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SEMI G30-88 © SEMI 198 6, 1988 2 efficient interface between the package and the primary heat sink. The m ounting sur face temperature is determined with a th er mocouple attached from the side or bottom of the adapter w…
SEMI G30-88 © SEMI 1986, 19881
SEMI G30-88
TEST METHOD FOR JUNCTION-TO-CASE THERMAL RESISTANCE
MEASUREMENTS OF CERAMIC PACKAGES
1 Purpose
The purpose of this test is to determine the thermal
resistance of ceramic packages using thermal test chips.
This test method deals only with junction-to-case or
mounting surface measurements of thermal resistance
and limits itself to heat sink and fluid bath testing
environments. Following the guidelines outlined in this
test method, junction-to-case thermal resistance
measurements of ceramic packages using the heat sink
and fluid bath methods should give the same results
only under certain limited conditions (i.e., under
conditions that approximate unidirectional heat flow
through the chip and substrate to the preferred heat
removal surface). If discrepancies occur, the heat sink
mounting technique shall be considered as the referee
test method. The heat sink mounting method for
measuring junction-to-case thermal resistance will be a
conservative measure of the package’s ability to
transfer heat to the ambient environment because heat
sinking is provided only on one side of the package,
whereas the fluid bath mounting method has the
potential for equally cooling both sides of the package.
1.1 Definitions — The following d efinitions and
symbols shall apply for the purpose of this test:
a. case temperature, T
C
, in degrees Celsius. The case
temperature is the temperature at a specified
accessible reference point on the package in which
the microelectronic chip is mounted.
b. mounting surface temperature, T
M
, in degrees
Celsius. The mounting surface temperature is the
temperature of a specified point at the device-heat
sink mounting interface (or primary heat removal
surface).
c. junction temperature, T
J
, in degrees Celsius. The
term is used to denote the temperature of the
semiconductor junction in the microcircuit in which
the major part of the heat is generated. For purposes
of this test, the measured junction temperature is
only indicative of the temperature in the immediate
vicinity of the element used to sense the
temperature.
d. power dissipation, P
H
, in watts, is the heating power
applied to the device causing a junction-to-reference
point temperature difference.
e. thermal resistance, junction-to-specified reference
point, R
ΘJR
, in degrees Celsius/watt. The thermal
resistance of the microcircuit is the temperature
difference from the junction to some reference point
on the package divided by the power dissipation P
H
.
f. temperature-sensitive parameter, TSP, is the
temperature-dependent electrical characteristic of
the junction under test which can be calibrated with
respect to temperature and subsequently used to
detect the junction temperature of interest.
2 Apparatus
2.1 The apparatus required for the se tests shall include
the following as applicable to the specified test
procedures.
a. Thermocouple material shall be copper-constantan
(type T) or equivalent, for the temperature range
-100 to +300°C. The wire size shall be no larger
than AWG size 30. The junction of the
thermocouple shall be welded to form a bead rather
than soldered or twisted. The accuracy of the
thermocouple and associated measuring system
shall be ± 0.5°C.
b. Suitable electrical equipment as required to provide
controlled levels of conditioning power and to make
the specified measurements. The instrument used to
electrically measure the temperature-sensitive
parameter shall be capable of resolving a voltage
change of 0.5 mV.
c. Controlled temperature chamber, fluid bath, or heat
sink capable of maintaining the specified reference
point temperature to within ± 0.5°C of the preset
(measured) value. Typical temperature-controlled
heat sink and fluid bath assemblies are presented for
illustrative purposes only.
2.2 Heat Sink Assembly — A typical heat sink
assembly for mounting the microelectronic device
under test is shown in Figure 1. The primary heat sink
is water cooled using a temperature-controlled fluid
circulator bath. An adapter socket/heat sink is fastened
to the heat removal surface of the primary heat sink,
and has a special geometry to handle specific size
packages (e.g., flat packs, dual-in-line packages, chip
carriers). This adapter provides a repeatable and
SEMI G30-88 © SEMI 1986, 1988 2
efficient interface between the package and the primary
heat sink. The mounting surface temperature is
determined with a thermocouple attached from the side
or bottom of the adapter with a thermal conducting
adhesive or grease at or near the interface between the
adapter and the package. It is at this point that the
device-under-test temperature is specified and
controlled. The adapter also contains the socket or other
electrical interconnection scheme. A thin coating (about
25 – 50 mm thick) of a thermal heat-sinking compound,
such as zinc-oxide-loaded silicone thermal grease, is
used at the interface to provide a reliable thermal
contact.
2.3 Fluid Bath Assembly — A typical temperature-
controlled fluid bath for thermally characterizing the
microelectronic device under test is shown in Figure 2.
In this figure, the package is mounted in a fluid bath
separate from the fluid circulator, although it can be
immersed directly in an integrated fluid circulator/bath
unit. The fluid in the bath should be continuously
stirred or agitated to ensure the required temperature
stability. Since this working fluid is being used as an
infinite heat sink, the case-to-fluid (ambient)
temperature difference at the case temperature reference
point of interest should be minimized, i.e., less than or
equal to 20°C. For case-to-fluid temperature differences
greater than 20°C, accuracy and repeatability
difficulties may occur due to a large variable
temperature gradient in the fluid film boundary layer at
the package-fluid interface. The case-to-fluid
temperature difference can be minimized by increasing
the fluid velocity and by decreasing the power density
seen by the fluid. The device under test should be
mounted such that heat transfer to the fluid is not
impeded. For leaded devices, the leads should be
oriented in such a manner so as not to interfere with the
heat transfer to the fluid and provide freedom to any
thermal currents caused by the power dissipation within
the package. The case temperature of the device under
test should be measured with a thermocouple that is
attached to the package and should not be assumed to
be at the fluid temperature. Care should be taken to
minimize exposure of the thermocouple bead to the
high temperature gradient in the fluid film boundary
layer at the package-fluid interface. The working fluid
should have a thermal conductivity at 25°C of at least
0.0006 W/cm°C. Working fluids such as inert
fluorocarbon liquids and silicone oils are suitable as a
cooling media.
3 Procedure
3.1 Direct Measurement of Refere nce Point
Temperature — T
C
. For the purpose of measuring a
microelectronic device thermal resistance, the reference
point temperature shall be measured at the package
location of highest temperature which is accessible
outside the package. This reference point location is
determined with the device operating in free air and
with no external heat-sinking. In general, this reference
point is found to be on the outside surface of the
package substrate directly underneath the chip in the
major path of heat flow from the chip to the heat sink or
ambient. Examples of the reference point location for
both cavity-up and cavity-down ceramic packages are
depicted in Figure 3. The package surface may be
altered to facilitate this measurement, provided that
such alteration does not affect the original heat transfer
paths and, hence, the thermal resistance, within the
package by more than a few percent. For packages with
an integral heat dissipater attached to the outside
surface of the package substrate, the case temperature
reference point shall be on the surface of the heat
dissipater at a point opposite the backside of the chip as
indicated in Figure 4.
3.1.1 Case temperature, T
C
. The microelectronic
device under test shall be mounted under specified
conditions so that the case temperature can be held at
the specified value. A thermocouple shall be attached
on the surface of the device package directly under the
chip. A conducting epoxy may be used for this purpose.
The thermocouple bead should be in direct mechanical
contact with the case of the microelectronic device
under test. For devices which, in their normal
application, are intimately connected (by pressure
contact, adhesive, soldering, or other means) to an
external heat sink, the mounting surface temperature, as
measured directly below the primary heat removal
surface of the case, may be used as the equivalent case
temperature.
If it is found that attaching the thermocouple directly to
the case is impractical, an alternate approach utilizing a
thermocouple welded to one side of a thin metal disk
should be used. This can be accomplished by parallel
gap welding the crossed thermocouple wires to one side
of a 0.25 cm (0.094 in) diameter, 0.02 cm (0.008 in)
thick beryllium-copper disk and then, with a thin layer
of adhesive, bonding the other side of the disk to the
case at the point of interest.
3.1.1.1 Mounting surface temperature , T
M
. The
mounting surface temperature is measured directly
below the primary heat removal surface of the case. It is
measured with a thermocouple at or near the mounting
surface of the heat sink. A typical mounting
arrangement is shown in Figure 5. The surface of the
copper mounting base shall be nickel plated and free of
oxides.
The thermocouple hole shall be drilled into the
mounting base such that the thermocouple lead is
directly below the area on the case of interest. It is
SEMI G30-88 © SEMI 1986, 19883
recommended that the thermocouple be secured into the
mounting base with a thermal conducting adhesive (or
solder) and that particular attention be paid to
minimizing air voids around the ball or the
thermocouple. A thermal conducting compound (or
adhesive) should be used at the interfaces of the
mounting base and the device under test. The mounting
surface technique is application oriented in that it takes
into account the mounting surface interface.
3.2 Thermal Resistance, Junction-t o-Specified
Reference point, R
ΘJR
3.2.1 General Considerations — The thermal
resistance of a semiconductor device is a measure of the
ability of its carrier or package and mounting technique
to provide for heat removal from the semiconductor
junction. The thermal resistance of a microelectronic
device can be calculated when the case/mounting
surface temperature and power dissipation in the device
and a measurement of the junction temperature are
known.
When making the indicated measurements, the package
shall be considered to have achieved thermal
equilibrium when halving the time between the
application of power and the taking of the reading
causes no error in the indicated results within the
required accuracy of measurement.
3.2.2 Indirect Measurement of Junction Temperature
for the Determination of R
ΘJR
— The purpose of the test
is to measure the thermal resistance of integrated
circuits by using particular semiconductor elements on
the chip to indicate the device junction temperature. In
order to obtain a realistic estimate of the operating
junction temperature, the whole chip in the package
should be powered in order to provide the proper
internal temperature distribution. During measurement
of the junction temperature the chip heating power
(constant voltage source) shall remain constant while
the junction calibration current remains stable. It is
assumed that the calibration current will not be affected
by the circuit operation during the application of
heating power.
The temperature-sensitive device parameter is used as
an indicator of an average (weighted) junction
temperature of the semiconductor element for
calculations of thermal resistance. The measured
junction temperature is indicative of the temperature
only in the immediate vicinity of the element used to
sense the temperature.
The temperature-sensitive electrical parameters
generally used to indirectly measure the junction
temperature are the forward voltage of diodes and the
emitter-base voltage of bipolar transistors. Other
appropriate temperature-sensitive parameters may be
used for indirectly measuring junction temperature for
fabrication technologies that do not lend themselves to
sensing the active junction voltages.
3.2.2.1 Steady-state technique for mea suring T
J
. The
following symbols shall apply for the purpose of these
measurements:
I
M
Measuring current in milliamperes.
V
MH-
Value of temperature-sensitive parameter in
millivolts, measured at I
M
, and corresponding to the
temperature of the junction heated by P
H
.
T
MC-
Calibration temperature in degrees Celsius,
measured at the reference point.
V
MC-
Value of temperature-sensitive parameter in
millivolts, measured at I
M
, and specific value o
f
T
MC
.
The measurement of T
J
using junction forward voltage
as the TSP is made in the following manner:
Step 1 — Measurement of the temperature coefficient
of the TSP (calibration).
The coefficient of the temperature-sensitive parameter
is generated by measuring the TSP as a function of the
reference point temperature, for a specified constant
measuring current, IM, by externally heating the device
under test in an oven or in a fluid bath. The reference-
point temperature range used during calibration shall
encompass the temperature range encountered in the
power application test (see Step 2). The measuring
current is generally chosen such that the TSP decreases
linearly with increasing temperature over the range of
interest, and that negligible internal heating occurs in
the silicon and metal traces. For determining the
optimum TSP calibration or measuring current, V
MC
vs.
log I
M
curves for two temperature levels that encompass
the calibration temperature range of interest should be
plotted. The optimum measuring current, I
M
, is then
selected such that it resides on the linear portion of the
two V
MC
vs. log I
M
curves that were generated. A
measuring current ranging from 0.05 to 5 mA is
generally used, depending on the specifications and
operating conditions of the device under test, for
measuring the TSP. The value of the TSP temperature
coefficient, V
MC
/T
MC
, for the particular measuring
current used in the test, is calculated from the
calibration curve, V
MC
vs. T
MC
. At least three points
should be used to generate the voltage vs. temperature
curve for the determination of the TSP temperature
coefficient.
Step 2 — Power application test.
The power application test is performed in two parts.
For both portions of the test, the reference point
temperature is held constant at a preset value. The first