First Published in EOS/ESD Technology
Semi-Rigid Plastic Trays
a test method that evaluated ESD protection
P.O. Box 748
Geneva, IL 60134
Cecil W. Deisch
Suppose a board (circuit pack) is
enclosed in a static-protective package and this package is sitting
on a conductive or static-dissipative surface. Someone who is
electrically charged reaches to pick up this package and as his
fingers approach, a spark discharge to the package occurs (Fig
1). Are any of the electronic circuits on the board inside harmed?
|Figure 1: A spark discharge to a static-protective
package (containing a circuit board) on a conductive or dissipative
Several boards of one kind in a specific
package can be tested and a known range of discharge voltages
can be used to come up with meaningful results for this one kind
of board. But how do you design tests where you can draw generalized
conclusion and make meaningful predictions? With such a variety
of circuit packs (each with different circuits and different components
of varying sensitivities), the number of combinations of all the
variables makes it nearly impossible to test. This does not include
the task of thoroughly instrumenting the board to accurately measure
harmful voltages on or around sensitive devices. It also does
not include the variability of the dissipative package or the
effects of varying the physical separation between the package
and the high points of sensitive components on the board.
A Test Setup
Instead of tackling this challenge
head-on, we designed a test setup which closely mimics the electrical
environment of a sensitive component in a worst-case ESD situation.
Because ESD events contain high voltages and high-frequency components,
we went to great lengths to thoroughly design the test fixture
and the measuring probe to give an accurate time vs. voltage scope
readout without being sensitive to stray pickup. Then we took
data on many varied parameters and came up with a lower bound
on the predicted effectiveness of ESD protection provided by a
package. This data gives valuable insight into how to design the
package in general, and allows a prediction of the minimum protection
There are three classes
of material from which most packages are made.
Metal. A well-made
metal package can give near-perfect ESD and electrical overstress
(EOS) protection. This is because there is virtually no resistive
voltage drop anywhere on the surface. Furthermore, no flux lines
can penetrate the metal at the high frequencies of a spark discharge
so there can be no induced voltage between any two points on the
inside of the package. If there is no voltage differential
anywhere inside the package, then there can be no harmful voltages
to damage sensitive parts. Metal containers, however, are generally
impractical because they are expensive and their contents cannot
A package made of an insulator (e.g., untreated polyethylene)
also gives excellent protection to the enclosed board but only
for the experiment previously outlined. A charged person who touches
the package will not generate a spark. Further, polyethylene can
sustain several kilovolts per mil of thickness, so there is no
breakdown from the outside to the inside surface. However, a charged
person who picks up this package will remain fully charged because
the package does not bleed away any charge during the time the
fingers touched the package while it was still on the conductive
surface. If the charged person now opens the package and touches
the board, there will be a spark discharge, possibly harming the
The ESD performance of a package made of static-dissipative material
falls somewhere between the two materials above. A charged person
can discharge a spark to this material. However, even though its
surface resistivity (105> to 1011 ohms/sq.)
is not nearly as low as metal, a properly-designed static-dissipative
package can still give excellent EOS and ESD protection. In contrast
to the insulator above, a static-dissipative package can quickly
bleed away the charge on a person, lessening the chance of subsequent
ESD damage. Many varieties of static-dissipative material are
transparent, making the contents of the package clearly visible.
Discharge to a Dissipative Material
|Figure 2: When a charged
body approaches a surface, a thin column of air becomes ionized
and a spark is discharged to a small area.
Consider the static-dissipative package
(Fig 2) which sits on a conductive surface (earth ground) and
totally encloses a circuit pack. As a charged body approaches
the surface of the package, a thin column of air becomes ionized
and a spark discharge is limited. This thin column of highly-conductive
ionized gas forms exceedingly fast (in a nanosecond or less) and,
once formed, it behaves electrically very mush like an ordinary
small diameter wire. In other words, most all the voltage across
it is caused by inductive voltage (L di/dt). Very quickly, electric
charge flows through this ionized column and is deposited in a
small area on the surface of the package. However, as the charge
accumulates on this surface spot, the voltage of the spot is rapidly
rising and approaches that o fthe charged body.
Once this voltage difference gets
small enough (about several hundred volts), then the ionized column
can no longer support itself and it extinguishes. This whole sequence
of events, from initiation to quenching, happens very quickly
(approximately a few nanoseconds). The main parameter that limits
the speed is the inductance of the thin column of ionized gas.
resistivity. What happens on the surface of the package during
this short event is strongly dependent on its surface resistivity.
For a material with surface resistivity near the high end (1011
ohms/sq.), there are many tiny, faint sparks during a discharge.
For each tiny discharge, the point on the surface directly beneath
the spark has very high voltage; however, the voltage just a short
distance away may be many hundreds of volts less during the actual
spark discharge. This is because the high surface resistivity
slows down the diffusion of the spark-deposited charge. The point
where the spark hits rises to several thousand volts, but just
a short distance away (e.g. 1 in.) the voltage may be in the low
hundreds or less.
effect. Once the spark extinguishes, the concentrated charge on
the surface quickly spreads out through surface conduction and
the voltage peak quickly decays. These events are quite analogous
to dropping a pebble into a pool of water. A column of water of
substantial height forms right at the center, but the height of
the rest of the water quickly drops off as the radius increases.
As time passes, the height of the center column and the water
immediately around it are quickly restored to the average surface
height of the pool.
resistivity. Packages with lower surface resistivity can have
fatter and more aggressive spark discharges to the surface; but
lower resistivity actually improved ESD protection. Since any
discharge to the surface of the package has to charge a larger
area, more charge must flow for a longer time to attain the same
surface voltage. The longer it takes to charge up this larger
area, the more time is available for the charge to bleed away
because of the lower resistivity material. The more charge removed
from the charged human body during the spark, the more body voltage
is reduced. The net result is that, although a larger area of
the package rises in voltage, the peak voltage at any point on
the surface is less.
on Board Components
As pointed out above,
when a spark discharge occurs, a small area on the surface of
the package around the point of discharge rapidly rises to several
thousand volts. Most of the rest of the surface of the package
distant from the spark rises very little. For all practical purposes,
the worst-case scenario is to assume all the rest of the package
remains at ground. Since the board inside takes on a voltage which
is the average of all voltages on the surface of the package,
and since only a small post rises to several thousand volts while
the rest is (worst-case) ground, the board takes on the average
voltage of nearly ground.
In many situations,
a sensitive component may sit right beneath the high-voltage spot
on the surface of the package while the board has an average voltage
near ground. The danger is analogous to a person standing in an
open field (grounded) while there is a highly charged thunder
|Figure 3: Worst-case circuit
model. The worst-case equivalent circuit model is shown here in
Fig. 3. Here the circuit board is replaced by a large grounded
metal plate; one or more leads of the sensitive device are connected
to this ground plane. Another lead of this sensitive device is
directly beneath the high-voltage spot overhead which is momentarily
caused by the spark. The parasitic capacitance to ground represents
not only the capacitance of the sensitive device itself, but also
the parasitic capacitance of routing conductor(s) connected to
At low spark discharge
voltages, the "hot spot" on the surface of the package
rises to perhaps a thousand or more volts. This hot spot behaves
much like a plate of a capacitor suddenly yanked to a high voltage.
An isolated conductor between the hot spot and the ground plane
sees a fraction of the hot spot rise in voltage. This fraction
depends on the size of the exposed conductor and how close it
is to the hot spot; the larger the conductor and/or the closer
it is to the hot spot, the larger the fraction. For a fixed configuration,
as the discharge voltage increases, so does the capacitively-coupled
voltage on the sensitive lead. This holds true up to a certain
voltage when another phenomenon takes over.
Discharge: Package Surface to Board
As the spark discharge voltage is
raised, the coupled voltage also rises almost linearly. Often
a point is reached where, if the spark voltage is increased just
slightly more, the coupled voltage suddenly jumps up (often by
a hundred or more volts). Once the discharge voltage gets high
enough and/or the surface of the package gets close enough to
a sensitive part on the circuit board, a secondary spark occurs
between the inner surface of the package and a component on the
This secondary spark is the hazard
to avoid because it can be extremely damaging. (Note: This secondary
spark effect applied only to materials that have relatively homogeneous
volume resistivity properties. The volume resisitivity allows
charge to freely flow between the outer and inner surface. In
particular, this internal spark phenomenon probably does not apply
to sandwich type materials such as dissipator-insulator-dissipative
the hazard. Since the internal spark discharge is the hazard to
avoid, what are the parameters that are most likely to aggravate
this hazard? Not surprisingly, the smaller the physical spacing
between the package surface and the sensitive part, the more likely
an internal spark. Also, a sharp point (such as a sheared end
of a component lead) is much more likely to initiate a spark because
of the ionizing voltage gradients at the surface of a small-radius
|Figure 4: The tip of a scope probe can be
used to measure transient voltage waveforms.
Instead of trying to accurately instrument
a sensitive device to measure the voltage transient during a spark
event, we let the tip of a scope probe represent the electrical
equivalent of a sensitive device. The probe tip also allows direct
and accurate measurements of transient voltage waveforms (Fig
The scope probe tip is an excellent model of a worst-case
sensitive part because it has only 12.5 pF between the tip and
ground. An actual sensitive part may have as little as six to
ten pF of input capacitance, but when it is mounted on the board
it is connected to at least one other component through several
inches of printed copper conductor. The actual total capacitance
at a sensitive node is likely to be at least 15 pF. When considering
immunity to ESD events, generally the higher the capacitance to
ground, the lower the transient voltages will be. This is because
it takes more coupling charge to raise the voltage on a larger
Based on the discussion
above, the most important parameters of the package that needed
to be tested are:
discharge model and the maximum discharge voltage. Since human
handling will be the source of any discharge, the human body model
source was also used. Also, if reasonable ESD precautions were
taken by the handler, there should never be a discharge. However,
once in a while mistakes are made and someone will become inadvertently
charged. Although we tested to 25 kV, we chose 12 kV as the maximum
discharge directly over the probe tip. The spark was discharged
directly over the probe tip. As the discharge was moved away from
the tip, the measured maximum probe voltage dropped quickly.
maximum voltage a sensitive part can withstand. Since this is
to be a worst-case test, the most sensitive Class 0 parts (able
to withstand ESD volts of 0-200V) were chose. Parts falling in
the middle of this range were chosen and declared a failure if
a peak of above 100V was measured.
shape of the sensitive node. Theory indicates and tests confirm
that sharp points are much more likely to initiate the damaging
internal secondary sparks. Tests with smooth, gently curved noded
showed no tendency to draw internal sparks, even with close spacing.
The sharp tip of the probe is a good model because it has a radius
of less than 8 mils (0.008 in.)
physical space between the inner surface of the package and the
closest component or conductor on the enclosed board. Preliminary
testing showed 0.5-in. Clearance was adequate for all materials.
Clearances in the range of 0.125 in to 0.5 in were tested.
resistivity of package material. Not surprisingly, low resistivity
gave the best protection for a given spacing and a given spark
voltage. We tested a wide variety of commercial proprietary materials
which have surface resistivities that span the range of 105
to 1011 ohms/sq.
the Low-Inductance Probe Ground
As mentioned above, spark discharges
are rich in very high frequency components. Most o the energy
in sparks falls in the region of 1MHz up to about 1 gHz. This
implies that to get accurate measurements:
1. Inadvertent coupling and crosstalk
must be held to a minimum
2. Ground inductance to the measuring
probe must be of the order or 1 nH or less.
This last requirement absolutely
rules out ground clip leads because they may have inductance in
the 100 NH or less. At the high frequencies being measured, any
transient currents through the ground lead can cause voltages
that thoroughly overwhelm any voltage being measured. (See Sanity
Since we wanted to adjust the position
of the probe in the hole in the ground plane, we machined a brass
barrel (or sleeve) to fit over the ground ring of the probe end.
This tight-fitting, low- inductance barrel provided a smooth outer
surface, allowing the probe to be inserted or withdrawn over a
range of about 0.8 in.
To complete the low ground inductance
connection to the plate, near its center a hole was punched which
has six flexible metal tangs that make a multi-point ground connection
to the barrel. This allows the probe to be moved in and out of
the plate while maintaining a low grounding connection.
of the Test Fixture
The metal plate is
a piece of galvanized sheet steel about 3 ft. square. For convenience
of use, it was mounted vertically. A small metal frame about 4-in.
square was attached to the metal plate, with the probe tip in
the middle. The outer surface of the frame is about 0.75 in. above
the metal plate and allows flat samples of static-dissipative
materials to be laid on top and held down with strip magnets (Fig
5). The metal frame has a low-inductance connection to the plate
and the static-dissipative sample also has multi-point contact
to the frame because of the compliant force of the strip magnets.
With this frame and the special ground barrel on the probe, the
spacing can be adjusted between the end of the probe and the inner
surface of the static-dissipative sample from zero (touching)
to about 0.7 in. (Fig. 6).
|Figure 5: Flat samples of static-dissipative
material were put on top of a frame that sits about 0.75 in.
above the metal plate.
||Figure 6: The frame and the ground barrel
on the probe allowed the spacing between the end of the probe
tip and the inner surface of the sample to be adjusted.
|Figure 7: The scope and probe lead were placed
on one side of a metal plate, and the spark tests were performed
on the other side. The plate provided shielding of the sensitive
Mounting the ground palate vertically
also provided the benefit of shielding. The scope and the probe
lead were set on one side of the plate and all the high-voltage
spark tests were performed on the other side. The large metal
plate shielded the sensitive measuring scope (Fig. 7).
When measuring high frequencies,
especially in the presence of high voltage spikes, it is best
to perform a "null measurement." This sets up the test
fixture in such a way that the expected voltage is zero. In never
will be zero, but the results give an indication of how much background
noise is contaminating the measurements. For the null test, the
metal frame was covered with aluminum foil and then sparks discharged
directly to the foil above the probe tip. Scope-measured voltage
peaks were typically a fraction of a volt, but were always less
than two volts even for 25 kV discharges. Since we are interested
in measuring voltages above approximately 10 V, the low null voltage
showed we had a good test setup.
Results, and Conclusions
About 10 static-dissipative materials
of different composition from several manufacturers were tested
over the full spark range of 5 to 25 kV. Each material was tested
after having been prepared in two ways:
1) Aged for several days at 20 deg.
C. and 50% RH
2) Aged for 3 days at 20 deg. C and 12% RH.
For each test coupon, the surface
resistivity and charge bleed-down were first measured and then
placed on the test fixture with a tip spacing of 0.5 in. Several
discharges were performed at each voltage and the maximum peak
voltage at the probe tip was recorded. The spacing was lowered
to 0.25 in. and the same was reduced to 0.125 in. If peaks of
more than approximately 100V were measured or if the secondary
internal spark event was observed, we stopped the test and went
on to the next test material.
taking hundreds of measurements of many different materials with
various spacings to the probe tip, we found that there was great
variation in the data because of the randomness of spark discharges.
Nonetheless, we can break the results down to some simple guidelines
The damage level of a sensitive device is 100V.
The static-dissipative material has a surface resistivity of no
more than 1 x 1011 W/sq.
It is highly unlikely that the human would be charged to more
than 12 kV.
If these guidelines
are met, then a package designed with a 0.25-in. clearance from
all components on the top of the board and from anything on the
bottom of the board will give adequate protection at least 99.9%
of the time.
clearance can be modified, depending on circumstances. For example,
if your circuit devices are much less sensitive to ESD, if the
humidity is kept high, and/or any ESD is likely to be much less
than 12 kV, then the package might be designed with an 0.125-in.
Clearance all around. On the other hand, circuit packs with exceedingly
sensitive devices, which are shipped or handled in very dry environments
and which maybe subjected to substantial ESD, may need packages
with as much as 0.5-in. clearances.
Since the clearance
guidelines are based on worst-case test measurements, the calculation
of the probability of adequate protection is based on extremely
1) The probability
that the spark to the package occurs directly above the most sensitive
device(s) is no more than 10% (more likely, the probability is
1% even for a small 4 x 6-in. board).
2) The probability
that the minimum clearance occurs directly over the most sensitive
occurs directly over the most sensitive device(s) is no more than
25% (more likely it is less than 5%).
3) The probability
that the most sensitive device(s) has a sharp point is no more
than 25%. Sharp points are more susceptible than rounded surfaces.
(It is more likely that the probability of the most sensitive
devices having sharp points less than 10%).
4) The probability
that the most sensitive device has a node of only 12.5 pF (the
tested value), is no more than 50%. The most sensitive node is
likely to have at least 15-20 pF. The higher the node capacitance,
the lower the susceptibility to ESD. (The probability of a 12.5
pF node is more likely less than 20%.
5) The probability that the package
will have a surface resistivity as high as 1 x 1011
ohms/sq. is less than 70%. Spark protection increases as surface
resistivity goes down. (A more reasonable assumption is that surface
resistivity will be at its upper limit of 1 x 10 11
ohms/sq. less than 25% of the time).
6) The probability that the spark
will be its most intense at 12 kV is no more than 50%. (Most discharges
will be in the low-kilovolt region and the chances of a 12 kV
spark is more likely less than 10%)
Taking the very conservative probabilities
of failure above and multiplying them gives an overall probability
of failure of about 0.10 of 1% or an overall probability of protection
of 99.9%. Using the more reasonable probabilities, the estimate
is about 1 failure in 4,000,000 or a protection rate of 99.9999+%.
A dissipative plastic tray with the
correct resisitivity (surface and volume) and spacing from the
tray outside to critical components will provide an excellent
shield as well as physical and contamination protection. Based
on published data it is reasonable to expect that more than 50%
of early field failures could be eliminated by using this type