommend you buy a surface-resistivity meter.
It sits on a surface and measures surface resistivity at about 30
volts or so. Some people would say these testers have limitations,
but the results, in my opinion, are good and quick. For example,
when used on a plastic surface, instead of a solid surface, the
questions about reliability arise. But, sit the meter on some pink
poly and it will read 1012. Showing there's some feeble
current flowing over the bag.
"The next item you will need is a megohmmeter. This instrument
doubles as a power supply and goes from 10 to 1000 volts, with 500
volts the current popular level of measurement. With this meter,
you can charge up a one-foot-square plate and calibrate your field
meters at 1000 volts.
"Incidentally, some people object to hand-held field meters,
but I like the tuning-fork type. Calibrated with 100 volts on the
plate, these instruments are good enough for most practical purposes...If
you worry and split hairs on whether the charge being measured is
500 or 2000 volts, then you are not going to use a handheld meter.
If you're checking for orders of magnitude, the handheld meter does
"The good thing about the tuning-fork type of handheld meter
is that it does not charge up in a field, such as from an ionizer.
You can even check your ionizer points with this meter.
"To use the megohmmeter to measure static-dissipative tabletops,
a NFPA99A electrode will be needed. The electrode is a 2 1/2-inch-diameter
aluminum foil surface with a layer of rubber and then a five-pound
weight. With six electrodes on the tabletop surface, resistance
to ground is measured.
"Your contact with the surface can vary the readings: smooth
tops give lower surface resistance readings, all things being equal,
than a textured surface. In my company, tabletops are checked annually.
"There are a number of divice-sensitivity testers on the market
for ranking devices into their classes. Buying one of these is important.
"I recommend testing packaging. We use our own test for this.
We put a MOSFET in a bag, and then, with a high-voltage power supply
and a pair of electrodes, we zap it at 1500 ohms.
"Last of all, you will need a continuity checker and a wrist-strap
tester. Of course, I'd also recommend the continuous monitoring
of wrist straps. The continuity checker will test wrist straps,
and the wrist-strap tester can be used a a megohmmeter.
For start-ups, Kolyer recommends using this
Surface Resitivity Meter
Handheld Electrostatic Meter
NFPA 99A Electrode
Device Sensitivity Tester
High-Voltage Power Supply
A Pair of Electrodes
A Supply of MOSFETS
Continuous Wrist Strap Monitor
Kolyer, a member of the technical staff at Rockwell International,
graduated from Hofstra Univ. and completed his Ph.D. in chemistry
at the Univ. of Pennsylvania. He has authored a number of papers,
many presented at the EOS/ESD Symposia, and is a member of the
EOS/ESD Association and the American Chemical Society.
Betty Smith is a senior engineer at
General Dynamics. She had these suggestions. "My equipment
can be used in a lab setting or as a moving lab. If I want to, I
can go to the manufacturing floor and do practical tests. In fact,
some test equipment stays in the plant.
"I put continuous monitors at the workbench to check wrist
straps, the most important item in our program. This eliminates
doing the work in the lab.
"In the lab, I test the continuous monitors. We calibrate the
monitors with a 2.5-megohm maximum and 275 Kohm as a minimum. This
approach eliminates the need for a 9-volt or the 20-volt tester,
which I put a big cross-on.
"Work-surfaces, which we test every 60 manufacturing days,
are the next most important items to be tested on the manufacturing
floor. For this test, I use a Biddle megohmmeter with one five-pound
weight. The biddle megohmmeter is also used to measure walking surfaces-
conductive floors, floor finishes and even rugs. In this case, you
use two five-pound weights, one meter apart, and put 100 volts through
"Still out on the floor, I measure the balance of ionizers
with a charge plate monitor. For the bench-top ionizers, I recommend
an ionometer. They are portable and easy to use.
"To test garments, I use the EOS/ESD standards as a guide.
You'll need a high-voltage power supply, and electrometer and a
lot of little clamps, probes and Teflon plates.
"Of course, back in the lab, we keep an oscilloscope and a
surface-resistivity meter for testing bags, boxes, DIP tubes and
other items with first purchased.
For startups, Smith recommends
using this test equipment:
Charge Plate Monitor
Biddle Megohmmeter (use with one or two 5-lb weights)
Surface Resistivity Meter
High-Voltage Power Supply
Electrometer Recorder (or Oscilloscope)
Continuous Wrist Strap Monitor
|Betty Smith, a senior
engineer in the manufacturing support area of materials at General
Dynamics, graduated from Indiana State University, pursued graduate
studies abroad in Spain, and is completing studies for a degree
in electrical engineering. Listed in the Who's Who of Outstanding
Young Women of America, she is a member of the EOS/ESD Association,
Society of Manufacturing Engineers and Society of Women Engineers.
Smith has presented two technical papers and will be chairperson
of the EOS/ESD Standards Committee on walking surfaces.
These are the comments made by Melissa
Feeney, a member of the technical staff at TRW Inc."I've proceeded
based on the point of view that materials need to be tested in a
lab, on the manufacturing floor and, sometimes, outside of my plant
at a vendor's place. So, some of my equipment needs to be portable.
"Equipment should be selected based on what properties
need to be tested. We look at rate of charge generation (tribocharging),
rate of charge removal (surface or volume resistivity), length of
time to dissipate charge (static decay time) and voltage suppression.
We also test charge attenuation in shielded bags.
"We use published standards as guides in doing our tests. For
example, we use standards such as EIA IS-5-A for tribocharging and
charge attenuation, and ASTM D257 (the latest version) for surface
resistivity and volume resistivity tests. In addition, we have established
test limits as follows: less than 1.4 nanoseconds for tribocharging,
1012W per square for surface and volume resistivity,
tow seconds maximum for static decay time, and less than 100 volts
for voltage suppression.
"In addition, we have modified some for the basic equipment.
Some tumeric (silver-filled) rubber was put on the bottom of the
NFP 56-A weight, so I don't have to mess with the aluminum foil
or any of the rubber. Now, I don't have any contact-resistance problems
and there aren't any residual black spots left on the surface. Secondly,
the weight is goldplated, as well as the alligator clips on the
electrometer. This gives me good contact.
"I believe that in the lab you should have an ionizer and a
dry box, for humidity control, to keep a constant environment in
the test lab,when that is necessary. Also, some device for measuring
capacitance is necessary so you can always go back to Q = CV relationship
to see what you are really dealing with in terms or a deliverable
charge off a part."
For Startups, Feeney recommends
using this test equipment:
Two Surface-Resistivity Meters:
1. Hewlett-Packard 4329A with HP1600A, Probes NFPA 56A and
Electro Tests Systems Tester
Ecco Ground Tester
Feeney, a member of the technical staff at TRW Inc., graduated
from California State University, Long Beach and is doing graduate
studies at University Southern California in the Claremont College.
Her duties include the selection of ESD protective materials
and implementation on proper handling procedures. She has presented
training classes at her company, at ASPRO and at University
of California, Los Angeles. Feeney is a member of the EOS?ESD
Association, the EOS/ESD Education Committee, the Society of
Manufacturing Engineers and the UCLA Society of Manufacturing
Engineer's Curriculum Committee.
Here are the highlights of the comments
made by Ben Baumgartner, Lockheed Corp. "As I put my list and
reasons on paper, I realized there were areas where equipment is
used to measure things that shouldn't be measured. So I started
again and produced two minimum ESD testing equipment lists; one
for a small, commercial plant and the other for a large plant. In
the small plant, the parts are not hi-rel parts and they probably
won't be doing research or analysis on the sensitivity of their
"First of all, they would need a high-resistance meter. It
doesn't have to be fancy, just any handheld meter that gives an
indication of whether the material is way up in the insulative range
or in the low-dissipative range.
"The second-iten is a voltmeter, a common item. It should be
capable of delivering 4.5 volts or higher at the terminals. Digital
meters should not be used for this purpose because they deliver
very low voltages, as low as 0.5 volts, and you can get misleading
results. Also, an electronic environment might interfere with these
meters. So stick with the standard, old-fashioned voltmeter.
"A third item that one should have is an electrostatic voltmeter
where one knows the input capacitance and the resistance. And this
is an appropriate spot to mention that I do not like handheld meters
because they lack the capability to tell the capacitance for a given
"A power supply that goes up to at least 2000 volts is the
last item. With this item, you can calibrate the electrostatic voltmeter
and charge up materials to see how fast they discharge.
"My second list, for a large plant doing hi-rel work, includes
a resistance meter with a wide range of settings, from 10 volts
to 1000 volts. Similar to a small plant, a resistance meter will
be needed and a voltmeter- but with a chart recorder, so you can
observe and record voltage readings and enter them in your reports.
"Last of all, in order to simulate the human-body capacitance,
you will need a human-body tester, an oscilloscope and a high-voltage
probe. The probe, used to calibrate the tester, permits you to tell
that a pulse was actually delivered and at what voltage the device
broke down. You can observe the method of breakdown: was it sudden
or was it slow?"
For Startups, Baumgartner recommends
using this basic test equipment:
A. In Small commercial/nonmilitary plants:
Volt-Ohmmeter > 4.5 V
Power Supply: up to 2000 v
B. For large plants:
Resistance Meter: range 10 to 1000 v
|Ben Baumgartner, a
lead engineer at Lockheed, has a BS in electrical engineering
form San Jose State University, A charter member of the EOS/ESD
Association, he presents papers frequently at the EOS/ESD Symposia.
Baumgartner is very active on committees for writing standards.
Before joining Lockheed, he worked with electronic devices,
microwave devices and test instrumentation at Stanford Research
Institute, Stanford Electronic Laboratories and Sylvania Electric.
Can a Measurement
from testing offers a haven for making decisions, a safe way around
the subjectivity in judging the reliability of the ESD-control materials
used to improve product yields. Measurements, at the heart of all
science and technology, offer a golden promise of true information.
Thus, it is disconcerting
when tests results aren't repeatable, as is the case in some ESD
testing. When the anticipated measure of objectivity fails, one
feels duped. The following analysis of why surface resistivity measures
are often not repeatable is enlightening.
Why aren't measurements
more reliable? Is the equipment a problem? Are there special techniques
or measuring tricks that a neophyte needs to learn? Are the measures
simply not applicable to the materials? The panel looked at this
issue by examining the test equipment, the testing conditions, the
testing techniques, the meaning of surface resistivity, and the
goals of the test.
measurements have been made for a long time in physics," Said
Ben Baumgartner. "The problem is adapting this measure to the
materials we use. The definition of surface resistivity is derived
from homogeneous materials. But what is homogeneous? Is pink poly
homogeneous? Is a foam surface that is porous homogeneous?
That's why I propose
that people use a two-point resistance method and call the measurement
an apparent resistance and apparent surface resistivity."
In addition to Baumgartner's
two-point method, there are two other types of testers: one uses
a guarded ring and another has two parallel bars. "There are
three different ways to measure surface resistivity, and each way
gives different results," said Betty Smith.
who had done an extensive study on surface resistivity testers,
concluded "if I get as much as an order of magnitude of repeatability,
that is as good as you can expect." She had compared testers
that have different distances between the guarded rings and the
parallel bars, as well as equipment from different suppliers and
the varied voltages put across the materials.
Kolyer suggested that
numbers can't be compared unless one agrees before testing on an
accepted order of magnitude based on what the number should mean.
"What are you looking for? If it is a piece of polyethylene,
we only need t know if a feeble current will run across it. So trying
to split hairs on the exact resistance is not very productive thinking."
In addition, there
is always the human equation: How much care was taken when doing
the tests? Good training should overcome this problem, but it doesn't.
And, did the tester intend to keep all environmental conditions,
while testing, constant; or did he intend to include changing environmental
conditions as part of the test?
These last question
involve more than testing technique. They relate to deeply held
convictions on how a testing lab fits into an overall ESD-control
program. The following section records the thoughts of panel members
on these questions.
Labs an Ivory Tower?
Items such as continuous wrist-strap
monitors and ionometers measure the reliability of ESD-control tools.
These items are placed, not in an isolated lab, but on the manufacturing
floor. The majority of the panel felt the materials should be
tested where the materials and equipment are used: where there is
changing temperatures and humidity, and people of all different
sizes, shapes and attitudes.
Although an isolated test lab is one component of a testing program,
the lack of confidence in any given material and the huge variation
implicit in a manufacturing-floor environment dictate that some
testing equipment be carried and used there.
Baumgartner, who runs his program from a lab, disagrees with the
others. He has engineering specs and logic to support his view.
Here is his reasoning.
"We tell engineering that our limit for control is 100 volts
and then we guarantee that no device will ever see anything higher
than 100 volts. No matter what voltage you might put on a worker,
using ESD control tools, he will never show more than 100 volts,"
"Let me use a hot-water heater as an analogy," he continued.
"If I want to be sure that the temperature of the water in
a hot-water heater will never rise beyond a certain point where
it could burn somebody, I'd install a controller. If I wanted to
be fancy, I could even calculate heat losses and so on to take into
account heat and temperature variation and have the controller work
with great precision. But the point is, I could guarantee that nobody
would ever get scalded.
"My ESD control system is like the controller on the hot-water
heater. I calculate what voltage is possible when a worker wears
a wrist strap. Now if the worker takes the wrist strap off, all
bets are off; but from an engineering point of view, I can set a
So the question about where one test involves whether one can trust
the engineering of a hot-water-heater controller. And can you trust
the engineering of your ESD control system? Melissa Feeney thinks
she'd want to do some tests on the hot-water controller, such as
checking the temperature of the water in the basin now and then,
and she and the others intend to keep checking their ESD control
So your degree of faith in your materials will dictate how you define
your job. And that will determine what test equipment you'll buy
and how you'll use it.