Texas Instruments | How to Evaluate Junction Temperature Properly with Thermal Metrics (Rev. B) | Application notes | Texas Instruments How to Evaluate Junction Temperature Properly with Thermal Metrics (Rev. B) Application notes

Texas Instruments How to Evaluate Junction Temperature Properly with Thermal Metrics (Rev. B) Application notes
Application Report
SLUA844B – December 2017 – Revised March 2019
How to Properly Evaluate Junction Temperature with
Thermal Metrics
Andy Chen
ABSTRACT
With the technology development of the semiconductor process, the chip integration level continuously
increases while the package size gets smaller, so the semiconductor devices face higher thermal stress
challenges. The high junction temperature not only derate the device electrical characteristics, but
increases the metal migration and other degeneration changes which cause accelerated aging and higher
failure rate. According to the electronic design rules, every 10°C rise in temperature reduces the average
life by 50%, so it is important to properly evaluate the thermal stress or junction temperature of the
semiconductor devices.
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Contents
Common Thermal Metrics for Junction Temperature Estimation ...................................................... 2
Why Thermal Resistance Parameters Are Often Misused ............................................................. 3
Using The ψ Thermal Characterization Parameter to Estimate Junction Temperature ............................ 7
About Temperature Measurement ......................................................................................... 9
Conclusion .................................................................................................................. 10
References .................................................................................................................. 10
List of Figures
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Older Device in Metal Can Package ...................................................................................... 3
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TO92 Package ............................................................................................................... 3
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SO8 Package
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8
................................................................................................................
RθJA Vs PCB Size ............................................................................................................
RθJC Measurement ...........................................................................................................
RθJB Measurement ...........................................................................................................
Complex Model ..............................................................................................................
Ψ Versus PCB Size ..........................................................................................................
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List of Tables
....................................................................................................
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Typical Thermal Metrics
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Quick Summary............................................................................................................. 10
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Trademarks
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Common Thermal Metrics for Junction Temperature Estimation
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Common Thermal Metrics for Junction Temperature Estimation
Some semiconductor devices are integrated with a dedicated thermal diode precisely measure the
junction temperature according to the calibrated forward voltage versus the temperature curve. Since most
devices do not have this design, the estimation of the junction temperature depends on the external
reference point temperature and the thermal metric of the package. The commonly used package thermal
metrics are the thermal resistance and thermal characterization parameter. Table 1 shows the typical
thermal metrics of LMR14030, which is a 40-V, 3.5-A step-down converter in the SO8 package with
thermal pad.
Table 1. Typical Thermal Metrics
THERMAL METRIC
RθJA
RθJC
(top)
Junction-to-case (top) thermal resistance
LMR14030SDDA
UNIT
42.5
°C/W
56.1
°C/W
25.5
°C/W
RθJB
Junction-to-board thermal resistance
ΨJT
Junction-to-top characterization parameter
9.9
°C/W
ΨJB
Junction-to-board characterization parameter
25.4
°C/W
•
•
2
Junction-to-ambient thermal resistance
Thermal resistance parameters, such as RθJA and RθJC, are the most common thermal metrics, almost
all of the semiconductor device specifications provide such information, although it is the most common
and misused metrics to the engineers.
Thermal characterization parameter, such as ΨJT and ΨJB, are the thermal metrics defined by the Solid
State Technology Association (JEDEC) in the 1990s. These metrics are more convenient to estimate
the junction temperature of devices in modern package type. More semiconductor manufacturers are
providing these thermal metrics.
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Why Thermal Resistance Parameters Are Often Misused
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Why Thermal Resistance Parameters Are Often Misused
The early generation semiconductor devices are normally packaged in a metal can. Figure 1 shows an
example a device in a metal can.
Figure 1. Older Device in Metal Can Package
The device with lead stands on the PCB after assembly, so the device heat is almost completely
distributed through the metal case to the air environment. The heat conduction has a single path and less
relationship with the PCB, so the thermal resistance parameters RθJA and RθJC are defined on this kind of
application conditions. Nowadays, many new packaging types, especially in the SMD packages, have
diverse heat conduction paths and more relationship with the PCB design, leading to erroneous results if
you continue to simply estimate the junction temperature with the resistance parameter.
RθJA is the thermal resistance of the junction to the environment in still air conditions and is the most
common thermal parameter for the semiconductor package. In most cases, the total heat of the device is
eventually distributed into the air, so the air temperature is easy to measure or pre-determined and the
junction temperature can be easily estimated with RθJA as long as the power consumption is known.
Figure 2 shows a normal transistor in a TO-92 package that is assembled on a PCB.
Figure 3. SO8 Package
Figure 2. TO92 Package
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Why Thermal Resistance Parameters Are Often Misused
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The heat is mainly conducted through the package body and has less relationship with the PCB as it
stands off the PCB. Using its typical RθJA, you can get accurate estimation results of junction temperature
based on the ambient temperature (or case temperature) and device power consumption. Figure 3 shows
the results from the RθJA versus the PCB size. Since the bottom exposed pad of the package is soldered to
the PCB, most of the heat is conducted through the PCB. The device and the PCB form a thermal
subsystem, then RθJA is a system-level thermal resistance parameter. Any PCB design differences, such
as board type, size, layer, copper foil thickness, number of vias, and so forth have a significant effect on
the final thermal performance.
Figure 4. RθJA Vs PCB Size
Figure 4 shows the relationship between the RθJA and the PCB size of TI package type TO-263 (KTW) and
VQFN-20 (RGW). Figure 4 clearly shows how the board size affects the RθJA.
The RθJA provided on data sheet specification is usually measured or simulated on a standard PCB
defined by JEDEC or device vendor. If your PCB design and application environment are similar to the
conditions defined by JEDEC or device vendor, then you can use the data sheet RθJA parameter, or use
the actual RθJA parameter. These are measured in a previous similar product system to an estimated
junction temperature. Otherwise, the difference in the PCB design can result in a large error on junction
temperature estimation and a higher estimated junction temperature results in a design constraint. A lower
estimated value directly leads to poor reliability.
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According to the recommendations of JEDEC, the RθJA parameter is not suitable for estimating junction
temperature in actual applications. It is more suitable for comparing the thermal performance of different
devices in the same package type, and it must be noted that the comparison is based on a similar PCB
design. Never just compare the value.
RθJC is defined as the thermal resistance between the junction to the case surface (top or bottom), as
defined by JEDEC.
Figure 5. RθJC Measurement
The testing method forces almost all of the dissipated heat through the single surface of the device (case
top or bottom of package), so RθJC applies to the condition that the chip dissipated power conducts through
the single surface of the device package (case top or bottom). This means that the RθJC parameter is
generally suitable for the condition that the only heatsinking is attached at the top (or bottom) of the
package where more than 90% of the heat is distributed from the top (or bottom), which is very similar to
JEDEC test conditions. For a package with bottom thermal pad, the bottom metal pad is allowed to be
soldered onto the PCB, but for the RθJC-top parameter, it must be ensured that the top is the main path of
heat sinking.
For a SMD device in typical plastic package without top heatsinking, it is incorrect to estimate the junction
temperature with RθJC by simply measuring the case top temperature and calculated device power
dissipation. This can result in a much higher result than the actual junction temperature.
The difficulty of using the RθJC parameter is how to accurately measure the surface temperature of the
package attached with the heatsink. The general method is to drill a ≤1 mm diameter through hole in the
central portion of the heatsink where it contacts with the chip package. Insert a thin thermocouple wire and
well contact with the package surface to measure the case temperature.
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RθJB parameter is defined later than the above two parameters. RθJB is defined as the junction to the PCB
(NOT package bottom) thermal resistance, which means that the PCB must be the main path of the
device heat dissipation. Figure 6 shows the JEDEC test method. It uses the circular cooling plate to clamp
PCB top and bottom side. It also uses a thermal insulation material to cover the chip top and bottom
position to ensure that almost all of the heat flow to the PCB.
Figure 6. RθJB Measurement
The RθJB parameter is more suitable for devices in SMD package types to estimate the junction
temperature, especially the package bottom that has an exposed thermal pad. Pay attention to the PCB
design in an actual application since JEDEC tests RθJB with high-thermal conductivity PCB (High-K PCB)
with Material FR4, 4 layers, and 1.6 mm thickness. The top layer of the bottom layer of copper foil is 2 oz.
The middle two layers of is 1 oz. The board size is normally 11.4-cm x 7.6-cm or 11.4-cm x 10.2-cm. If the
PCB design in the actual application is similar or more optimized, then use this parameter to estimate the
junction temperature. Sometimes it is not convenient to measure the PCB temperature since the
thermalcouple needs to be placed close to the edge of the package in 1 mm space, so the JEDEC
defined Ψ parameters to be more easy to use.
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Using The ψ Thermal Characterization Parameter to Estimate Junction Temperature
Using The ψ Thermal Characterization Parameter to Estimate Junction Temperature
According to the definition of thermal resistance, it is the ratio of the temperature difference and the actual
conducted thermal power between the two points. Figure 7 shows that the actual application system has
diverse cooling path.
Figure 7. Complex Model
The model is equivalent to a complex series/parallel circuit network since it is difficult to simply calculate or
determine the actual thermal power conducted in one certain path. JEDEC tests the thermal resistance in
a way that always forces almost all of the device heat flow to the reference point. The simple reason is
that it uses the total power consumption of the chip to calculate thermal resistance. The complexity of the
thermal conduction path in an actual system determines that junction temperature cannot be simply
estimated by the total dissipation power and thermal resistance parameters. For this purpose, JEDEC
defines the thermal characterization parameter, Ψ, which only represents the ratio of the temperature
difference (between the junction and reference point ) and the total dissipated power of the chip. It is only
a coefficient, although the calculation formula and the units (°C/W) is very similar to Rθ, but Ψ is not the
true thermal resistance parameters.
Now there are two common thermal characterizations parameters: ΨJT for junction to the package top and
ΨJB for junction to PCB. ΨJT is more often used because:
• It is more convenient to measure case top temperature.
• The SMD chip spreads heat mainly through PCB and only a small amount of the heat flows to the top.
The temperature difference between the junction to the top is usually smaller than junction to the PCB.
In other words, ΨJT is smaller than ΨJB, so using the ΨJT parameter has relatively smaller error. Table 1
shows the quick summary data.
If you compare thermal resistances, one of the key advantages of Ψ parameter is the smaller dependency
to PCB size. Figure 8 shows this relationship. The Ψ parameter overcomes the θ parameter dependency
to PCB, so it is more convenient in the thermal design and has more accurate estimation results.
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Figure 8. Ψ Versus PCB Size
Note that if the chip is attached to heat-sinking, then RθJC or ΨJS (junction to the heat sink) must be used to
estimate the junction temperature.
8
How to Properly Evaluate Junction Temperature with Thermal Metrics
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About Temperature Measurement
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About Temperature Measurement
Thermocouple and infrared thermal imager are commonly used for temperature measurement. When a
thermocouple measures the temperature, a fine gauge wire (36 to 40 gauge, J or K type wire) must be
used to minimize the local cooling from the thermocouple. It must be attached to the center of the package
surface (± 1 mm) with a bead of thermally conductive epoxy no larger than 2-mm x 2-mm on a side.
Taping the thermocouple to the package surface is not recommended. To minimize the heat sinking
impact of the thermocouple, the wire must be dressed along the diagonal of the package, down to the
PCB surface, and over a minimum distance of 25 mm before lifting from the PCB. The thermocouple wire
can be tacked to the PCB for this routing purpose with tape. Use of improper thermocouple wire gauge
can create errors in the measurements of 5-50%.
Temperature measurement with an infrared thermal imager is simple, convenient, fast, and accurate.
Depending on the ambient temperature or test environment changes, the machine requires a sufficient
preheat stability time before the test. Be sure to correct the reading for the emissivity of the surface being
investigated. Choose a high emissivity surface (non-metal, rough, low reflective) on the measured object
as a test point. For low emissivity surfaces, apply black insulating tape, spray paint, or apply it with a black
water-based stylus pen. Test it vertically at the top of the test point where the test area is well-focused and
filled with full display windows.
Regarding air ambient temperature measurement, the PCB must be placed horizontally. Measure the air
temp at point 2.54 cm underneath the PCB center and 2.54 cm to the horizontal side. Take the average
temperature of two points.
Regarding the PCB temperature testing point, choose the middle position of package side closest to the
inside chip. Measure the board temp within 1 mm to the edge of the package, or at the test point
recommended by the device vendor. It is best to choose a point where the copper foil is routed and
connected to the package. Scratch the solder mask before measurement. For practical operation with a
leaded package device, select the center pin on the package long side, measuring temperature at the lead
end that is soldered to PCB. If the pin solder pad space to the package edge is larger than 1.5 mm,
directly measure the temperature on the board as described above.
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Conclusion
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Conclusion
The theory of thermal is correct. The thermal resistance parameter can be used to estimate the
temperature difference if the heat conduction path and the thermal power conducted between the two
points of the system is clear. In actual applications, the thermal conduction path is diverse, and the heat
spreads through multiple channels. Unlike the total power consumption, it is difficult to estimate the power
consumption of a particular path. For these reasons, the thermal characterization parameter Ψ is more
suitable for estimating the junction temperature. Table 2 shows a summary.
Table 2. Quick Summary
THERMAL
METRIC
TYPE
USAGE FORMULA
RθJA
Thermal Resistance
TJ = TA + RθJA × PT
RθJC
Thermal Resistance
TJ = TC + RθJC × PD
REMARK
1.
2.
PT is the total chip dissipated power
Used to rank package thermal performance
1.
PD is the thermal power conducted from junction to
case (top or bottom) surface
Used to rank package thermal performance
2.
1.
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TJ = TB + RθJB × PD
PD is the thermal power conducted from junction to
board.
Used to rank package thermal performance and
estimate TJ of devices on application PCB
RθJB
Thermal Resistance
ΨJT
Thermal Characterization
Parameter
TJ = TC-top + ΨJT × PT
1.
2.
PT is the total chip dissipated power
Used to estimate TJ of devices on application PCB
ΨJB
Thermal Characterization
Parameter
TJ = TB + ΨJB × PT
1.
2.
PT is the total chip dissipated power
Used to estimate TJ of devices on application PCB
2.
References
1.
2.
3.
4.
5.
Darvin Edwards, Semiconductor and IC Package Thermal Metrics
Masashi Nogawa, Using New Thermal Metrics.
JESD51-1, Integrated Circuits Thermal Measurement Method – Electrical Test Method
JESD51-2, Integrated Circuits Thermal Test Method Environmental Conditions – Natural Convection
JESD51-5, Extension of Thermal Test Board Standards for Packages with Direct Thermal Attachment
Mechanisms
6. JESD51-12, Guidelines for Reporting and Using Electronic Package Thermal Information
10
How to Properly Evaluate Junction Temperature with Thermal Metrics
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Revision History
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Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from A Revision (March 2018) to B Revision .................................................................................................. Page
•
Edited application report for clarity. ..................................................................................................... 1
Changes from Original (December 2017) to A Revision ................................................................................................ Page
•
Changed Typical Thermal Metrics table. ............................................................................................... 2
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