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Charged Device Modelling
INTRODUCTION: Microelectronic products, semiconductor devices, are used in almost
any kind of equipment. These devices may suffer from electronic discharge damage. This
can occur, when a device acquires charge through some turboelectric process and then abruptly
touches grounded objects. Discharge damage also may occur, when charged personnel is
touching grounded devices. To avoid the human body discharge, considerable effort has
been made by employing wrist straps, static dissipative flooring, or avoiding human handling
completely. But ICs may even already suffer from transportation. Often they are packaged in
plastic shipping tubes, made from PVC (Polyvinyl Chloride). Left untreated, the material
acts like a charge generator. PVC must be dipped in antistatic solution, but the coating
deteriorates with age and wear.|
CDM MODELLING: In order to find out how safe the semiconductor devices are and what
kind of stress, or electronic discharge the semiconductor products can take, test methods
were needed to be developed. For simulating controlled discharge at semiconductor devices,
a special method, the Charged Device Modelling (CDM) became known.
THE CDM SIMULATOR: At a standard charged device modelling simulator the device
under test is placed on dielectric, charged by a power supply and discharged through a 1
Ohm resistor that serves to sample the discharge current. The process is then monitored via
an oscilloscope, connected to a parallel transmission line. The parasitic inductance, associated
with the 1 Ohm resistor, is critical to the proper operation of the circuit.
THE DISC RESISTOR: The problem of parasitic inductances can be avoided or at least
minimized by its geometrical shape that is most benificial to the circuit. If a disc resistor is
used, the current flows radially. By mounting the disc resistor on a coaxial line of same
dimensions a CDM (Charged Device Modelling) test can be performed by simply connecting
the protruding center conductor of the coaxial line with the device that has been charged.
CALIBRATION: For calibration purposes the coaxial probe needs a mating receptacle. It
must connect to a device that accomodates the protruding center conductor of the probe. The
structure of the probe/mating receptacle must maintain a perfect 50 Ohm transmission line,
with the only exception of the small area where the 1 Ohm resistor is placed.
THE HARDWARE: Spectrum Elektrotechnk GmbH has designed and manufactured a probe
that has a standard SMA female connector at one end, and therefore can easily connect to
standard transmission lines with SMA connectors. The other side of the unit is equipped with
a replaceable center conductor and has a narrow cylindrical area for the 1 Ohm resistor. The
replaceable center conductor is of utmost importance, as damages at the center conductor,
resulting from arcing when discharging devices, are unavoidable.
The mating device for calibration purposes accomodates the protruding center conductor of
the probe and maintains a perfect 50 Ohm line, with the exception of the area where the one
Ohm resistor has been placed. The SMA connector at the other end allows again for connection
with standard 50 Ohm transmission lines.
The mounting plate is necessary to accomodate the probe savely in order to mount it on a
positioner for accurate placement in relation to the device to be discharged.
CUSTOM PRODUCTS: The CDM Components are custom products, developed to one
customers' needs. Spectrum Elektrotechnk GmbH is a very innovative company. Our engineers
are constantly designing new products, or modifying existing products to customers'
requests. Therefore, our catalogs will never be complete. If you do not find exactly what you
need in our catalogs, or other catalogs, please contact our sales or engineering department.