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First Published in EOS/ESD
Technology Oct/Nov 1987
Tote
Box Material: How Good is It?
New
Materials for multilayer tote boxes measure up well in lab comparison
with tradition polyolefin plastic.
by J.M Kolyer, W.E.
Anderson and D.E. Watson
Rockwell International Corp., Electronics Operations
Anaheim, CA
Until recently, all
tote boxes for ESD control were made of polyolefin and either topically
applied antistat, extruded-in antistat, or extruded-in graphitic
(conductive) carbon. Each of these materials has its limitations.
Topically applied antistat
is only an expedient because the antistat wears off after some undetermined
period of use. Also, a wall of plain polyolefin, even with an antistatic
surface, provides little Faraday-cage shielding protection from
external static fields or discharges (Table 1).
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SHIELDING/DISCHARGE
TEST
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|
Tote
box type
|
Capacitance
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Resistance
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|
MOSFET
|
|
(pF)
|
W
|
V
|
damage*
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Antistatic
or carbon loaded polyolefin (0.140 in. wall)
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Human
|
Human
|
-8000
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3/3 (OS)
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|
Same as above
|
97
|
1500
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+5000
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2/2 (2S)
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Same, but with
air gap**
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97
|
1500
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+1500
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2/2 (2S)
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Carbon-loaded
polyolefin as above, but lined with 0.0003-in. Aluminum foil
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Human
Tesla coil
|
Human
Tesla coil
|
-8000
35,000
|
0/10
1/5 (1S)
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|
Corshield folded
to give two layers of the foil
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Tesla coil
|
Tesla coil
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35,000
|
0/5
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Vinyl (0.011
in.) on 20-gauge aluminum sheet (0.0375 in)
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Tesla coil
|
Tesla coil
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35,000
|
0/5
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Sandwich of
aluminum screen sandwiched between 0.060-in. sheets of "Forbon"
hard vulcanized fiber (NVF Co., Container Div.)
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Tesla
coil
|
Tesla
coil
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35,000
|
0/5
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*For
example, 2/5 (1S) would mean that five MOSFETs were tested,
two were damaged and one of those damaged was shorted.
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**
1-in. gap between each electrode and inner surface of box.
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Table
1
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Extruded-in antistat provides a longer life than
topically applied antistat because it continues to bleed to the
surface for some time, creating a weakly conductive sweat layer
from atmospheric moisture. However, handling, heat, contact with
paper products, or exposure to solvents will eventually deplete
all the antistat (Ref 1). Again, shielding is poor.
Extruded-in conductive carbon offers
the advantage of permanence, but it has several problems. A carbon-loaded
polyolefin tote-box wall is conductive enough to endanger people;
a current of over 100 mA, which is usually fatal (DOD-HDBK-263),
can be carried at 110 V (Table 2). If the conductive box itself
is charged it is more dangerous than antistatic or nonconductive
boxes (Table 3 and Ref 2).
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CURRENT
CARRIED BY TOTE BOXES
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|
|
Resistance
(W)
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Current
(ma)
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Tote box
type
|
100 V
|
200 V
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110 V
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220 V
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Antistatic
|
2 x 109
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2 x 109
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5 x 10-5
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1 x 10-4
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Carbon-loaded
polyolefin
|
(950 W at
1.5 V
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120
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230
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Corshield
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5 x 108
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4 x 108
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2 x 10-4
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5 x 10-4
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| Vinyl-aluminum
sheet |
> 1012
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2 x 1011
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< 10 -7
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1 x 10-6
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| Table
2
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DAMAGE
TO MOSFETs BY GROUNDED OPERATOR
TOUCHING CHARGED TOTE BOX
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|
Tote
box type
|
Voltage
of charged tote box |
Total
MOSFETs
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|
+ 500
|
+1000
|
-8000
|
damaged
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|
Antistatic
|
0/5
|
0/5
|
1/5
|
1/15
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|
Carbon-loaded
polyolefin
|
1/5
|
2/5
|
3/5
|
6/15
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|
Corshield
|
1/5
|
0/5
|
1/5
|
2/15
|
| Vinyl-aluminum
sheet |
0/5
|
0/5
|
1/5
|
1/15
|
| Table
3
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A high-conductivity
surface is also dangerous to devices (Ref 3), especially if the
operator should be charged and touches a sensitive lead to the box
(Table 4).
However, the wall of a carbon-loaded
polyolefin box is not conductive enough to be a good Faraday cage;
note that the volume resistivity of carbon-loaded polyoelfin is
on the order of 102 W-cm versus 106 W-cm for aluminum
foil. Refs 3 and 4 agree that highly ESD-sensitive components
should always be protected by metal-foil bags, not carbon-loaded
bags or bags with thin, see-through metallization. This conclusion
is applicable to other containers such as boxes. Futhermore, carbon-loaded
ployolefin imparts high triboelectric charges to nonconductors
stroked on it (Table 5 and Ref 1) and sloughs conductive particles
that could fall into open microelectronic devices and cause shorts.
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DAMAGE
TO MOSFETs BY CHARGED OPERATOR
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Tote box
type
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MOSFET
damage
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Antistatic
|
0/5
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Carbon-loaded
polyolefin
|
3/5 (1S)
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|
Corshield
|
0/5
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Vinyl-aluminum
sheet
|
0/5
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| Bare aluminum sheet |
4/5 (3S)
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| Table 4
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TRIBOELECTRIC
CHARGING DATA
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Tote box
type
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V on coupon
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nC on
coupon
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|
FR-4
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Acrylic
|
Al
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FR-4
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Acrylic
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Al
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| Antistatic |
70
|
700
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0
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0.6
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6.4
|
0.0
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| Carbon-loaded polyolefin |
400
|
800
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0
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3.8
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8.5
|
0.0
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| Corsheild |
200
|
200
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0
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1.0
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2.9
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0.1
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| Vinyl-aluminum sheet |
200
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200
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800
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3.3
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2.6
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2.5
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Table
5
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Looking at New Materials
The basic defect of conventional boxes
is that they are insufficient Faraday cages. So, two multilayer
designs with permanent ESD properties have been investigated: fiber-board-foil
and vinyl-metal sheet. Multiple layers are necessary because a homogeneous
wall is not capable of providing both a safe antistatic or nonconductive
surface and Faraday-cage protection.
Commercially available fiberboard-foil
construction ("Cornshield" by Conductive Containers, Inc.)
consists of aluminum foil sandwiched between layers of fiberboard.
The fiberboard has a naturally antistatic surface but is covered
with an antistatic coating to seal in sulfur-containing impurities
that might tarnish silver-plated leads. The foil provides superior
Faraday-cage protection. For example, a discharge from a key held
by a person charged to 8000 V caused no damage to field effect transistors
(MOSFETs), whereas an impractically heavy wall (0.140 in.) of carbon-loaded
polyolefin allowed transistor damage (Table 1).
Even the Tesla coil test was passed
when the Corshield box was folded to give two layers of foil (Table
1). In previous work (Ref 5), only constructions with heavy foil
or metal screen passed this test. Note that the Teslacoil test is
the worst case in electrical stress and positioning of the device
in the box. However, this test is not the worst case in statistical
significance because only five parts are tested for a "pass"
rating. Furthermore, our Tesla-coil test does not use worst-case
acceptance criteria because subtle damage may not be seen by the
curve tracer, and the MOSFETs used are sensitive to 100 to 200 V
whereas some new devices may be affected by only 20 volts. If a
cost-effective material can pass this test, we would use that material.
Fiberboard-foil costs
less than other materials. Also, it can be stored easily as flat
sheets and then folded into boxes when needed. Its only major defect
is its limited durability, but heavier fiberboard will probably
prove sturdy enough for most applications.
The vinyl-metal sheet
design should satisfy the market niche requiring extreme durability.
The metal, either steel or aluminum, provides high structural strengh
and is coated on both sides with tough vinyl, e.g. 0.010 in thick,
by either lamination or powder-coating. The resulting non-conductive
surface is safe for people. Also, in a contrived charge-device model
test (Table 6), the nonconductive surface was even safer for devices
than an antistatic surface.
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CHARGED-DEVICE
MODEL TEST
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Tote box
type
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MOSFET
damage
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| Antistatic |
2/5 (1S) |
| Carbon-loaded polyolefin |
2/5 (0S) |
| Corshield |
1/5 (1S) |
| Vinyl-aluminum |
0/5 |
| Bare aluminum sheet |
2/5 (1S) |
| Table 6 |
The relatively heavy metal wall, 0.0375-in.
aluminum (20 gauge), is a virtually impregnable Faraday cage and
suppresses the voltage of a static charge, no matter how much the
surface may be stroked, so that the box never has an appreciable
E field when the metal is grounded via bare metal feet on the bottom.
In our test, the effectiveness of draining off charges onto an antistatic
bench-top was better for a vinyl-metal sheet box than for a carbon-loaded
polyefin box (Table 7).
However, good contact with the bench
surface, creating flatness of the bottom of the box, can be critical.
The slightly flexible Corshield box benefited from being conformable
and lying flat, whereas the rigid antistatic or carbon-loaded polyolefin
boxes were slightly "dished" (concave) so that only edges
or corners made contact.
The nonconductive vinyl surface's only
defect is that it can tribolelectrically charge conductors, whereas
an antistatic or conductive surface cannot (Table 5). However, charging
of nonconductive surfaces, e.g., conformally coated circuit-board
modules, seems a more important issue, and nonconductive vinyl was
less of an offender than carbon-loaded polyolefin (Table 5).
Summary
Table 8 summarizes
our evaluations. Together, the two new multilayer boxes should satisfy
all in-plant handling needs for an ESD-control program. These boxes
are able to afford secure Faraday cage protection for even the most
sensitive items when electrically continuous lids are being used.
Cost advantage depends
on many factors, most notably the number of units produced and the
fabrication method. In general, a vinyl-aluminum sheet box would
be competitive with an injection molded carbon-loaded polyolefin
box. A vinyl-steel sheet box, though cheaper than aluminum, would
be almost three times heavier in the same gauge. In contrast with
the above choices, the fiberboard-foil box is inherently inexpensive
(Table 8).
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TOTE BOX
TYPE
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Characteristic
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Antistatic
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Carbon-loaded
polyolefin
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Cardboard-aluminum
foil
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Vinyl-metal
sheet
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ESD shielding
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Drain Time
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Triboelectric charging:
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a) Nonconductors
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b) Conductors
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Danger to devices:
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a) Box charged
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b) Operator charged
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Danger to people
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Permanence (ESD)
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Durability (physical)
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Cost
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| Table 8 |
Last of all, fiberboard boxes (Corshield)
are commercially available at this time; however, vinyl-metal sheet
designs are still in the prototype stage.
Another interesting multilayer design
is one which utilizes an aluminum screen or foil sandwiched between
layers of hard vulcanized fiber (Table 1). The vulcanized fiber
material is naturally antistatic, even at low humidity (5 x 1011W/sq
after seven weeks' storage at 72 deg. F and 12% RH). As in the case
of the vinyl-metal sheet box, the commercial success of this design
would depend upon the development of a practical fabrication method,
but both appear to be excellent alternatives for future ESD control.
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Test
Methods
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Below are the various test
methods that were used to derive the results shown in each
of the tables accompanying this article.
Shielding/Discharge Test (results in Table 1):
An electrode 1.5 in.sq. was taped against
the inside surface of one wall of the tote box and connected
to the substrate-case lead of a Motorola 2N4351 MOSFET, and
a similar electrode was taped against the inner surface of
the bottom of the box and connected to the gate lead. Then
a discharge was made to the outer wall of the box over the
electrode. This discharge was from a charged person holding
a key, from a capacitor connected to a resistor and a steel
probe, or from a Tesla coil operated for 30 sec. The box sat
on a grounded plate during the test.
Current Carried
by Tote Boxes Test (results in Table 2):
NFPA 56A electrodes were placed 1 in.
apart on the box surface, and the resistance was read with
a Beckman Model L-10 megohmmeter.
Damage to MOSFETS by Grounded Operator
Touching Charged Tote Box Test (results in Table 3): a
grounded operator held the substrate case lead of a MOSFET
and touched the gate lead to the charged tote box resting
on a nonconductive plastic stand-off.
Damage to MOSFETS
by Charged Operator Test (results in Table 4):
an operator charged to +1000 volts held a MOSFET (as in
Table 1) by the substratecase lead and touched the gate lead
to the grounded tote box.
Triboelectric Charging Test (results
in Table 5): 1.5-in. sq.
coupons of uncoated aluminum or FR-4 epoxy circuit-board laminate,
uncoated or coated with acrylic conformal coating, were shaken
in the tote box for 30 sec and then dropped into a Faraday
cup or measured with a static field meter. All charges were
positive.
Charged-Device
Model Test Test (results in Table 6): The
capacitor (1300 picofarads) representing a charged circuit
board was FR-4 epoxy laminate, 0.096 x 11 x 15 in., copper-plated
on both sides with 1-in. unplated borders. The substrate-case
led of a MOSFET (as in Table 1) was connected to the lower
side of the capacitor, which was suspended by nonconductive
twine. Then the upper plate was charged to +1000 volts, and
the gate lead of the MOSFET was touched to the grounded tote
box being tested.
Drain Time Test (results in Table
7): The
tote box, suspended by nonconductive twine, was charged to
-8000 volts, placed on a melamine-formaldehyde laminate table
top (10 W/sq.) for either 1 or 5 sec and lifted again; then
the field on the box was measured with a static meter.
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References
1. J.M. Kolyner and
W.E. Anderson, "Permanence of the Antistatic Property of commercial
Antistatic Bags and Tote Boxes," Reliability Analysis Center
EOS/ESD Symposium Proceedings, EOS-5 (1983): 87-94.
2. J.M. Kolyer, W.E.
Anderson and DE Watson, "Hazards of Static Charges and Fields
at the Work Station," Reliability Analysis Center EOS/ESD Symposium
Proceedings, EOS-6, Philadelphia, PA (1984): 7-19.
3. R.D. Enoch and R.N.
Shaw, "An Experimental Validation of the Field-Induced ESD
Model," Reliability Analysis Center EOS/ESD Symposium Proceedings,
EOS-8, Las Vegas, NV (1986): 224-231.
4. G.C. Holmes, P.J.
Huff and R.L. Johnson, "An Experimental Study of the ESD Screening
Effectiveness of Antistatic Bags," Reliability Analysis Center
EOS/ESD Symposium Proceedings, EOS-6, Philadelphia, PA (1984): 78-84.
5. J.M. Kolyner and
W.E. Anderson, "Perforated Foil Bags: Partial Transparency
and Excellent ESD Protection," Reliability Analysis Center
EOS/ESD Symposium Proceedings, EOS-7, Minneapolis, MN (1985): 111-117.
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