QWhat
is the difference between ground and a groundable point?
The ESD Association standard
ESD S6.1-1999 defines ground as:
(1) A conducting connection, whether
intentional or accidential, between an electrical circuit
or equipment and the earth, or to some conducting body
that serves in place of earth.
(2) The position or portion of an electrical
circuit at zero potential with respect to the earth.
(3) A conducting body, such as the
earth or the hull of a steel ship used as a return path
for electric currents and as an arbitrary zero reference
point.
Example: Your third wire electrical
outlet
Groundable Point: A designated connection location or assembly used on
an ESD protective material or device that is intended
to accommodate electrical connection from the device
to an appropriate ground.
Example: The snap on your worksurface
mat
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Q What is
the practical difference between measuring resistance
to ground and measuring inductance?
Inductance (L) is the current
(I) flow through a coil having of some number (N) turns
and the magnetic flux (ø) linked to the coil.
It is measured in Henry's (H) or mH, where
L= N(ø/I )
Resistance is the ratio of DC
voltage to Current passing between two electrodes, or
points, i.e., R = V/I.
In ESD control applications
we ask three questions (1) how much charge is generated?
(2) where does it go? - we want it to go to ground,
and (3) how quickly can it dissipate? Resistance to
ground is the opposition to current flow at a given
voltage. Thus, an accurate resistance (R) measurement
to ground (a) confirms a path to ground, and (b) depending
on the capacitance (C) of the discharging body, indicates
how quickly (t) a charge can dissipate to ground, i.e.,
t = RC, and 5t is the approximate total time in seconds
for dissipation.
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QWhat
is the effect of humidity in ESD control circuit assembly
and test environments?
High relative humidity
(Rh) causes moisture to deposit and be absorbed by some
materials. It does not eliminate triboelectric charge
generation. Rather, it lubricates surfaces to reduce
charge generation to some degree, and helps render material
surfaces slightly more dissipative. The end result is
lower charge generation and a lower residual charge.
Unfortunately, Rh does not reduce charge sufficiently
on ordinary materials to protect many ESD sensitive
devices in critical assembly or laboratory areas. Never
design your ESD control based on available humidity.
Rather, design your ESD control
program assuming a low humidity (or no humidity) environment.
The if Rh is available, it helps make a good program
more effective. Like a special sauce heightens the flavor
of well-prepared dish.
Finally, relative humidity and
moisture will not eliminate charge. If it did, you would
never see lightening during a thunderstorm.
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QMy
PHT-770 Hygro-Thermometer keeps displaying an "E2"
message. What does it mean?
An "E2" error
message represents a bad sensor. The message is displayed
continiously everytime the Hygro-Thermometer is turned
on.
Unfortunately, Prostat does
not repair or replace the sensor on the PHT-770. You
will need to replace it with our PHT-771 Digital Psychrometer.
To learn more about our PHT-771, click
here. If you wish to purchase one, please call our
sales department at +1 630-238-8883.
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QWhat is the difference between the PRF-914B Miniature Concentric Ring and the PRF-922B Two-Point Probe?
The PRF-914B and the PRF-922B are two different measurements. Two-Point electrodes measure few effective paths in parallel between them, while a concentric ring measures many paths in parallel, as a result the measurements are different. The PRF-914B will measure in excess of E14 ohms while the PRF-922B will measure up to E12 ohms, two orders of magnitude less.
The PRF-914B is best for evaluating the effects of compounding and processing. The PRF-922B meets a standard that users identify with on finished products at lower resistance levels.
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QTo calculate the capacitance of a packaged device, is there some special method necessary for maintaining a voltage on a packaged device?
First, for the lab environment, the device must be clean and dry. If you are making measurements in high Rh, charge drain will be amplified. Technically, the best way to make the measurement is to:
- Suspend the device in space using a fine thread and drop of super glue
- Ionize the device to eliminate all charge
- Charge the device lead frame using a known voltage
- Maintain device contact with the charging source while you zero your Faraday cup and raise the cup to the device, removing the charging source before the device enters the cup
- The device should fully rest and make total contact with the bottom of the cup
- Record the cup charge (Q), divide by the charge voltage (V) to get an approximate capacitance (C)
- Repeat several times, eliminate the min and max extremes and calculate the average capacitance.
In the field, I use a miniature CPM that is very clean and stable, having minimum leakage:
- Place the clean, ionized device on the small CPM, leads down in contact with the floating plate
- Charge the CPM to known voltage and “flip” the device into a zeroed Faraday cup
- Calculate capacitance as above
- Repeat several time and calculate average capacitance
this is simply an approximation of device capacitance. If your CPM “leaks”, or the humidity is high, it is difficult to obtain good measurements. However, you are approximating capacitance to estimate device voltage in your process. It takes some practice and one must develop a bit of skill and confidence in the measurement.
A new, and better way to assess device voltage is to measure it with a Contact Voltmeter (see http://www.prostatcorp.com/CVM780) or a very high impedance voltmeter equipped with a fine ceramic contact probe. This will give you an accurate measurement of device lead or subassembly actual voltage at the moment of contact without discharging the conductor and causing an ESD event. This technique is far more convenient, quite fast, and far more practical than the traditional calculations when analyzing a manufacturing process.
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QWe measure resistance of garment sleeve to sleeve and it's OK when the garment is properly grounded. Why do we see 200 Volts on my field meter when measuring this garment?
If the instrument & operator are not properly grounded, the difference in voltage between the meter and grounded garment will be displayed. Always ground & zero your instrument.
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