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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to test equipment for heating and cooling of
microelectronic devices to precise temperatures while under electrical
test, and specifically to apparatus for rapidly bringing microelectronic
devices to selected high or low temperatures.
2. Description of the Prior Art
Microelectronic circuits in production must be tested to determine their
characteristics over specified extremes of temperature. For example, such
circuits for military applications may require testing at -55.degree. C.
and at +125.degree. C. Equipments available in the past for maintaining
circuits under test at such temperatures generally fall into two classes:
chambers, such as ovens and refrigerators, into which the circuits under
test are placed; and jigs or fixtures arranged to hold the circuits with
temperature controlled gases or liquids circulated in or around the
fixture.
Typical test chambers are disclosed by Frick et al. in U.S. Pat. Nos.
3,408,565 and 3,412,333. The fluid transfer testers are illustrated by
U.S. Pat. No. 3,710,251 to Hagge et al. which teaches use of dry nitrogen;
and Pat. No. 3,979,671 to Melker et al. describing the use of liquid
injection for controlling temperature of a semiconductor chip. Each of
such prior art equipments is expensive, large, and inconvenient for rapid
testing. To obtain low temperatures, refrigeration equipment has been
necessary, which is inherently slow and requires frequent attention and
maintenance. Thus, the need clearly exists for a low-cost, easy to use,
and fast response device-under-test (D.U.T.) temperature controller.
SUMMARY OF THE INVENTION
This invention is a novel temperature controller for devices under test
that is extremely simple, convenient to use, and capable of quickly
bringing the unit under test to the desired temperature. My D.U.T.
temperature controller may be constructed at a fraction of the cost of
units involving fluid heat transfer and refrigeration units.
The D.U.T. temperature controller consists of two temperature heads or
probes which may be in the form of small metallic box-like structures with
thermal masses large compared to the mass of the units to be tested. A
typical size of a head is 11/2".times.11/2".times.11/4". To obviate the
need for cumbersome fixtures to maintain the probes in contact with the
case of a D.U.T., this typical size and shape provides adequate balance to
permit a probe to rest on the top of a case during tests. Additionally,
the weight of each probe is sufficient to allow gravity to maintain good
thermal contact between the probe and case when the probe is placed
thereon. The high temperature probe includes a small heater element and a
diode heat sensor. A small diameter flexible cable connects the probe to a
control and monitor section. A temperature reference diode, an operational
amplifier, and a heater proportional control amplifier are utilized in
combination with the high temperature head elements to monitor its
temperature and to control the temperature within very close limits to a
selected high temperature.
The cold probe is of similar construction to the high temperature probe but
has a reservoir for accepting dry ice. As the dry ice sublimates, it can
reduce the temperature of the cold head to near -65.degree. C.; however,
the heating element is controlled by its control and monitor section to
hold the temperature to a selected low value.
In operation, the monitor and control section circuit is calibrated for the
selected high temperature, for example, 125.degree. C. The operational
amplifier circuit and proportional control amplifier represent a tight
servo loop to hold the head at the selected temperature. After the head
has come up to its set temperature as indicated by the monitor, the
microelectronic unit under test is placed in its operating jig, with its
case in a horizontal plane.
The high temperature probe is placed on the case where it is held by
gravity in a heat transfer relationship to the device under test. The base
section of the head represents a heat sink, and as heat is drawn from its
thermal mass by the microcircuit under test, a slight dip in temperature
on the monitor indicator will be noted. The control circuit then functions
to increase the current in the heater element sufficient to replace the
lost heat. The monitor indicator will return to its preset voltage after
about 60 seconds, and the test can proceed.
For low temperature testing, preferably chips of dry ice are placed in the
cold head reservoir, and as it sublimates, the temperature of the probe
will drop. Assuming, for example, that a low temperature of -55.degree. C.
is desired. The monitor and control circuits are calibrated for that
temperature and will keep the cold head heater off until the temperature
approaches the preset value. At that point, current will begin to flow
through the cold head element causing the temperature to cycle about and
stabilize at -55.degree. C. as the dry ice continues to sublimate. After
the probe has stabilized at the preset low temperature, it is placed on
the device under test. Heat is drawn from the microcircuit by the thermal
mass of the cold head, causing a slight increase in the temperature
monitor reading. However, this heat is quickly absorbed by the dry ice,
bringing the indicator back to the preset value in about 70 seconds, and
the device tests can then proceed. During a series of tests, the dry ice
may be easily replenished as it is consumed.
The present invention thus provides a set of small, easily used temperature
probes that will bring microelectronic circuits under test to a selected
temperature quickly without use of bulky and difficult-to-use
refrigeration systems and heating chambers. The control circuits hold the
units under test within very narrow tolerances and are easily calibrated
for desired high and low temperatures. No inconvenient gases or liquids
are required as in some prior art systems.
Accordingly, it is a principal object of this invention to provide
apparatus for bringing microelectronic devices under electrical test to
selected high and low temperatures quickly and conveniently.
It is another object of the invention to provide a D.U.T. temperature
controller having separate hot and cold heads.
It is yet another object of the invention to provide a low cost D.U.T.
temperature controller having small, easily handled hot and cold heads.
It is still another object of the invention to provide a D.U.T. temperature
controller of convenient and economical construction, that does not
require the use of refrigeration systems or holding fixtures.
It is a further object of the invention to provide a D.U.T. temperature
controller of compact size that will maintain its hot and cold heads at
selected temperature within very close tolerances.
It is still a further object of the invention to provide a D.U.T.
temperature controller having a monitor to provide the operator with an
indication when the device under test is at the selected test temperature.
It is yet a further object of the invention to provide a D.U.T. temperature
controller comprising no moving parts, solid state monitor and control
circuits, and having high reliability with minimum maintenance required.
These and other objects and advantages of the invention will be apparent
from the following detailed description with reference to the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a top view of a preferred construction of a cold probe in
accordance with this invention, with the bottom of the reservoir and the
heat sink base partially cut away to show the heater element and heat
sensor;
FIG. 2 is a sectional view of the cold probe of FIG. 1;
FIG. 3 is an exploded perspective view of the cold probe of FIG. 1;
FIG. 4 is a top view of the high temperature probe of the invention, with
the potting compound cut away to show the heater element and the heat sink
base;
FIG. 5 is a sectional view of the high temperature probe of FIG. 4;
FIG. 6 is an exploded perspective view of the high temperature probe of
FIG. 5;
FIG. 7 is a schematic diagram of the monitor and control sections of the
invention; and
FIG. 8 is a plot of typical temperature variations of the hot and cold
probes under start up and operate conditions.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention utilizes two temperature heads or probes, one for
producing a high temperature and another for producing a low temperature.
Referring to FIGS. 1, 2 and 3, details of a preferred implementation of a
cold probe 10 in accordance with the invention is shown. The size of the
head 10 may be selected in accordance with the size of microelectronic
devices to be tested; however, a box having dimensions of
11/2.times.11/2.times.11/4 inches has been found well suited for use with
encapsulated LSI and integrated circuits. A heat sink base 14, which may
be of aluminum with a thickness of 1/4 inch as shown in FIGS. 1 and 2, has
a first cavity 16 for accepting heating element 22, and a second cavity 18
for accepting heat sensor diode 24, effectively embedding diode 24 in heat
sink 14. The heat sink 14 is preferably slightly larger in area than the
contact area for the D.U.T. Heating element 22 may be a 20 ohm, 6 watt
metal-encased resistor, and heat sensor 24 may be a 1 N 4153 diode. Heater
22 and diode 24 are secured in the cavities by electrically insulative,
heat-conductive epoxy 15.
A light weight flexible cable 30 connects the cold probe to its associated
monitor and control circuits with shielded cable 34 connecting to heater
22 and shielded cable 32 connecting to diode 24.
Outside shell 12 may be formed from 1/8" wall aluminum box tubing and is
insulated from heat sink base 14 by fiberglass separator 36. A cup 20 is
inserted in shell 12 with its bottom surface in firm contact with heat
sink base 14. Cup 20 forms a reservoir for holding dry ice chips during
operation of the invention.
FIG. 3 shows, in exploded view, the assembly of heat sink base 14,
fiberglass separator 36, shell 12, and cup 20.
Turning now to FIGS. 4, 5 and 6, the preferred embodiment of the high
temperature probe or head 40 is illustrated. The construction is similar
to that of cold probe 10 with the same general size; however, a larger
heating element is required. Heat sink base 44 may be 1/4" thick aluminum
approximately 11/4.times.11/4 inches and contains heat sensor diode 48 in
cavity 47. The exact size of heat sink 44 is determined from the size of
the devices to be tested. Electrically insulative, heat conductive epoxy
45 is used to secure diode 48 effectively embedded in heat sink 44.
Heating element 60 may be a 16 ohm, 20 watt metal encased resistor affixed
to heat sink base 44 by screws 61. Outside shell 42, formed from 1/8"
wall, aluminum box tubing is separated from base 44 by fiberglass
separator 46.
FIG. 6 shows an exploded view of hot probe 40 illustrating the assembly of
heat sink base 44 with resistor 60, separator 46, and shell 42. After
assembly, shell 42 may be filled with a ceramic potting compound 43 such
as Sauerisen 33. Lightweight, flexible cable 50, consisting of shielded
heater cable 52 and sensor diode cable 54, connects hot probe 40 to the
monitor and control circuits.
Having described the temperature probe means, the monitor and control
circuits will be explained with reference to the schematic diagram of FIG.
7. The hot and cold sections of the monitor/control system of FIG. 7 are
essentially the same, but are calibrated for different control
temperatures. Thus, the"hot" section 70 will be described in detail and
the "cold" section 80 variations specifically discussed. Although
particular circuit elements are recited, these are for exemplary purposes
only and many other types and values of elements may also be used in
accordance with the invention.
The high temperature heat sink 44 contains heat sensor diode 48, which may
be a 1 N 4153 semiconductor diode forward biased at about 1 ma from a +18
volt supply through resistor 73. As may be noted, diode 48 is in a bridge
circuit 79 with diode 71, also a 15 N 4153, biased through adjustable
potentiometer 74. When both diodes 48, 71 are held at ambient temperature,
for example at 23.degree. C., pot 74 is adjusted to produce the same
forward voltage for diode 71 or for diode 48 as evidenced by exact balance
of the bridge 79. Operational amplifier 72 which is preferably an Lm 741,
measures the differential voltage across the bridge circuit 79, which will
be zero when both diodes are at ambient. The amplifier is thus working in
a floating mode with respect to the power supply 93 ground potential. The
gain of operational amplifier 72 is set by dc feedback potentiometer 76
and may be set to produce an output voltage change of exactly 100
mv/deg.C. rise in heat sense diode 48. For the diode and bias current
specified above, the diode 48 voltage changes at the rate of -2 mV/deg.C.;
therefore, if the gain of operational amplifier 72 is set at 50, a one
degree change in the heat sense diode 48 will give +100 mv change in the
output.
For the example of both sense diode 48 and reference diode 71 being at
ambient temperature, the differential voltage output of operational
amplifier 72 will be zero volts. However, it is desired that meter 90 read
the ambient temperature in such case. Therefore, it is necessary to offset
the voltage. For a 0-15 volt voltmeter, a direct reading scale may be
used; that is, 23.degree. C. may register +2.3 volts. To this end,
potentiometer 75 is connected to operational amplifier 72 to provide means
of setting the offset voltage to the desired value. The meter 90 may now
be used to read .+-.150.degree. C. by connecting in the proper polarity.
Switch 91 therefore connects meter 90 to the hot probe operational
amplifier 72 to read positive voltages.
The output voltage from operational amplifier 72 appears across divider
potentiometer 77 with its wiper arm connected to amplifier 78. Input
transistor 79, which is a switching transistor and may be a 2 N 2222, has
a turn-on voltage of about 0.6 V. Potentiometer 77 is set to a point
greater than this value when the output voltage is +2.3 volts causing
transistor 89, which may be a 2 N 5038, to be on with current flowing in
probe heater 60. As this heater current flows, the hot probe temperature
rises and the forward voltage of sense diode 48 drops at the approximate
rate of -2 mV/deg.C. generating a differential voltage output from
operational amplifier 72.
As an example of a particular calibration, assume that the forward voltages
of diode 48 and diode 71 are each +0.585 volts at 23.degree. C. ambient
and the output of amplifier 72 is +2.3 V. If the hot probe temperature
desired is +125.degree. C., potentiometer 77, which may be a ten-turn
calibrated type, may be set to 0.6/12.5 of maximum or 4.8%. Since the base
voltage of transistor 79 is less than 0.6 volts, that transistor is fully
off, transistor 89 is fully on, and maximum current flows through heater
60. Assume the hot probe temperature rises to 73.degree. C. for example,
an increase of 50.degree. from ambient. At -2 mV/deg.C., this increase is
a change of -100 mV at the input to amplifier 72 and, for a gain of 50, an
output of 5 volts. The meter 90 now reads 2.3+5.0 volts or 7.3 volts,
indicating 73.degree. C. With potentiometer 77 set at 4.8% of 7.3 volts or
0.35 volts, transistor 79 remains off and full current flows in heater 60.
As the hot probe temperature rises to, say, 113.degree. C., meter 90 will
read 11.3 volts and the base voltage of transistor 79 is 0.525 volts and
is beginning to turn on, transistor 89 is beginning to turn off, and the
current in heater 60 is decreasing. When the temperature of heat sink 44
approaches the desired value of +125.degree. C. the threshold point of
transistor 79 is reached, at which slight changes in drive current causes
large proportional changes in the heater current. Therefore, only an
amount of current flow at +125.degree. C. results that will balance heat
loss due to radiation, and conduction to the D.U.T.
When the hot probe is applied to a cold device under test, heat will be
suddenly absorbed from the heat sink 44, causing a drop of a few degrees
in probe temperature. A large increase in heater current results, quickly
returning the probe to the set temperature. As may now be seen, the system
represents a tight dc servo loop and has been found to be capable of
maintaining the probe temperature (after stabilization) within .+-.1% of
the set temperature. Amplifier 78 may be noted to operate as a switch,
turning the heater current full on below about +113.degree. C. and a
sensitive proportional control around the examplary set temperature of
+125.degree. C.
Monitor and control section 80 for cold probe 14 is essentially identical
to hot probe section 70 described above except that it is calibrated to
operate below ambient and to hold current through heater 22 off until the
desired low temperature of probe 14 is approached. At this point,
Darlington amplifier 88 operates as a proportional amplifier, increasing
the current through heater 22 as the probe temperature decreases. Meter 90
will read a positive voltage at the output of operational amplifier 82 and
is connected thereto by switch 91. Zener 95 which may be a 1 N 746 is used
to prevent meter 90 from swinging negative when first switched into the
cold probe circuit 80 and the output of operational amplifier 82 has not
yet come up to zero volts from -2.3 volts.
For a selected lower temperature of -55.degree. C. as an example,
potentiometer 87 is set at 1.2/5.5 of maximum or 21.8% of its maximum
output. As the cold probe 14 drops from ambient toward -55.degree., the
difference voltage will go from -2.3 volts to zero volts at 0.degree. C.
Assume now that the cold probe temperature has dropped to -45.degree. C.,
the difference voltage is (23.degree.+45.degree.) (2 mV/deg.C.)=136 mv
causing an output change of 6.8 volts. Subtracting the -2.3 volt offset, a
reading on meter 90 is therefore 4.5 volts or 45.degree. C. The Darlington
circuit 88 is then driven by 21.8% of this voltage or 0.981 volts. This
causes input transistor 96 and heater control transistor 97 to begin to
turn on. The resulting current through heater 22 slows the rate of cooling
as the heat sink 14 temperature approaches -55.degree. C.
At -55.degree. C., the input to current control amplifier 88 is 21.8% of
5.5 volts or 12 volts, causing a substantial current flow through heater
22. This temperature represents an equilibrium point at which the heater
supplies the heat required for maintaining the preset temperature during
continued sublimation of the dry ice. When the cold probe 10 is placed on
a warm device under test, the probe heat sink 14 absorbs the heat
therefrom and sensor 24 will sense a slight rise in temperature, causing a
reduction in current through heater 22. The dry ice will again cool probe
10 to the -55.degree. C. equilibrium point.
As may now be seen, the control circuit 80 provides a very accurate dc cold
temperature control loop. A typical response during a typical device test
has been found to be approximately 60 seconds for the -55.degree. C.
equilibrium point to be obtained; thus, the device is brought to the test
temperature quickly and conveniently.
The hot and cold control systems, as may be recognized, are direct coupled
from their sense diodes through the feedback loops to the heat elements
and therefore have a frequency response down to direct current.
The two reference diodes 71, 81 are installed in a heat sink in the
controllor chassis and room air is circulated around the heat sink,
resulting in a stable ambient reference temperature. If the heat sink
temperature changes slightly, the reference diodes will sense such change,
the control circuits respond, and the meter will still read the correct
probe temperatures.
Although not illustrated, it is desirable to cover the four sides of each
probe 10, 40 with thermal insulating material such as asbestos cloth to
allow handling of the probe heads without danger. Additionally, such cover
reduces frost formation on the outside of cold probe 10.
FIG. 8 shows a plot of the variations in temperature of the heat sinks 14
and 44 of probes 10 and 40 for a typical apparatus constructed in
accordance with the invention. The unit was calibrated for an ambient
temperature of 23.degree. C., a high test temperature of 85.degree. C. and
a low test temperature of -55.degree. C. The graph indicates the times
required to reach the desired preset temperatures from initial turn on of
the apparatus, and the times for the heat sinks to stabilize after being
placed on a typical device-under-test.
Starting with an ambient temperature of 23.degree. C., the hot probe heat
sink is seen to stabilize at 85.degree. C. in approximately 11.5 minutes.
Applying the hot probe to a D.U.T., a slight drop in temperature of about
2 degrees occurred, and stabilized at 85.degree. C. in about 60 seconds
from application. The cold probe heat sink reached its preselected
temperature of -55.degree. C. in about 4 minutes from 23.degree. ambient.
Applying the cold probe to a D.U.T. resulted in about 2 degrees rise and
about 70 seconds to restabilize at -55.degree.. Thus, once the probes have
initially attained their preselected test temperatures, a device under
test can be brought to the required temperature in about one minute. The
operator may determine from observing the indicator 90 when the D.U.T. and
the probe are stabilized so that the electrical testing of the device can
proceed. For the unit represented by the graph of FIG. 8, the cycling time
between hot and cold tests is thus seen to be on the order of 60-70
seconds.
As will now be recognized, the invention provides a small, easily applied
and low cost apparatus to enable microelectronic circuit devices and the
like to be quickly and easily brought to selected low and high
temperatures during electrical tests of the devices, with the apparatus
requiring no refrigeration equipment, nor special chambers. The simplicity
of the apparatus insures very high reliability, long life, and freedom
from continual maintenance.
It is to be understood that the preferred embodiment described hereinabove
is presented for exemplary purposes only and is not to be considered
restrictive. Many variations and modifications will be obvious to those of
ordinary skill in the art. For example, the specific probe temperatures
may be easily set to other values, and the size of the probes may be
varied to suit the physical dimensions of the devices to be tested. In
addition, other control and monitor circuits may be substituted without
departing from the spirit and scope of the invention. As is also obvious,
various changes in transistors such as other types and substitution of NPN
for PNP, can be made without deviating from the invention.
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