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SEMI G38-0996 © SEMI 1987, 2004 3 thermocouple beneath the DUT. For purposes of measuring forced air junc tion-to-ambient thermal resistance in a wind tunnel , the ambient tem perature should be measured with a thermocou…
SEMI G38-0996 © SEMI 1987, 2004 2
wind tunnel assemblies are presented for reference
purposes only.
5.1.4 Still-Air Enclosure Assembly — The microcircuit
shall be mounted in a cubic enclosure of not less than
0.028 m
3
(1.0 ft
3
). There shall be no radiation sources
other than the microcircuit under test in the enclosure.
The interior enclosure wall shall have a high reflectance
finish (emissivity < 0.1). The ambient temperature
should be measured by means of a thermocouple
mounted at a distance of approximately 2.54 cm (1.0 in)
beneath the DUT and 1.27 cm (0.5 in) from the test
board or socket.
5.1.4.1 The microcircuit shall be mounted in such a
manner that conduction cooling through the leads or the
test socket or both shall be small compared to the other
cooling mechanisms. No. 36 AWG wire should be
connected to the device test socket. The air flow (by
natural convection) should be unrestricted above and
beneath the device. An alternative approach would be
to use a mounting arrangement that approximates an
application environment. Such a reference mounting
configuration can be found in Specification, Thermal
Test Board Standardization for Measuring Junction-to-
Ambient Thermal Resistance of Semiconductor
Packages. Device mounting and test board positioning
inside the measuring chamber are depicted in Figure 1.
5.1.5 Wind Tunnel Assembly — A typical wind tunnel
design is shown with its dimensions in Figure 2.
5.1.5.1 The fan or blower should be placed
downstream of the DUT as depicted in Figure 2. A
static pressure differential measurement across a
calibrated nozzle is used to calculate the wind tunnel air
velocity, while a thermocouple is used to measure the
ambient temperature upstream of the DUT. Both of
these devices should be placed at a specified location
upstream of the DUT, as shown in Figure 2. The
thermocouple is located in the center of the wind
tunnel, 5.08 cm (2.0 in) from the test section and 2.54
cm (1.0 in) above the center plane of the DUT.
5.1.5.2 The microcircuit shall be mounted in such a
manner that conduction cooling through the leads or the
test socket, or both, shall be small compared to the
other cooling mechanisms. To minimize conduction
through the leads, No. 36 AWG wire should be
connected to the device test socket. The DUT should be
aligned so that the air front is parallel to the longer edge
of the package (air front hitting the package side). An
alternative approach would be to use a mounting
arrangement that approximates an application
environment. Such a referee mounting configuration
can be found in SEMI G42. Device mounting and test
board orientation inside the wind tunnel are depicted in
Figure 3.
5.1.5.3 Flow straighteners should be placed upstream
of the DUT, as shown in Figure 2. The flow
straighteners should provide a flat velocity profile
across the test section of the wind tunnel. This will
ensure that the DUT is exposed to a uniform velocity
across its entire cross section. A typical velocity profile
for well-developed turbulent flow is depicted in Figure
3.
5.1.5.4 Calibrated hot wire anemometer or nozzle with
suitable pressure gauges (p
1
and p
2
) for measuring the
pressure difference across the nozzle is used to
calculate the wind tunnel air velocity. When using the
nozzle, the pressure differential across the calibrated
nozzle is measured using a liquid monometer, typically
in inches of water.
5.1.5.5 Hot wire anemometer capable of verifying the
air velocity profile with an accuracy of ± 5%. The
velocity sensor should disrupt the airflow as little as
possible.
6 Procedure
6.1 Measurement of Wind Tunnel Air Velocity, v
A
—
The air velocity measurement techniques are direct
method and indirect method.
6.1.1 Direct Measurement of Wind Tunnel Air Velocity
— The direct method is based on a measurement by a
hot wire anemometer in wind tunnel.
6.1.2 Indirect Measurement of Wind Tunnel Air
Velocity, vA — The air velocity measurement technique
is an indirect method based on a pressure differential
measurement across a calibrated nozzle. The actual
velocity of interest is derived from a calibration curve
relating the pressure differential of the nozzle to the
volume (or mass) flow rate of the air through the
nozzle. The required air velocity (in linear feet per
minute) is then calculated by dividing the volume flow
rate (in cubic feet per minute) by the cross sectional
area at the entrance of the DUT test section (in square
feet). The volume flow rate shall be determined to
within an accuracy of ± 10%.
6.1.2.1 Air Flow Profile — Hot wire anemometer
should be used to verify that the flow profile of the air
front, measured within 5.08 cm (2.0 in.) of the test
section, does not vary by more than 10% across the
center 90% of the test section. The DUT and mounting
board/socket should not be in the test section when the
flow profile is determined.
6.2 Direct Measurement of Reference Point
Temperature, T
S
— For the purpose of measuring the
still-air junction-to-ambient microelectronic device
thermal resistance in a chamber, the ambient
temperature (T
R
= T
A
) should be measured with a
SEMI G38-0996 © SEMI 1987, 2004 3
thermocouple beneath the DUT. For purposes of
measuring forced air junction-to-ambient thermal
resistance in a wind tunnel, the ambient temperature
should be measured with a thermocouple upstream of
the DUT in the section of the tunnel that experiences
fully developed flow.
6.3 Thermal Resistance, Junction to Specified
Reference Point, R
θJR
.
6.3.1 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 ambient 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.
6.3.2 Indirect Measurement of Junction Temperature
for the Determination of R
θJA
— 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.
6.3.3 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.
6.3.4 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.
6.3.4.1 Steady-State Technique for Measuring 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 parameters 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 of
T
MC
.
6.3.4.2 The measurement of T
J
, using junction forward
voltage as the TSP, is made in the following manner:
6.3.4.2.1 Step 1 — Measurement of the temperature
coefficient of the TSP (calibration).
6.3.4.2.2 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, I
M
, by externally
heating the device under test in a controlled temperature
oven or 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
blotted. 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.
6.3.4.3 Step 2 — Power application test
6.3.4.3.1 The power application test is performed in
two parts. For both portions of the test, the reference
point temperature and the specified air velocity are held
constant at a preset value. The first measurement to be
made is that of the temperature-sensitive parameter
(i.e., V
MC
, under operating conditions with the
SEMI G38-0996 © SEMI 1987, 2004 4
measuring current, I
M
, used during the calibration
procedure). The DUT shall then be operated with
heating power (P
H
) applied. The temperature-sensitive
parameter, V
MH
, shall be measured with constant
measuring current, I
M
, that was applied during the
calibration procedure (see Step 1).
6.3.4.3.2 The heating power, P
H
, shall be chosen such
that the calculated junction-to-reference point
temperature difference as measured at V
MH
is greater
than or equal to 20°C. In accomplishing this, the device
under test should not be operated at such a high heating
power level that the on-chip temperature sensing and
heating circuitry is no longer electrically isolated. Care
should also be taken not to exceed the design ratings of
the package-interconnect system, as this may lead to an
overestimation of the power being dissipated in the
active area of the chip due to excessive power losses in
the package leads and wire bonds. The values of V
MH
,
V
MC
, and P
H
are recorded during the power application
test.
6.3.4.3.3 The following data shall be recorded for these
test conditions:
a. Temperature-sensitive electrical parameters (V
F
,
V
EB
, or other appropriate TSP).
b. Junction temperature, T
J
, is calculated from the
equation:
T
J
= T
R
+ V
MH
− V
MC
()
∆V
MC
∆T
MC
−
1
where T
R
= T
A
c. Ambient (air) temperature, T
A
.
d. Ambient (air) velocity, v
A
, (v
A
= 0 for still-air
enclosure).
e. Power dissipation, P
H
.
f. Mounting arrangement (including offset from test
board).
6.4 Calculations of R
θJR
6.4.1 Calculation of Package Thermal Resistance —
The thermal resistance of a microelectronic device can
be calculated when the junction temperature, T
J
, has
been measured in accordance with procedures outlined
in Sections 6.1 through 6.3. With the data recorded
from each test, the thermal resistance shall be
determined from:
R
θ
JR
=
T
J
−
T
R
P
H(Package)
where : R
θ
JR
=
R
θ
JA
and T
R
=
T
A
.