Freescale Semiconductor
Application Note
AN1955
Rev. 1, 4/2014
Thermal Measurement Methodology
of RF Power Amplifiers
By: Mali Mahalingam and Edward Mares
INTRODUCTION
This document explains the methodology used by
Freescale for thermal measurement of high power RF (Radio
Frequency) power amplifiers (RFPA). Semiconductor device
reliability heavily depends on device operating temperature so
the accurate thermal characterization of these high power
devices is crucial in establishing the reliability of the systems
that use such devices.
procedure used by the IR microscope. The emissivity
correction procedure recommended by IR microscope
manufacturers is ineffective at compensating for the
translucent nature of silicon [1]. With the IR microscopy
procedure, the maximum die surface temperature (“hot spot”)
in the measurement field can be located. The hot spot
temperature is selected as the die temperature (T
J
) for thermal
resistance
JC
). The thermal resistance calculations are
described later.
DIE SURFACE TEMPERATURE (T
J
)
MEASUREMENT
Infrared (IR) microscopy is used to determine the die
surface temperature (T
J
) during amplifier operation. Because
this IR measurement method requires a direct view of the die,
the protective ceramic lid is removed and replaced with a
modified lid that has an opening to view the die. In the case of
overmolded plastic packages, the center portion of the mold
compound is etched away until the die is sufficiently exposed.
Because the heat flow from the device to the heatsink is
dominated by conduction, the measurement error caused by
the removal of the lid or removal of the mold compound around
the die surface is negligible. The exposed die is coated with
a high emissivity coating (see Appendix) to obtain a fixed
emissivity value for IR thermal measurement. This coating
greatly improves the accuracy of the IR measurement
because it eliminates the need for any emissivity correction
Die Temperature (T
J
) Measured
with IR Microscope
CASE TEMPERATURE (T
C
) MEASUREMENT
The case temperature (T
C
) of the package is measured by
a 0.020 diameter stainless steel sheath thermocouple (Type
J; Omega part # JMQSS--020G--12) that is mounted within the
heatsink of the RF circuit. It is mounted from the bottom and
protrudes through the mounting interface to contact with the
bottom surface of the package (Figure 1). A 0.032 diameter
hole is drilled through the circuit heatsink to permit
thermocouple passage. This small hole provides minimal
disturbance to the heat flow path and interface integrity. The
thermocouple model is selected based on its sensitivity
combined with excellent durability. A spring mechanism is
added to the thermocouple to guarantee constant mechanical
contact with the bottom side of the flange. The placement for
this thermocouple is centered relative to the centermost active
transistor in the package (Figure 2).
4 Active Die
Die
Package
Heatsink
Temperature of Case (T
C
) Measured with Spring--
Loaded Thermocouple Making Direct Contact
Thermocouple Centered Relative to
Centermost Active Die in Package
Figure 1. Case Temperature Measurement
Figure 2. Positioning of Thermocouple
Freescale Semiconductor, Inc., 2004, 2006, 2014. All rights reserved.
AN1955
1
RF Application Information
Freescale Semiconductor
#4--40 Stainless Cap Screws
Die Temperature (T
J
) Measured
with IR Microscope
Die
Package
Heatsink
Heatsink Temperature (T
H
) Measured with Thermocouple
Buried in Heatsink 0.010 Below Mounting Interface
DuPont Delrin Clamp
Package
Figure 3. Heatsink Temperature Measurement
Figure 4. Exploded View of Clamping Scheme
for Metal Ceramic Devices
HEATSINK TEMPERATURE (T
H
) MEASUREMENT
The heatsink temperature (T
H
) directly beneath the
mounting interface of the RFPA to the circuit heatsink
(Figure 3) must be measured in certain situations. In these
cases, the 0.032 diameter hole drilled for thermocouple
passage stops 0.010 from the circuit heatsink surface. This
method of heatsink temperature measurement is particularly
useful in the following cases:
If the RFPA is soldered into place where the
spring--loaded TC cannot be used.
For thermal resistance measurements that include
various interface materials, such as thermal greases
or thermal pads, to determine their performance in the
thermal resistance stack--up.
during power testing. When the device is secured into the test
circuit, the IR scan is initiated and the desired RF signal and
power are applied. Once the desired case temperature is
reached for the part and is stable, the IR scan image is
captured along with all corresponding electrical data. This
data is recorded, and the corresponding thermal resistance
value is calculated.
THERMAL RESISTANCE,
JC
, CALCULATION
The method for determining junction--to--case thermal
resistance
JC
) under a chosen RF test condition is described
for both multi--die RFPA transistor products and multi--stage
RFIC products. For a multi--die RFPA transistor product for a
specified RF test condition, a single value is reported for the
junction--to--case thermal resistance. For a multi--stage RFIC
product, the junction--to--case thermal resistance (
JC--stage
)
is reported for each stage.
For a multi--die RFPA transistor product, the highest die
surface temperature (“hot spot”) measured by the IR scan is
used as T
J
in the thermal resistance calculation. Total power
dissipated in the product is calculated as
P
diss
= (RF input power + DC power (I
D
* V
D
))
– (RF output power + RF reflected power)
Junction--to--case thermal resistance is calculated as
JC
= (T
J
-- T
C
) / P
diss
For a multi--stage RFIC product, the highest die surface
temperature for each stage is measured by the IR scan and
used in the thermal resistance (
JC--stage
) calculation for that
stage. Power dissipated in each stage is determined and used
in the thermal resistance calculation for that stage.
THERMAL MEASUREMENT SEQUENCE
Before inserting each boltdown metal--ceramic part into the
RF test fixture, a layer of thermal grease (Dow Corningr
340--heatsink compound) is applied to the bottom of the flange
by a roller. A DuPonttDelrinr material clamp is used to apply
downward force to the ears and leads of the package
(Figure 4). This clamp fastens the device to the heatsink using
two #4--40 stainless steel cap screws, each tightened to
5 lb.--in. of torque.
For nonboltdown parts, a solder that is liquid at room
temperature is used instead of thermal grease as the interface
material.
The stage on which the RF circuit is secured has the ability
to be electrically heated and cooled by liquid. The temperature
of this stage is adjusted so that the desired case temperature
(usually between 70_C and 90_C) for the part is achieved
AN1955
2
RF Application Information
Freescale Semiconductor
DATA SHEET
JC
VALUE
Thermal resistance data reported on a Freescale RFPA
technical data sheet is based on performance tests done on
a sample size of ten parts, taken from different manufacturing
lots. Each part is powered to the desired RF condition and
measured. The mean thermal resistance value of that group
is then used for the data sheet.
Using infrared microscopy to accurately determine the
die temperature (T
J
) at high frequency (~2 GHz)
under RF test conditions
Using thermocouple measurements to accurately
determine the case temperature (T
CASE
)
Using the maximum die temperature of the device to
calculate the junction--to--case thermal resistance
(
JC
Establishing confidence level in the measured
JC
CONFIDENCE IN
JC
DATA
A Gauge R&R (Reproducibility and Repeatability)
assessment was used to demonstrate the methodology
employed in measuring and reporting accurate thermal
characterizations of Freescale high power RF power
amplifiers. This assessment showed that the measured
standard deviation (part--to--part variation plus measurement
variation) expressed as a percentage of the measured mean
is around 5%.
value
Implementing this methodology to determine the
JC
data for the Freescale RFPA technical data sheet.
This thermal measurement methodology is applied to both
multi--die RF power transistor products and multi--stage
power RFIC products.
SUMMARY
Thermal measurement methodology has been developed
and implemented to accurately characterize high power RF
power amplifiers. Integral to this measurement methodology
are:
REFERENCES
1. M. Mahalingam and E. Mares, “Infrared Temperature
Characterization of High Power RF Devices,”
Proceedings of IEEE MTT--S International Microwave
Symposium, May 2001.
AN1955
RF Application Information
Freescale Semiconductor
3
Appendix: Coating Methodology for Infrared Thermal
Measurement of RF Power Amplifiers
This appendix describes the coating recipe used to fix the
emissivity value of target objects in the infrared (IR) thermal
measurement methodology of metal ceramic and overmolded
plastic RF power amplifiers. We have assessed that an IR
microscope’s emissivity correction procedure does not work
well when applied to uncoated IR translucent targets, such as
an Si device [1]. In some cases, the nonactive regions of the
die layout show up as higher temperature regions than the
active regions. This issue is resolved once the device is
coated with a high emissivity coating. Another issue is that the
temperature is measured lower for noncoated devices in
comparison to the same devices when coated and measured
under identical operating conditions.
SELECTION OF COATING APPLIED TO DEVICE
Based on an internal study comparing six different
coatings, the one with the least impact on RF performance
(gain, efficiency and intermodulation distortion at both 1 GHz
and 2 GHz) was chosen. This coating is applied with an
airbrush.
APPLICATION OF COATING TO DEVICE USING
AN AIRBRUSH
The use of an airbrush to apply the coating permits accurate
and even coverage of paint onto the device. An air pressure
of about 20--25 PSI is supplied to the airbrush for spraying.
The airbrush is held about 1/2 away from the units during
application. A few passes of paint application are made to all
of the units. To expedite the drying process between
application coats, the paint supply is shut off to allow only air
to pass through. The units are then sprayed with air only until
the paint is dry. The coating process is repeated until the active
die has been adequately covered with paint. An emissivity
measurement was run using this coating process and was
determined to be 0.98. Therefore, a constant emissivity value
of 0.98 is input into the IR microscope when performing
thermal measurements with coated devices.
REMOVAL AND REPLACEMENT OF PROTECTIVE
LID FOR METAL CERAMIC PRODUCT
MEASUREMENT
To allow viewing of the die for IR thermal measurement, the
protective lid of the metal ceramic package must be removed.
To do this, a metal ceramic package is placed on a hot plate
at a temperature of ~ 280_C for about 45 seconds. It remains
on the hot plate until the epoxy seal of the protective ceramic
lid has sufficiently melted to allow removal. The unit is relidded
with a modified lid that has an opening. The part is then ready
for application of paint.
REFERENCES
1. Mahalingam and E. Mares, “Infrared Temperature
Characterization of High Power RF Devices,”
Proceedings of IEEE MTT--S International Microwave
Symposium, May 2001.
REMOVAL OF MOLD COMPOUND FOR PLASTIC
PRODUCT EVALUATION
To permit thermal evaluation of an overmolded plastic
package, the mold compound is etched away from the middle
portion of the package without damaging the die and
interconnects.
AN1955
4
RF Application Information
Freescale Semiconductor
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E
2004, 2006, 2014. Freescale Semiconductor, Inc.
AN1955
RF Application Information
AN1955
Rev. 1, 4/2014
Freescale Semiconductor
5
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