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Study on the methods to measure the junction-to-case thermal resistance of IGBT modules and press pack IGBTs
MR-12407; No of Pages 9 Microelectronics Reliability xxx (2017) xxx–xxx
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Study on the methods to measure the junction-to-case thermal resistance of IGBT modules and press pack IGBTs Erping Deng a,b,⁎, Zhibin Zhao a, Peng Zhang b, Jinyuan Li b, Yongzhang Huang a a b
State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Changping District, Beijing 102206, China Global Energy Interconnection Research Institute, Changping District, Beijing 102211, China
a r t i c l e
i n f o
Article history: Received 15 February 2017 Received in revised form 24 April 2017 Accepted 24 May 2017 Available online xxxx Keywords: Press pack IGBTs IGBT modules Thermocouple method Transient dual interface method Thermal resistance measurement
a b s t r a c t The accurate measurement of the junction-to-case thermal resistance of Insulated Gate Bipolar Transistor (IGBT) devices is notably important for manufacturers to optimize the internal structure of packaging in order to improve its reliability and for users to take full advantage of the devices. The existing differences between IGBT modules and press pack IGBTs (PP IGBTs) not only in their packaging styles, but also in their working conditions may lead to some differences in the methods to measure their junction-to-case thermal resistance. In this paper, the junction-to-case thermal resistances of both IGBT modules and PP IGBTs have been measured using the traditional thermocouple method (steady-state method) and transient dual interface method (transient method). The applicability of these two methods for the measurement of junction-to-case thermal resistance of IGBT modules and PP IGBTs is summarized based on the experimental results. The steady-state method is suitable for the measurement of junction-to-case thermal resistance of IGBT modules, but not for PP IGBTs, because of the thermocouple inserted to measure the case temperature. The transient method is appropriate for the measurement of junction-to-case thermal resistance of not only IGBT modules, but also PP IGBTs, as a thermocouple is not required to measure the case temperature. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction It is well known that the junction-to-case thermal resistance is the most important thermal parameter of a power semiconductor device and also the criterion to evaluate its heat dissipation ability [1]. Thus, accurate measurement is notably important for manufacturers to optimize the internal structure in order to improve its reliability and for users to take full advantage of the devices. Currently, the most commonly used method to measure the thermal resistance is the traditional thermocouple method, which was proposed by IEC, MIL standard [2], and JEDEC51-1 standard [3], and it is referred to as the steady-state method in this paper. The junction-to-case thermal resistance can be calculated using Eq. (1) the junction temperature Tj, case temperature Tc, and power dissipation P of the device under test (DUT) are known [4]. Rjc ¼
T j −T c P
ð1Þ
⁎ Corresponding author at: State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Changping District, Beijing 102206, China. E-mail address: [email protected] (E. Deng).
The junction temperature Tj can be indirectly measured using an electrical method. The relationship between the voltage drop caused by a notably small constant current and the junction temperature, also called the K factor, is the most commonly used method, [5]. Although this method is notably simple and easy to perform, its accuracy is significantly affected by the thermocouple position, clamping force for fixture, thermal grease, etc. Thus, this method is often not sufficiently reproducible and accurate [6]. In 2010, the JEDEC51-14 standard [7] specified a new method (the transient dual interface method, which is referred to as the transient method in this paper) to measure the junction-to-case thermal resistance of power semiconductor devices without measuring the case temperature. The transient method defined in this paper is different from the traditional transient method used to measure the transient thermal impedance, which is included in the datasheet. This method considerably improves the accuracy and reproducibility because a thermocouple is not required to measure the case temperature. The steady-state method is the most commonly used method to measure the junction-to-case thermal resistance of wire-bonded Insulated Gate Bipolar Transistor (IGBT) modules, and the transient method has been recently introduced by some manufacturers, such as Infineon [8]. The differences between the packaging styles (the internal structure is shown in Fig. 1) and working conditions of press pack IGBTs (PP IGBTs) and wire-bonded IGBT modules render the method suitable for
http://dx.doi.org/10.1016/j.microrel.2017.05.032 0026-2714/© 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: E. Deng, et al., Study on the methods to measure the junction-to-case thermal resistance of IGBT modules and press pack IGBTs, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.05.032
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Fig. 1. The internal structure of different IGBT packaging styles.
wire-bonded IGBT modules, but not for PP IGBTs. Furthermore, an external clamping pressure of approximately 1.2 kN/cm2 is required to maintain the normal operation of PP IGBTs [9], which is much higher than the recommended clamping pressure of 10 N/cm2 for the fixture during the thermal resistance measurement [7]. Therefore, the thermocouple located between the case and the heatsink will be damaged under such a high clamping force, and the heat path will also be influenced by the inserted thermocouple. Consequently, the accurate measurement of junction-to-case thermal resistance of PP IGBTs remains a significant challenge. Only the junction-to-heatsink thermal resistance (but not the junction-to-case thermal resistance) is provided in the datasheet provided by the manufacturers of PP IGBTs such as Westcode [10] and Toshiba [10]. In this paper, the junction-to-case thermal resistances of IGBT modules and PP IGBTs are measured using both the steady-state method and transient method. The applicability of these two methods to IGBT modules and PP IGBTs is compared based on the experimental results and packaging styles. This paper is structured as follows. The steady-state and transient method to measure the junction-to-case thermal resistance of power semiconductor devices are introduced in Section 1. The principle of the transient method and the determination of thermal
resistance are described in Section 2. A half-bridge IGBT module manufactured by Infineon is introduced to measure the thermal resistance using the two aforementioned methods and both the experimental results are compared with the values provided in the datasheet in Section 3. Furthermore, the junction-to-heatsink thermal resistance of PP IGBTs is measured using the steady-state method as the thermocouple cannot be placed in the interface between the case surface and heatsink and the value of junction-to-heatsink thermal resistance thus obtained is compared with the value provided in the datasheet. The junctionto-case thermal resistance of the PP IGBTs is measured using the transient method in Section 4. Section 5 concludes the paper and describes the applicability of the steady-state method and transient method to IGBT modules and PP IGBTs according to the experimental results. 2. Measurement principle Between the two aforementioned methods, the steady-state method is relatively simple and the most commonly used method to measure the junction-to-case thermal resistance of power semiconductor devices. Therefore, in this paper, only the principle of the transient method is presented.
Please cite this article as: E. Deng, et al., Study on the methods to measure the junction-to-case thermal resistance of IGBT modules and press pack IGBTs, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.05.032
E. Deng et al. / Microelectronics Reliability xxx (2017) xxx–xxx
The transient method requires two measured transient thermal impedance (Zth) curves of a power semiconductor device in contact with a temperature-controlled heatsink. The first test is performed with no thermal interface material between the DUT and the heatsink, i.e., direct contact or without grease. For the second measurement, a thin layer of thermal grease or oil is applied at the interface to reduce the thermal contact resistance, i.e., contact with grease. The thermal grease in the interface changes the thermal contact resistance and ensures a clear separation of the transient thermal impedance curves. The transient thermal impedance that corresponds to the splitting point is defined as the junction-to-case thermal resistance [7]. 2.1. Transient thermal impedance curve The principle of the transient thermal impedance curve measurement is shown in Fig. 2 [11]. One of two direct current sources provides the heating current and the other provides sensing current to the device under test (DUT). A heating current I drive flow into the DUT in order to heat it to thermal equilibrium; subsequently, the heating current Idrive is switched to a sensing current Isense in order to measure the voltage drop during the cooling phase. The junction temperature T j during the cooling phase is acquired through the recorded voltage drop and K factor, which is the relationship between the voltage drop and junction temperature [11]. Finally, the thermal impedance Zth , cooling (t) of the DUT during the cooling
3
phase can be obtained using Eq. (2). Furthermore, the junction behaviour during the heating phase is similar to that of the cooling phase; hence, the thermal impedance Zth , heating(t) during the heating phase can be calculated using Eq. (3) [12]. T j ðt Þ−T j ðt ¼ 0Þ P
ð2Þ
Z th;heating ðt Þ ¼ Z th;cooling ðt ¼ 0Þ−Z th;cooling ðt Þ
ð3Þ
Z th;cooling ðt Þ ¼
2.2. Determination of the junction-to-case thermal resistance The principle of the determination of the junction-to-case thermal resistance of power devices is shown in Fig. 3. Two Zth curves are required for the measurement under two different contact interface conditions with and without thermal grease which causes the two Zth curves to separate at some point of time ts in Fig. 3(b). Since the heat paths only differ at the case surface, these two Zth curves are always consistent from the junction-to-case surface. Therefore, the transient thermal impedance Zth(ts), which is specified at the splitting point, is assumed to be the junction-tocase thermal resistance Rjc of the DUT. Generally, it is very difficult to accurately determine the splitting point of the two Zth curves. Thus, a series of mathematical transformations is proposed to obtain the splitting point more
Fig. 2. Principle diagram of the transient thermal impedance measurement curve.
Please cite this article as: E. Deng, et al., Study on the methods to measure the junction-to-case thermal resistance of IGBT modules and press pack IGBTs, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.05.032
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Fig. 3. Principle of the transient dual interface method
precisely. A variable substitution is performed to the two Zth curves in order to simplify the process; subsequently, a differential is performed to these substitution curves. Furthermore, in order to eliminate the influence of the steady-state thermal resistance difference Δ θ, the difference between these differential curves is normalized as δ(Zθjc), and the mathematical processing is shown in Eqs. (4) and (5). aðzÞ¼ Z θjc ðt Þ;
for z ¼ ln ðt Þ
ð4Þ
ΔðdaðzÞ=dtÞ ðda1 ðzÞ=dtÞ−ðda2 ðzÞ=dtÞ ¼ δ Z θjc ¼ Δθ Δθ
ð5Þ
3. Experiments on IGBT modules In this section, the thermal resistance tester Phase 11 is used to measure the IGBT chip junction-to-case thermal resistance of the half-bridge module manufactured by Infineon (FF100R12RT4). Both
Please cite this article as: E. Deng, et al., Study on the methods to measure the junction-to-case thermal resistance of IGBT modules and press pack IGBTs, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.05.032
E. Deng et al. / Microelectronics Reliability xxx (2017) xxx–xxx
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Table 1 Junction-to-case thermal resistance of the IGBT module measured using the steady-state method under different conditions. No
Ic (A)
Tj (°C)
Tc (°C)
Rjc (K/W)
1# 2# 3# 4# 5# 6# 7# 8#
99.999 100.03 100.02 100.06 99.951 99.965 100.01 99.978
93.7 93.6 93.9 93.7 93.7 93.7 93.8 116.6
39.9 40.0 40.0 40.0 40.0 32.4 41.3 50.5
0.27270 0.27154 0.27128 0.27193 0.27099 0.30963 0.26415 0.32250
the steady-state method and transient method are applied to measure the thermal resistance and the experimental results are compared.
3.1. Steady-state method The IGBT chip junction-to-case thermal resistances of the half-bridge module shown in Table 1 are measured by steady-state method under different conditions. A thin layer of thermal grease is applied to the interface between the DUT and heatsink to reduce the thermal contact resistance. Herein, the input current for heating up the module under test is 100 A. A clamping pressure of about 10 N/cm2 is applied to 1#–6# for fixture. All the external test conditions of 1#–5# are the same and the measurement is repeated five times. Meanwhile, the thermocouple used for the measurement of case temperature is placed exactly under the measured IGBT chip as much as possible. The external test conditions of 6# are maintained to be consistent with 1#–5#, except for a slight change in the location of the thermocouple. Further, the clamping force for the half-bridge fixture of 7# between the module and heatsink is slightly higher than the normal clamping pressure of about 10 N/cm2 which is applied to 1#–6#. The clamping pressure of fixture for 8# is slightly lower than the normal clamping pressure. From the data, it can be observed that the repeatability of measurement is acceptable under the conditions that all the external test conditions are the same (1#–5#). However, the results obtained under different conditions are significantly different, especially when the clamping force required for fixture is changed, since the external conditions—such as the location of thermocouple, the clamping force for fixture etc.—have a significant influence on the accuracy of the thermocouple. In this paper, an average value of 0.272 K/W for 1#–5# is assumed to be the junction-to-case thermal resistance of the half-bridge.
Fig. 4. Transient thermal impedance curves of the IGBT module with/without thermal grease. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
3.3. Experimental results analysis From the Zth curve of FF100R12RT4 shown in Fig. 5 it can be observed that the junction-to-case thermal resistance provided in the datasheet is 0.27 K/W [13]. Table 2 shows the experimental results and comparison with the value provided in the datasheet. The result the steady-state method is consistent with the value in the datasheet, but the result of obtained using the transient method is slightly lower than the value in the datasheet. This can be attributed to the fact that the splitting point of the Zth curves will be advanced when the package contains a layer with larger thermal resistance, for example Al2O3 in DBC [7,14]. Thus, the experimental result obtained using the splitting point, which is also called the transient thermal resistance, is slightly smaller than the thermal resistance measured using the steady-state method. Furthermore, the transient thermal resistance is not necessarily consistent with the steady-state junction-to-case thermal resistance because the steady-state (which means a long time) heat flow distribution inside DUT differs from the transient heat flow distribution at time ts [7].
3.2. Transient method The transient thermal impedance curves of the IGBT module with/without thermal grease are measured via the thermal tester Phase 11 and the junction-to-case thermal resistance is determined by using the splitting point, as shown in Fig. 4. The red and black lines shown in Fig. 4 denote the experimental results measured with/without thermal grease, respectively. Thus, the thermal contact resistance between the contact interface of the module and heatsink is reduced by using thermal grease, and eventually leads to a separation of the Z th curves at the contact interface. All the external test conditions, especially the input current and reference temperature, must be the same during the test to ensure accuracy. The junction-to-case thermal resistance of 0.242 K/W can be obtained using the splitting point.
Fig. 5. Transient thermal impedance curve from FF100R12RT4 datasheet.
Please cite this article as: E. Deng, et al., Study on the methods to measure the junction-to-case thermal resistance of IGBT modules and press pack IGBTs, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.05.032
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Table 2 Comparison of the IGBT module experiment results obtained by the steady-state method and transient method with the value in the datasheet.
Thermal resistance (K/W) Deviation (%)
Datasheet
Steady-state
Transient
0.27 –
0.272 –0.7
0.242 10.4
4. Experiments on PP IGBTS 4.1. Test bench The fixture of the commercial thermal resistance tester is not suitable for PP IGBTs because of their special packaging style and working conditions. In this paper, we designed two fixtures for PP IGBTs and used the thermal resistance tester Phase 11 to measure the junctionto-case thermal resistance and the Zth curve of the PP IGBT. According to the principle of the steady-state method and transient method, the K factor [5], which accounts for the relationship between the junction temperature and voltage drop, should be measured to predict the junction temperature during the test. Subsequently, the transient thermal impedance is recorded. The fixtures for the measurement of K factor and transient thermal impedance are shown in Fig. 6. Four nuts are used to constrain the displacement to sustain the desired clamping force, which is provided by a standard pressure machine. A disc spring is needed to compensate the physical movements during the process of clamping and thermal expansion. The clamping force has negligible influence on the K factor provided the studied PP IGBT is in good contact because the measurement current is very small [15]. Fig. 7 shows the results of the 3300 V/360 A PP IGBT manufactured by Westcode under clamping forces of 5 kN and 8 kN with a small constant sense current of 20 mA. The results demonstrate that the fixture can satisfy the measurement requirements because the slopes (°C/V) are exactly the same, with only a difference of 0.02 (°C) in the intercept. The fixture to measure the transient thermal impedance consisted of heatsinks, a DUT, a disc spring, a clamping force sustaining plate, a hydraulic pump and a pressure sensor. Dual heatsinks were used to clamp and cool the DUT on both collector and emitter sides. The hydraulic pump provided a continuously adjustable clamping force ranging up to 50 kN, which satisfied the requirements of all the current ratings of PP IGBTs. A pressure sensor measured the clamping force applied to the DUT, which was displayed on the screen in real time. 4.2. Steady-state method The PP IGBT studied in this paper is 2500 V/360 A with the recommended clamping force of approximately 8–12 kN, which is manufactured by Westcode [10]. The thermal contact resistance between the multi-layers within PP IGBTs is sensitive to the clamping force and is reduced when the clamping force is increased [16]. A clamping force of 8 kN is applied in this paper to measure the junction-to-heatsink thermal resistance in order to compare the experimental results with the values provided in the datasheet. The other reason for the measurement of the junction-to-heatsink thermal resistance is that the thermocouple to measure the case temperature will be destroyed under the high clamping force of 8 kN. Meanwhile, the inserted thermocouple also changes the heat flow and affects thermal distribution eventually. In this paper, the thermocouple is placed at the outlet of the heatsink to record the heatsink temperature. Therefore, the measured junctionto-heatsink thermal resistance will be slightly higher because the heatsink temperature recorded by the thermocouple is smaller than the equivalent heatsink temperature. The values of junction-to-heatsink thermal resistance of PP IGBTs obtained from the experimental results and datasheet are shown in Table 3.
Fig. 6. Measurement fixtures fabricated in our lab for PP IGBTs.
From the data, it is evident that the change trend of the experimental results is consistent with the values in the datasheet, although the error of the value is slightly higher. This can be attributed to the fact that the test conditions are different from those employed by the manufacturer, especially the location of the thermocouple for the measurement of heatsink temperature. As the emitter side of PP IGBTs has a pedestal to assure insulation and packaging for gate pin, the heat flow path will
Please cite this article as: E. Deng, et al., Study on the methods to measure the junction-to-case thermal resistance of IGBT modules and press pack IGBTs, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.05.032
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Fig. 7. Comparison between voltage-temperature calibration curves (K factor) measured with clamping forces of 5 kN and 8 kN.
be much longer than that of the collector side; therefore, the thermal resistance obtained with emitter side cooling is much larger than the value obtained with collector side cooling. When the DUT is double side cooled, the thermal resistance is much smaller and this is one of the most important advantages of PP IGBTs. From the experimental results, it can be observed that when the steady-state method is applied to PP IGBTs, it is difficult to measure even the heatsink temperature accurately, let alone the case temperature. 4.3. Transient method Contrary to the steady-state method, a thermocouple is not required to record the case temperature in the transient method. However, the reference temperature must be the same during the measurement of the Zth curves with/without thermal grease. Thus, a thermocouple is positioned in a constant temperature chamber during the measurement. Transient thermal impedance curves with/without thermal grease under a clamping force of 8 kN are measured and shown in Fig. 8. From the data shown in Fig. 8, it is very difficult to identify the splitting point. Furthermore, the steady-state thermal resistance difference of these two transient thermal impedance curves is also smaller than 0.01 K/W. This phenomenon is primarily caused by the high clamping force and the thermal grease, which fills the interface. The small steady-state thermal resistance difference Δ θ shows that the thermal grease has a limited effect on the interface for reducing the thermal contact resistance. Furthermore, thermal contact resistance between multi-layers within PP IGBTs and the interface between the DUT and heatsink are sensitive to the clamping force [16]. Fig. 9 shows the junction-to-heatsink thermal resistance under different clamping forces with double side cooling. From the data, it can be observed that the clamping force has a significant influence on the thermal resistance, which is caused by the thermal contact resistance. Further, the thermal contact
7
Fig. 8. Transient thermal impedance curves of the PP IGBT with emitter side cooling under a clamping force of 8 kN.
resistance tends to steady with the increment of clamping force. Thus, the thermal contact resistance of the interface between the DUT and heatsink may change negligibly under such a high clamping force caused by the thermal grease. In other words, the variation of thermal contact resistance of the interface caused by thermal grease is too small to separate the thermal impedances measured under two different test conditions. Furthermore, the substrate is used for heat dissipation and electrical insulation between the semiconductor chips and the heatsink in typical wire-bonded IGBT modules or the packaging of TO-247. Thus, thermal grease (mostly electrically insulating) is the most commonly used interface material in contact interfaces to improve thermal conductivity. Nevertheless, the packaging of PP IGBTs is entirely different. Heatsinks are used to dissipate heat and conduct current in PP IGBTs as shown in Fig. 1(b). The electrically insulated thermal grease is no longer suitable for changing the interface conditions. In this study, a suitably thick metal object (such as a bus bar) is proposed to be inserted into the interface to increase the difference of the interface between the DUT and heatsink. Two transient thermal impedance curves under two different interface conditions between the DUT and heatsink with emitter side cooling are measured and shown in Fig. 10. Based on the principle mentioned above, two Zth curves shown in Fig. 10(a) can be modified into the differential curves shown in Fig. 10(b) through variable substitution and differential transformation.
Table 3 Comparison of the PP IGBT experiment results measured by the steady-state method under different cooling conditions with datasheet. Cooling conditions
Ic (A)
P (W)
Tj (°C)
Emitter side cooling Collector side cooling Double side cooling
140 234 320
265.2 534 821.2
74.2 70.8 93.4
Thermal resistance (K/W) Experimental
Datasheet
Error
0.19283 0.09115 0.07775
0.152 0.0843 0.0541
26.8% 8.1% 43.6%
Fig. 9. Junction-to-heatsink thermal resistance of the PP IGBT under various clamping forces with double side cooling.
Please cite this article as: E. Deng, et al., Study on the methods to measure the junction-to-case thermal resistance of IGBT modules and press pack IGBTs, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.05.032
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E. Deng et al. / Microelectronics Reliability xxx (2017) xxx–xxx Table 4 Application comparison of the steady-state and transient methods to IGBT modules and PP IGBTs.
IGBT modules PP IGBTs
Steady-state
Transient
Yes No
Yes Yes
4.4. Experimental results analysis According to the experimental results, the junction-to-heatsink thermal resistance measured by the steady-state method is similar to the value provided in the datasheet, especially the change trend. The difference between the experimental results and value in the datasheet is mainly caused by the heatsink cooling conditions and the location of the thermocouple. Although the steady-state method is relatively simple and easy to perform, only the junction-to-heatsink thermal resistance (but not the junction-to-case thermal resistance) of PP IGBTs can be measured. However, the junction-to-heatsink thermal resistance is not sufficiently accurate for users in applications because of the difference between the heatsink cooling conditions during the thermal resistance measurement and the application. The junction-to-case thermal resistance measured by the transient method shows the same change trend as the value provided in the datasheet. Although the transient method is relatively complex owing to the need to measure two Zth curves, this method can accurately measure the junction-to-case thermal resistance and provides guidance for the design and application. 5. Conclusions According to the experimental results above, it can be observed that the methods used for IGBT modules may not be suitable for PP IGBTs because of the differences in their packaging styles and working conditions. The applicability of the steady-state method and transient method to the measurement of junction-to-case thermal resistance of IGBT modules and PP IGBTs is summarized in Table 4.
Fig. 10. Determination of the junction-to-case thermal resistance of emitter side cooling under a clamping force of 8 kN.
Subsequently, the junction-to-case thermal resistance can be obtained using the criterion k = 0.0045 ∗ Zth + 0.003 [7] for high power semiconductors. From the data shown in Fig. 10(c), it can be observed that the junction-to-case thermal resistance with emitter side cooling is approximately 0.048 K/W. Similarly, the junction-tocase thermal resistance with collector side cooling is approximately 0.032 K/W, which is smaller than the value with emitter side cooling.
• Both the steady-state method and transient method are suitable for the measurement of junction-to-case thermal resistance of IGBT modules. However, the accuracy of the steady-state method is significantly influenced by the clamping force for fixture, thermocouple position, thermal grease etc. The transient method rectifies these disadvantages of the steady-state method without a thermocouple to record the case temperature and has a great reproducibility. • The steady-state method is not suitable for PP IGBTs to measure the junction-to-case thermal resistance because of the thermocouple inserted to record the case temperature. Such a high external clamping force during the normal working of the PP IGBTs destroys the thermocouple. Moreover, the inserted thermocouple will distort the heat flow path and affect the results eventually. And last but not least it is extremely difficult to identify the temperature as the case temperature and not the heatsink or some value in between. • The transient method is applicable for PP IGBTs to measure the junction-to-case thermal resistance without requiring a thermocouple to measure the case temperature. This is a great advantage for PP IGBTs because the thermocouple required to measure the case temperature cannot be placed in the interface between the case surface and heatsink. Acknowledgements The work presented in this paper has been supported by the National Natural Science Foundation of China (51477048), National Key R&D Program of China (2016YFB0901800), and Fundamental Research Funds for the Central Universities 2016XS04.
Please cite this article as: E. Deng, et al., Study on the methods to measure the junction-to-case thermal resistance of IGBT modules and press pack IGBTs, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.05.032
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References [1] F. Wakeman, D. Hemmings, W. Findlay, G. Lockwood, Pressure Contact IGBT, Testing for Reliability, Westcode Semiconductors Ltd., March 2012. [2] MIL-STD-883G, United States Department of Defense Test Method Standard: Microcircuits, Method 1012.1 Thermal Characteristics, 1980. [3] Electronic Industries Association, Integrated Circuit Thermal Measurement Method – Electrical Test Method, EIA/JEDEC Standard, JESD51-1, 1995 (www.jedec.org). [4] F.F. Oettinger, D.L. Blackburn, Semiconductor Measurement Technology: Thermal Resistance Measurements, 1, Semiconductor Electronics Div, 1990. [5] U. Scheuermann, R. Schmidt, Investigations on the vce(t) - method to determine the junction temperature by using the chip itself as sensor, PCIM Europe 2009, pp. 802–807. [6] D. Schweitzer, H. Pape, L. Chen, et al., Transient dual interface measurement — a new JEDEC standard for the measurement of the junction-to-case thermal resistance, Semiconductor Thermal Measurement and Management Symposium (SEMITHERM), 2011 27th Annual IEEE, IEEE 2011, pp. 222–229. [7] Electronic Industries Association, Transient Dual Interface Test Method for the Measurement of the Thermal Resistance Junction-to-case of Semiconductor Devices With Heat Flow Through a Single Path, EIA/JEDEC Standard, JESD51-14, 2010 (www.jedec.org). [8] H. Pape, D. Schweitzer, L. Chen, et al., Development of a standard for transient measurement of junction-to-case thermal resistance, Microelectron. Reliab. 52 (7) (2012) 1272–1278. [9] Recommendations Regarding Mechanical Clamping of Press Pack High Power Semiconductors Apr. 2004, ABB. [10] http://www.westcode.com/igbt1.html. [11] L. Yafei, K. Yasushi, H. Tomoyuki, Thermal transient test based thermal structure function analysis of IGBT package, ICEP2014 Proceeding 2014, pp. 596–599. [12] A. Hensler, D. Wingert, C. Herold, et al., Thermal impedance spectroscopy of power modules, Microelectron. Reliab. 51 (51) (2011) 1679–1683. [13] http://pdf1.alldatasheet.com/datasheetpdf/view/416895/INFINEON/FF100R12RT4. html. [14] D. Schweitzer, Transient dual interface measurement of the Rth-JC of power packages, Proc. 14th THERMINIC, Rome 2008, pp. 14–19. [15] T. Poller, J. Lutz, S. D'Arco, M. Hernes, Determination of the thermal and electrical contact resistance in press-pack IGBTs, European Conference on Power Electronics and Applications, Lille, France, vol. 75, 2013, pp. 1–9. [16] E. Deng, Z. Zhao, P. Zhang, et al., Optimization of the thermal contact resistance within press pack IGBTs, Microelectron. Reliab. 69 (2017) 17–28.
9 Erping Deng was born in Hunan province, China, in 1989. He received the bachelor degree in electrical engineering from Harbin Institute of Technology, Harbin, China, in 2013. Currently, he is a Ph.D. student of State Key Laboratory of Alternate Electrical, North China Electric Power University. His main research interest is the packaging and reliability of high voltage power electronics devices.
Zhibin Zhao was born in Hebei province, China, in 1977. He received the Ph.D. degree in electrical engineering from North China Electric Power University, Baoding, China, in 2005. Currently, he is a Professor of State Key Laboratory of Alternate Electrical, North China Electric Power University. His main research interest is computational electromagnetics and electromagnetic compatibility in power electronic.
Yongzhang Huang was born in Guangxi province, China, in 1962. He received the B.S. degree from the Department of Engineering Physics, Tsinghua University, Beijing, China, in 1984, and the Ph.D. degree in physics from the Chinese Academy of Sciences in 1991.He is currently a professor in the Department of Electrical Engineering at North China Electric Power University, Beijing, China. He is also a Chinese distinguished expert of “thousand talents program” and the deputy director of the State Key Laboratory of new energy power system.
Please cite this article as: E. Deng, et al., Study on the methods to measure the junction-to-case thermal resistance of IGBT modules and press pack IGBTs, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.05.032
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Study on the methods to measure the junction-to-case thermal resistance of IGBT modules and press pack IGBTs Erping Deng a,b,⁎, Zhibin Zhao a, Peng Zhang b, Jinyuan Li b, Yongzhang Huang a a b
State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Changping District, Beijing 102206, China Global Energy Interconnection Research Institute, Changping District, Beijing 102211, China
a r t i c l e
i n f o
Article history: Received 15 February 2017 Received in revised form 24 April 2017 Accepted 24 May 2017 Available online xxxx Keywords: Press pack IGBTs IGBT modules Thermocouple method Transient dual interface method Thermal resistance measurement
a b s t r a c t The accurate measurement of the junction-to-case thermal resistance of Insulated Gate Bipolar Transistor (IGBT) devices is notably important for manufacturers to optimize the internal structure of packaging in order to improve its reliability and for users to take full advantage of the devices. The existing differences between IGBT modules and press pack IGBTs (PP IGBTs) not only in their packaging styles, but also in their working conditions may lead to some differences in the methods to measure their junction-to-case thermal resistance. In this paper, the junction-to-case thermal resistances of both IGBT modules and PP IGBTs have been measured using the traditional thermocouple method (steady-state method) and transient dual interface method (transient method). The applicability of these two methods for the measurement of junction-to-case thermal resistance of IGBT modules and PP IGBTs is summarized based on the experimental results. The steady-state method is suitable for the measurement of junction-to-case thermal resistance of IGBT modules, but not for PP IGBTs, because of the thermocouple inserted to measure the case temperature. The transient method is appropriate for the measurement of junction-to-case thermal resistance of not only IGBT modules, but also PP IGBTs, as a thermocouple is not required to measure the case temperature. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction It is well known that the junction-to-case thermal resistance is the most important thermal parameter of a power semiconductor device and also the criterion to evaluate its heat dissipation ability [1]. Thus, accurate measurement is notably important for manufacturers to optimize the internal structure in order to improve its reliability and for users to take full advantage of the devices. Currently, the most commonly used method to measure the thermal resistance is the traditional thermocouple method, which was proposed by IEC, MIL standard [2], and JEDEC51-1 standard [3], and it is referred to as the steady-state method in this paper. The junction-to-case thermal resistance can be calculated using Eq. (1) the junction temperature Tj, case temperature Tc, and power dissipation P of the device under test (DUT) are known [4]. Rjc ¼
T j −T c P
ð1Þ
⁎ Corresponding author at: State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Changping District, Beijing 102206, China. E-mail address: [email protected] (E. Deng).
The junction temperature Tj can be indirectly measured using an electrical method. The relationship between the voltage drop caused by a notably small constant current and the junction temperature, also called the K factor, is the most commonly used method, [5]. Although this method is notably simple and easy to perform, its accuracy is significantly affected by the thermocouple position, clamping force for fixture, thermal grease, etc. Thus, this method is often not sufficiently reproducible and accurate [6]. In 2010, the JEDEC51-14 standard [7] specified a new method (the transient dual interface method, which is referred to as the transient method in this paper) to measure the junction-to-case thermal resistance of power semiconductor devices without measuring the case temperature. The transient method defined in this paper is different from the traditional transient method used to measure the transient thermal impedance, which is included in the datasheet. This method considerably improves the accuracy and reproducibility because a thermocouple is not required to measure the case temperature. The steady-state method is the most commonly used method to measure the junction-to-case thermal resistance of wire-bonded Insulated Gate Bipolar Transistor (IGBT) modules, and the transient method has been recently introduced by some manufacturers, such as Infineon [8]. The differences between the packaging styles (the internal structure is shown in Fig. 1) and working conditions of press pack IGBTs (PP IGBTs) and wire-bonded IGBT modules render the method suitable for
http://dx.doi.org/10.1016/j.microrel.2017.05.032 0026-2714/© 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: E. Deng, et al., Study on the methods to measure the junction-to-case thermal resistance of IGBT modules and press pack IGBTs, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.05.032
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Fig. 1. The internal structure of different IGBT packaging styles.
wire-bonded IGBT modules, but not for PP IGBTs. Furthermore, an external clamping pressure of approximately 1.2 kN/cm2 is required to maintain the normal operation of PP IGBTs [9], which is much higher than the recommended clamping pressure of 10 N/cm2 for the fixture during the thermal resistance measurement [7]. Therefore, the thermocouple located between the case and the heatsink will be damaged under such a high clamping force, and the heat path will also be influenced by the inserted thermocouple. Consequently, the accurate measurement of junction-to-case thermal resistance of PP IGBTs remains a significant challenge. Only the junction-to-heatsink thermal resistance (but not the junction-to-case thermal resistance) is provided in the datasheet provided by the manufacturers of PP IGBTs such as Westcode [10] and Toshiba [10]. In this paper, the junction-to-case thermal resistances of IGBT modules and PP IGBTs are measured using both the steady-state method and transient method. The applicability of these two methods to IGBT modules and PP IGBTs is compared based on the experimental results and packaging styles. This paper is structured as follows. The steady-state and transient method to measure the junction-to-case thermal resistance of power semiconductor devices are introduced in Section 1. The principle of the transient method and the determination of thermal
resistance are described in Section 2. A half-bridge IGBT module manufactured by Infineon is introduced to measure the thermal resistance using the two aforementioned methods and both the experimental results are compared with the values provided in the datasheet in Section 3. Furthermore, the junction-to-heatsink thermal resistance of PP IGBTs is measured using the steady-state method as the thermocouple cannot be placed in the interface between the case surface and heatsink and the value of junction-to-heatsink thermal resistance thus obtained is compared with the value provided in the datasheet. The junctionto-case thermal resistance of the PP IGBTs is measured using the transient method in Section 4. Section 5 concludes the paper and describes the applicability of the steady-state method and transient method to IGBT modules and PP IGBTs according to the experimental results. 2. Measurement principle Between the two aforementioned methods, the steady-state method is relatively simple and the most commonly used method to measure the junction-to-case thermal resistance of power semiconductor devices. Therefore, in this paper, only the principle of the transient method is presented.
Please cite this article as: E. Deng, et al., Study on the methods to measure the junction-to-case thermal resistance of IGBT modules and press pack IGBTs, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.05.032
E. Deng et al. / Microelectronics Reliability xxx (2017) xxx–xxx
The transient method requires two measured transient thermal impedance (Zth) curves of a power semiconductor device in contact with a temperature-controlled heatsink. The first test is performed with no thermal interface material between the DUT and the heatsink, i.e., direct contact or without grease. For the second measurement, a thin layer of thermal grease or oil is applied at the interface to reduce the thermal contact resistance, i.e., contact with grease. The thermal grease in the interface changes the thermal contact resistance and ensures a clear separation of the transient thermal impedance curves. The transient thermal impedance that corresponds to the splitting point is defined as the junction-to-case thermal resistance [7]. 2.1. Transient thermal impedance curve The principle of the transient thermal impedance curve measurement is shown in Fig. 2 [11]. One of two direct current sources provides the heating current and the other provides sensing current to the device under test (DUT). A heating current I drive flow into the DUT in order to heat it to thermal equilibrium; subsequently, the heating current Idrive is switched to a sensing current Isense in order to measure the voltage drop during the cooling phase. The junction temperature T j during the cooling phase is acquired through the recorded voltage drop and K factor, which is the relationship between the voltage drop and junction temperature [11]. Finally, the thermal impedance Zth , cooling (t) of the DUT during the cooling
3
phase can be obtained using Eq. (2). Furthermore, the junction behaviour during the heating phase is similar to that of the cooling phase; hence, the thermal impedance Zth , heating(t) during the heating phase can be calculated using Eq. (3) [12]. T j ðt Þ−T j ðt ¼ 0Þ P
ð2Þ
Z th;heating ðt Þ ¼ Z th;cooling ðt ¼ 0Þ−Z th;cooling ðt Þ
ð3Þ
Z th;cooling ðt Þ ¼
2.2. Determination of the junction-to-case thermal resistance The principle of the determination of the junction-to-case thermal resistance of power devices is shown in Fig. 3. Two Zth curves are required for the measurement under two different contact interface conditions with and without thermal grease which causes the two Zth curves to separate at some point of time ts in Fig. 3(b). Since the heat paths only differ at the case surface, these two Zth curves are always consistent from the junction-to-case surface. Therefore, the transient thermal impedance Zth(ts), which is specified at the splitting point, is assumed to be the junction-tocase thermal resistance Rjc of the DUT. Generally, it is very difficult to accurately determine the splitting point of the two Zth curves. Thus, a series of mathematical transformations is proposed to obtain the splitting point more
Fig. 2. Principle diagram of the transient thermal impedance measurement curve.
Please cite this article as: E. Deng, et al., Study on the methods to measure the junction-to-case thermal resistance of IGBT modules and press pack IGBTs, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.05.032
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Fig. 3. Principle of the transient dual interface method
precisely. A variable substitution is performed to the two Zth curves in order to simplify the process; subsequently, a differential is performed to these substitution curves. Furthermore, in order to eliminate the influence of the steady-state thermal resistance difference Δ θ, the difference between these differential curves is normalized as δ(Zθjc), and the mathematical processing is shown in Eqs. (4) and (5). aðzÞ¼ Z θjc ðt Þ;
for z ¼ ln ðt Þ
ð4Þ
ΔðdaðzÞ=dtÞ ðda1 ðzÞ=dtÞ−ðda2 ðzÞ=dtÞ ¼ δ Z θjc ¼ Δθ Δθ
ð5Þ
3. Experiments on IGBT modules In this section, the thermal resistance tester Phase 11 is used to measure the IGBT chip junction-to-case thermal resistance of the half-bridge module manufactured by Infineon (FF100R12RT4). Both
Please cite this article as: E. Deng, et al., Study on the methods to measure the junction-to-case thermal resistance of IGBT modules and press pack IGBTs, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.05.032
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Table 1 Junction-to-case thermal resistance of the IGBT module measured using the steady-state method under different conditions. No
Ic (A)
Tj (°C)
Tc (°C)
Rjc (K/W)
1# 2# 3# 4# 5# 6# 7# 8#
99.999 100.03 100.02 100.06 99.951 99.965 100.01 99.978
93.7 93.6 93.9 93.7 93.7 93.7 93.8 116.6
39.9 40.0 40.0 40.0 40.0 32.4 41.3 50.5
0.27270 0.27154 0.27128 0.27193 0.27099 0.30963 0.26415 0.32250
the steady-state method and transient method are applied to measure the thermal resistance and the experimental results are compared.
3.1. Steady-state method The IGBT chip junction-to-case thermal resistances of the half-bridge module shown in Table 1 are measured by steady-state method under different conditions. A thin layer of thermal grease is applied to the interface between the DUT and heatsink to reduce the thermal contact resistance. Herein, the input current for heating up the module under test is 100 A. A clamping pressure of about 10 N/cm2 is applied to 1#–6# for fixture. All the external test conditions of 1#–5# are the same and the measurement is repeated five times. Meanwhile, the thermocouple used for the measurement of case temperature is placed exactly under the measured IGBT chip as much as possible. The external test conditions of 6# are maintained to be consistent with 1#–5#, except for a slight change in the location of the thermocouple. Further, the clamping force for the half-bridge fixture of 7# between the module and heatsink is slightly higher than the normal clamping pressure of about 10 N/cm2 which is applied to 1#–6#. The clamping pressure of fixture for 8# is slightly lower than the normal clamping pressure. From the data, it can be observed that the repeatability of measurement is acceptable under the conditions that all the external test conditions are the same (1#–5#). However, the results obtained under different conditions are significantly different, especially when the clamping force required for fixture is changed, since the external conditions—such as the location of thermocouple, the clamping force for fixture etc.—have a significant influence on the accuracy of the thermocouple. In this paper, an average value of 0.272 K/W for 1#–5# is assumed to be the junction-to-case thermal resistance of the half-bridge.
Fig. 4. Transient thermal impedance curves of the IGBT module with/without thermal grease. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
3.3. Experimental results analysis From the Zth curve of FF100R12RT4 shown in Fig. 5 it can be observed that the junction-to-case thermal resistance provided in the datasheet is 0.27 K/W [13]. Table 2 shows the experimental results and comparison with the value provided in the datasheet. The result the steady-state method is consistent with the value in the datasheet, but the result of obtained using the transient method is slightly lower than the value in the datasheet. This can be attributed to the fact that the splitting point of the Zth curves will be advanced when the package contains a layer with larger thermal resistance, for example Al2O3 in DBC [7,14]. Thus, the experimental result obtained using the splitting point, which is also called the transient thermal resistance, is slightly smaller than the thermal resistance measured using the steady-state method. Furthermore, the transient thermal resistance is not necessarily consistent with the steady-state junction-to-case thermal resistance because the steady-state (which means a long time) heat flow distribution inside DUT differs from the transient heat flow distribution at time ts [7].
3.2. Transient method The transient thermal impedance curves of the IGBT module with/without thermal grease are measured via the thermal tester Phase 11 and the junction-to-case thermal resistance is determined by using the splitting point, as shown in Fig. 4. The red and black lines shown in Fig. 4 denote the experimental results measured with/without thermal grease, respectively. Thus, the thermal contact resistance between the contact interface of the module and heatsink is reduced by using thermal grease, and eventually leads to a separation of the Z th curves at the contact interface. All the external test conditions, especially the input current and reference temperature, must be the same during the test to ensure accuracy. The junction-to-case thermal resistance of 0.242 K/W can be obtained using the splitting point.
Fig. 5. Transient thermal impedance curve from FF100R12RT4 datasheet.
Please cite this article as: E. Deng, et al., Study on the methods to measure the junction-to-case thermal resistance of IGBT modules and press pack IGBTs, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.05.032
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Table 2 Comparison of the IGBT module experiment results obtained by the steady-state method and transient method with the value in the datasheet.
Thermal resistance (K/W) Deviation (%)
Datasheet
Steady-state
Transient
0.27 –
0.272 –0.7
0.242 10.4
4. Experiments on PP IGBTS 4.1. Test bench The fixture of the commercial thermal resistance tester is not suitable for PP IGBTs because of their special packaging style and working conditions. In this paper, we designed two fixtures for PP IGBTs and used the thermal resistance tester Phase 11 to measure the junctionto-case thermal resistance and the Zth curve of the PP IGBT. According to the principle of the steady-state method and transient method, the K factor [5], which accounts for the relationship between the junction temperature and voltage drop, should be measured to predict the junction temperature during the test. Subsequently, the transient thermal impedance is recorded. The fixtures for the measurement of K factor and transient thermal impedance are shown in Fig. 6. Four nuts are used to constrain the displacement to sustain the desired clamping force, which is provided by a standard pressure machine. A disc spring is needed to compensate the physical movements during the process of clamping and thermal expansion. The clamping force has negligible influence on the K factor provided the studied PP IGBT is in good contact because the measurement current is very small [15]. Fig. 7 shows the results of the 3300 V/360 A PP IGBT manufactured by Westcode under clamping forces of 5 kN and 8 kN with a small constant sense current of 20 mA. The results demonstrate that the fixture can satisfy the measurement requirements because the slopes (°C/V) are exactly the same, with only a difference of 0.02 (°C) in the intercept. The fixture to measure the transient thermal impedance consisted of heatsinks, a DUT, a disc spring, a clamping force sustaining plate, a hydraulic pump and a pressure sensor. Dual heatsinks were used to clamp and cool the DUT on both collector and emitter sides. The hydraulic pump provided a continuously adjustable clamping force ranging up to 50 kN, which satisfied the requirements of all the current ratings of PP IGBTs. A pressure sensor measured the clamping force applied to the DUT, which was displayed on the screen in real time. 4.2. Steady-state method The PP IGBT studied in this paper is 2500 V/360 A with the recommended clamping force of approximately 8–12 kN, which is manufactured by Westcode [10]. The thermal contact resistance between the multi-layers within PP IGBTs is sensitive to the clamping force and is reduced when the clamping force is increased [16]. A clamping force of 8 kN is applied in this paper to measure the junction-to-heatsink thermal resistance in order to compare the experimental results with the values provided in the datasheet. The other reason for the measurement of the junction-to-heatsink thermal resistance is that the thermocouple to measure the case temperature will be destroyed under the high clamping force of 8 kN. Meanwhile, the inserted thermocouple also changes the heat flow and affects thermal distribution eventually. In this paper, the thermocouple is placed at the outlet of the heatsink to record the heatsink temperature. Therefore, the measured junctionto-heatsink thermal resistance will be slightly higher because the heatsink temperature recorded by the thermocouple is smaller than the equivalent heatsink temperature. The values of junction-to-heatsink thermal resistance of PP IGBTs obtained from the experimental results and datasheet are shown in Table 3.
Fig. 6. Measurement fixtures fabricated in our lab for PP IGBTs.
From the data, it is evident that the change trend of the experimental results is consistent with the values in the datasheet, although the error of the value is slightly higher. This can be attributed to the fact that the test conditions are different from those employed by the manufacturer, especially the location of the thermocouple for the measurement of heatsink temperature. As the emitter side of PP IGBTs has a pedestal to assure insulation and packaging for gate pin, the heat flow path will
Please cite this article as: E. Deng, et al., Study on the methods to measure the junction-to-case thermal resistance of IGBT modules and press pack IGBTs, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.05.032
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Fig. 7. Comparison between voltage-temperature calibration curves (K factor) measured with clamping forces of 5 kN and 8 kN.
be much longer than that of the collector side; therefore, the thermal resistance obtained with emitter side cooling is much larger than the value obtained with collector side cooling. When the DUT is double side cooled, the thermal resistance is much smaller and this is one of the most important advantages of PP IGBTs. From the experimental results, it can be observed that when the steady-state method is applied to PP IGBTs, it is difficult to measure even the heatsink temperature accurately, let alone the case temperature. 4.3. Transient method Contrary to the steady-state method, a thermocouple is not required to record the case temperature in the transient method. However, the reference temperature must be the same during the measurement of the Zth curves with/without thermal grease. Thus, a thermocouple is positioned in a constant temperature chamber during the measurement. Transient thermal impedance curves with/without thermal grease under a clamping force of 8 kN are measured and shown in Fig. 8. From the data shown in Fig. 8, it is very difficult to identify the splitting point. Furthermore, the steady-state thermal resistance difference of these two transient thermal impedance curves is also smaller than 0.01 K/W. This phenomenon is primarily caused by the high clamping force and the thermal grease, which fills the interface. The small steady-state thermal resistance difference Δ θ shows that the thermal grease has a limited effect on the interface for reducing the thermal contact resistance. Furthermore, thermal contact resistance between multi-layers within PP IGBTs and the interface between the DUT and heatsink are sensitive to the clamping force [16]. Fig. 9 shows the junction-to-heatsink thermal resistance under different clamping forces with double side cooling. From the data, it can be observed that the clamping force has a significant influence on the thermal resistance, which is caused by the thermal contact resistance. Further, the thermal contact
7
Fig. 8. Transient thermal impedance curves of the PP IGBT with emitter side cooling under a clamping force of 8 kN.
resistance tends to steady with the increment of clamping force. Thus, the thermal contact resistance of the interface between the DUT and heatsink may change negligibly under such a high clamping force caused by the thermal grease. In other words, the variation of thermal contact resistance of the interface caused by thermal grease is too small to separate the thermal impedances measured under two different test conditions. Furthermore, the substrate is used for heat dissipation and electrical insulation between the semiconductor chips and the heatsink in typical wire-bonded IGBT modules or the packaging of TO-247. Thus, thermal grease (mostly electrically insulating) is the most commonly used interface material in contact interfaces to improve thermal conductivity. Nevertheless, the packaging of PP IGBTs is entirely different. Heatsinks are used to dissipate heat and conduct current in PP IGBTs as shown in Fig. 1(b). The electrically insulated thermal grease is no longer suitable for changing the interface conditions. In this study, a suitably thick metal object (such as a bus bar) is proposed to be inserted into the interface to increase the difference of the interface between the DUT and heatsink. Two transient thermal impedance curves under two different interface conditions between the DUT and heatsink with emitter side cooling are measured and shown in Fig. 10. Based on the principle mentioned above, two Zth curves shown in Fig. 10(a) can be modified into the differential curves shown in Fig. 10(b) through variable substitution and differential transformation.
Table 3 Comparison of the PP IGBT experiment results measured by the steady-state method under different cooling conditions with datasheet. Cooling conditions
Ic (A)
P (W)
Tj (°C)
Emitter side cooling Collector side cooling Double side cooling
140 234 320
265.2 534 821.2
74.2 70.8 93.4
Thermal resistance (K/W) Experimental
Datasheet
Error
0.19283 0.09115 0.07775
0.152 0.0843 0.0541
26.8% 8.1% 43.6%
Fig. 9. Junction-to-heatsink thermal resistance of the PP IGBT under various clamping forces with double side cooling.
Please cite this article as: E. Deng, et al., Study on the methods to measure the junction-to-case thermal resistance of IGBT modules and press pack IGBTs, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.05.032
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E. Deng et al. / Microelectronics Reliability xxx (2017) xxx–xxx Table 4 Application comparison of the steady-state and transient methods to IGBT modules and PP IGBTs.
IGBT modules PP IGBTs
Steady-state
Transient
Yes No
Yes Yes
4.4. Experimental results analysis According to the experimental results, the junction-to-heatsink thermal resistance measured by the steady-state method is similar to the value provided in the datasheet, especially the change trend. The difference between the experimental results and value in the datasheet is mainly caused by the heatsink cooling conditions and the location of the thermocouple. Although the steady-state method is relatively simple and easy to perform, only the junction-to-heatsink thermal resistance (but not the junction-to-case thermal resistance) of PP IGBTs can be measured. However, the junction-to-heatsink thermal resistance is not sufficiently accurate for users in applications because of the difference between the heatsink cooling conditions during the thermal resistance measurement and the application. The junction-to-case thermal resistance measured by the transient method shows the same change trend as the value provided in the datasheet. Although the transient method is relatively complex owing to the need to measure two Zth curves, this method can accurately measure the junction-to-case thermal resistance and provides guidance for the design and application. 5. Conclusions According to the experimental results above, it can be observed that the methods used for IGBT modules may not be suitable for PP IGBTs because of the differences in their packaging styles and working conditions. The applicability of the steady-state method and transient method to the measurement of junction-to-case thermal resistance of IGBT modules and PP IGBTs is summarized in Table 4.
Fig. 10. Determination of the junction-to-case thermal resistance of emitter side cooling under a clamping force of 8 kN.
Subsequently, the junction-to-case thermal resistance can be obtained using the criterion k = 0.0045 ∗ Zth + 0.003 [7] for high power semiconductors. From the data shown in Fig. 10(c), it can be observed that the junction-to-case thermal resistance with emitter side cooling is approximately 0.048 K/W. Similarly, the junction-tocase thermal resistance with collector side cooling is approximately 0.032 K/W, which is smaller than the value with emitter side cooling.
• Both the steady-state method and transient method are suitable for the measurement of junction-to-case thermal resistance of IGBT modules. However, the accuracy of the steady-state method is significantly influenced by the clamping force for fixture, thermocouple position, thermal grease etc. The transient method rectifies these disadvantages of the steady-state method without a thermocouple to record the case temperature and has a great reproducibility. • The steady-state method is not suitable for PP IGBTs to measure the junction-to-case thermal resistance because of the thermocouple inserted to record the case temperature. Such a high external clamping force during the normal working of the PP IGBTs destroys the thermocouple. Moreover, the inserted thermocouple will distort the heat flow path and affect the results eventually. And last but not least it is extremely difficult to identify the temperature as the case temperature and not the heatsink or some value in between. • The transient method is applicable for PP IGBTs to measure the junction-to-case thermal resistance without requiring a thermocouple to measure the case temperature. This is a great advantage for PP IGBTs because the thermocouple required to measure the case temperature cannot be placed in the interface between the case surface and heatsink. Acknowledgements The work presented in this paper has been supported by the National Natural Science Foundation of China (51477048), National Key R&D Program of China (2016YFB0901800), and Fundamental Research Funds for the Central Universities 2016XS04.
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9 Erping Deng was born in Hunan province, China, in 1989. He received the bachelor degree in electrical engineering from Harbin Institute of Technology, Harbin, China, in 2013. Currently, he is a Ph.D. student of State Key Laboratory of Alternate Electrical, North China Electric Power University. His main research interest is the packaging and reliability of high voltage power electronics devices.
Zhibin Zhao was born in Hebei province, China, in 1977. He received the Ph.D. degree in electrical engineering from North China Electric Power University, Baoding, China, in 2005. Currently, he is a Professor of State Key Laboratory of Alternate Electrical, North China Electric Power University. His main research interest is computational electromagnetics and electromagnetic compatibility in power electronic.
Yongzhang Huang was born in Guangxi province, China, in 1962. He received the B.S. degree from the Department of Engineering Physics, Tsinghua University, Beijing, China, in 1984, and the Ph.D. degree in physics from the Chinese Academy of Sciences in 1991.He is currently a professor in the Department of Electrical Engineering at North China Electric Power University, Beijing, China. He is also a Chinese distinguished expert of “thousand talents program” and the deputy director of the State Key Laboratory of new energy power system.
Please cite this article as: E. Deng, et al., Study on the methods to measure the junction-to-case thermal resistance of IGBT modules and press pack IGBTs, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.05.032