- Email: [email protected]
Power module thermal cycling tester for in-situ ageing detection
MR-12459; No of Pages 5 Microelectronics Reliability xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Power module thermal cycling tester for in-situ ageing detection Ph. Pougnet a,b,⁎, G. Coquery b,c, R. Lallemand c, A. Makhloufi d a
Valeo Siemens eAutomotive, Cergy, France VEDECOM Institute, Versailles, France c IFSTTAR, Versailles, France d INSA, Rouen, France b
a r t i c l e
i n f o
Article history: Received 20 May 2017 Received in revised form 13 June 2017 Accepted 20 June 2017 Available online xxxx
a b s t r a c t The success of the electric car depends heavily on battery technology and on compact, reliable and efficient power converters in which power modules are a key element. Power Module technology is evolving rapidly. New semiconductor chips are developed to improve electrical performance and reduce power losses. Innovative design architectures are introduced to improve thermal management. Before mass production launch, the performance and lifetime of these technological innovations needs to be evaluated to respect the longer warranty requirements. This is usually achieved by Accelerated Testing. In this case, the effect of thermal cycles on the lifetime of power modules is usually studied by placing unpowered power modules in a thermal chamber and checking that their performance matches the specification after a given number of thermal cycles. However this method provides no information on the power module performance degradation due to cyclic thermal stresses. The Thermal Cycling Tester presented in this paper reproduces the stresses created in use conditions by environmental thermal cycles. This approach makes it possible to monitor ageing by applying power cycles periodically and measuring thermal and electric parameters. These results are then combined with mechanical and acoustic microscopy characterizations and finite element modeling results to provide a better understanding of the failure mechanisms. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction High Power modules are used in electric cars to drive the electric motor. In use, these power modules are submitted to harsh thermal loads and the resulting thermo-mechanical stresses impact their lifetime. As these stresses depend on application specific mounting and cooling conditions, it is important to verify that power modules function correctly in the planned use conditions during the warranty period which lasts in some cases 15 years. To evaluate the effect of thermal cycles on lifetime, successive thermal cycles which can be found in worst case use conditions are applied on a sample of power modules. This Thermal Cycling Test reveals significant failures. To monitor the degradation caused by these thermal cycles, power cycles are periodically applied and electrical and thermal measurements are performed. These data provide information on the degradation of the thermal properties and performance of the power modules under test. Combining these data with characterisations performed at the end of TCT and 3D finite element modelling a good
⁎ Corresponding author. E-mail address: [email protected] (P. Pougnet).
understanding of the failure mechanism caused by thermal cycling is obtained. 2. Mission profile and use conditions 2.1. Power module technology Compact power modules make it possible to optimize the cost of power convertors while guaranteeing their performance in terms of mass, volume and energy conversion yield. The electronic circuitry of power modules is obtained by attaching electronic chips on an electrical insulating and thermal conductive substrate. The design architecture of power modules evolves rapidly. According to application requirements in terms of voltage and current, the electronic chips used are either Metal Oxide Semiconductor Field Effect Transistors (MOSFET) or Insulated Gate Bipolar Transistors (IGBT) combined with power diodes. Since 2011 appeared new semiconductor chips based on Silicon Carbide (SiC) and Gallium Nitride (GaN) materials. Power Module substrates often consist in a stack of insulating ceramic bonded to thick copper layers (Direct Bond Copper or DBC). There is a variety of die attach materials: SnAgCu, SnAg and silver sintered layers. Power chips and circuit interconnects are done by bonding aluminum wires (wedge bonding).
http://dx.doi.org/10.1016/j.microrel.2017.06.042 0026-2714/© 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: P. Pougnet, et al., Power module thermal cycling tester for in-situ ageing detection, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.06.042
2
P. Pougnet et al. / Microelectronics Reliability xxx (2017) xxx–xxx
end of a given number of thermal cycles that their performances match the specification. However, experience and simulation show that the thermal management of a power module depends strongly on the thermal interface material used (nature, thickness) and on the mechanical mounting conditions on the heat sink. The test approach presented in this paper considers power modules mounted and cooled in the same way as in the final automotive application. The measurement system provides information on the ageing of power modules under thermal cycling loads throughout the whole test duration and this without dismounting the devices.
3. Thermal cycling tester 3.1. Test protocol Fig. 1. Single side cooled power module.
However new technologies are developed: copper wire bonding, aluminum ribbon bonding, sintering layer interconnects. The thermal design of a power module uses often a water or air cooled base plate (Fig. 1). Usually in single side cooled power modules power chips are encapsulated in a silicon gel and the module is enclosed in a box. However new two side cooling architectures using cooling through the base plate and the top copper layer are proposed (Fig. 2). Before launching an electric vehicle into production it is recommended to check that power modules function correctly in the planned use conditions. The effect of use load on power module is usually assessed by Accelerated Testing. 2.2. Mission profile loads
The effect of thermal loads on power modules lifetime is studied by applying thermal cycles to the modules under test [2]. The test protocol is displayed in Fig. 3. Before test, the voltage drop as a function of temperature is measured on IGBT and diode chips. To obtain this calibration function, power modules are mounted on a water plate which makes it possible to set the modules at temperatures ranging between 20 °C and 150 °C. The calibration procedure consists in applying a low amplitude measuring current (100 mA) and measuring voltage drop at various temperature steps and once a thermal equilibrium is reached. This calibration function provides the equivalent junction temperature when the voltage drop is known. Thermal cycles are applied on the modules under test. The low and high temperature levels (Fig. 4) correspond to the mission profile worst case conditions: −20 °C/125 °C. Dwell time is 10 min.
In use, power modules are submitted to passive and active thermal cycles. Active thermal cycles are fast varying cycles caused by power losses [1]. Passive thermal cycles are slow varying cycles due to variation of temperature in the vicinity of the power module. Various materials are used in the assembly of power modules. Usually these materials are selected so that their thermal expansion coefficients match the thermal coefficient of the silicon power chips. However as thermal expansion mismatch of these materials remains, thermal cycle loads lead to thermo-mechanical stresses. These stress cycles damage the power modules and have an effect on their lifetime. 2.3. Accelerated tests The effect of passive thermal cycles is usually studied by placing unpowered power modules in a thermal chamber and checking at the
Fig. 2. Double side cooled power module.
Fig. 3. Thermal cycling test protocol.
Please cite this article as: P. Pougnet, et al., Power module thermal cycling tester for in-situ ageing detection, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.06.042
P. Pougnet et al. / Microelectronics Reliability xxx (2017) xxx–xxx
3
Equivalent junct ion t emperat ure ( ° C)
160 140 120 100 80 60 Variable initial * time (s) 350 * time 717 * time_2 1039 * time_3
40 20 0 0
1
2
3.2. Case study of a single sided cooled power module 3.2.1. Power modules under test Three power modules are tested simultaneously. They are labelled A, B and C. Their architecture is a single side cooled base plate (see Fig. 1). The chips under test (two chips in parallel) are IGBT chips (labelled (14 and 42) and power diodes (23). To improve cooling, a Thermal Interface Material (TIM) is coated between the base plate and the tester cooling plate This TIM is coated applying the production assembly procedures. The power modules are mounted on the cooling plate by tightening screws at the torque recommended by the power module supplier. Thermal cycles are applied varying the coolant temperature. The power modules under test are enclosed in a box filled with thermal insulation foam to protect them from humidity condensation. The voltage drop measuring current is set at 100 mA. 3.2.2. Results Characterization of thermal parameters and junction temperature is carried out before applying thermal cycles load, after 350 cycles, 717 cycles and 1038 cycles. Junction equivalent chip temperature (Tj) at the end of the 5 s power pulse increases with the number of applied thermal cycles (see Fig. 5). Before applying thermal stress, Tj for chips 14 and 42 of module A is 138.9 °C. After 1038 thermal cycles, Junction temperature at 4.8 s is 158.4 °C, which means thermal cycle loads provoke an 19.5 °C increase of the equivalent junction temperature The relative variation of junction temperature T j is obtained from measurement data on IGBT chips 14 and 42 of modules A, B and C and power diodes of modules A and C (sample size: 8). Anderson Darling normality test shows that
4
5
Fig. 5. Junction equivalent temperature variation of module A chip 14 during power cycle at various applied thermal cycles (initial, 350, 717, 1038).
these data come from a normal distribution. The 95% confidence for the mean of this relative T j increase is 8.1%–14.6% and its standard deviation is 3.9%. This means that the increase of the equivalent junction temperature due to thermal stress is statistically significant. Moreover, results obtained on IGBT chip 14 of module A before applying thermal cycles load, after 350 cycles, 717 cycles and 1038 cycles (see Fig. 6) show that thermal stress increases the thermal impedance of chips under study. The thermal resistance (Rth) is defined by the value of thermal impedance at the of the power cycle (4.8 s). Fig. 7 displays the thermal resistance as a function of the applied number of thermal cycles. An important variation of the thermal resistance (Rth) is observed between 0 and 350 cycles.
4. Discussion 4.1. Acoustic microscopy characterization Acoustic microscopy of the interfaces of the layers of modules A, B and C is carried out before applying thermal stress and after 1038 cycles. These characterizations reveal warping and delamination at the base plate solder – Direct Bond Copper (DBC) interface. This is observed on module A (see Fig. 8) and on module C (see Fig. 9). Further study of solder die attach does not provide clear results as warping makes it difficult to focus on the surface of the chips. To get a sound status of the die attach solder layer, physical analysis by microsection remains to be done.
0,7 0,6
Thermal impedance ( ° C/ W)
The slow ramp rate (1 °C/min) is close to the temperature variation rate in the vicinity of the power convertor in use conditions. In order to monitor degradation, the gate emitter leak current (Iges), the emitter-collector voltage (Vce) of IGBT chips and the forward voltage drop (Vd) of power diodes are measured constantly and logged. Periodically, in order to measure the thermal impedance (Zth) and the thermal resistance (Rth), power pulses are applied [3]. The current amplitude (Ice) is set at the worst case (250 A). The overall power current duration is 5 s. However every 0.5 s, this power pulse is switched off. Applying a low amplitude and constant current in short time intervals at 0, 5, 1, 1, 5, 2, 2.5, 3, 3.5, 4, 4.5 and 5 s from the start of the power current, Vce, Vd, Ice, and Iges are measured. These data are used to obtain the equivalent junction temperature as a function of time and thus the thermal impedance (Zth) of the IGBT and diode chips. When the thermal flow from the semiconductor junction to the cool plate is stabilized at the end of the power pulse the thermal resistance between junction and coolant water is measured. In this protocol, the total number of power cycles is low and does not contribute to ageing. Only thermal cycles loads damage the modules under test. As it is a common practice, end of life is detected when thermal resistance Rth or voltage drop Vce increase more than 20% and 5% respectively.
3 Time ( s)
Fig. 4. Applied temperature cycles as a function of time.
0,5 0,4 0,3 a) : Zth initial b) Zth after 1038 cy cles
0,2 0,1 0,0 0
1
2
3
4
5
Time ( s)
Fig. 6. Zth variation of module A IGBT chip 14 a) before thermal cycles, b) after 1038 thermal cycles.
Please cite this article as: P. Pougnet, et al., Power module thermal cycling tester for in-situ ageing detection, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.06.042
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P. Pougnet et al. / Microelectronics Reliability xxx (2017) xxx–xxx
Thermal resistance (K/W)
0,67 0,66 0,65 0,64
0,63 0,62 0,61 0,6 0,59 0
200
400
600
800
1000
1200
Number of thermal cycles Fig. 7. Thermal resistance of module A IGBT chip 14 as a function of applied thermal cycles.
4.2. Thermo-mechanical plastic strain distribution A finite element model of the power module under study is developed. The module base plate is coated with a thermal interface material layer and screwed to the heat sink. The contact boundary condition is modeled using GLUE command in ANSYS APDL software. The physical properties of most materials of the module assembly are assumed linear. However the die attach and base plate solder layers have visco-plastic properties and are modeled using the Anand approach and parameters of references [4,5]. The thermal load simulation consists in heating up the power module to the liquidus temperature of the solder (220 °C) and it is assumed a rigid connection is formed at this liquidus temperature. This makes it possible to take into account built in stress. Then a series of 6 consecutive thermal cycles ranging from − 20 °C to 125 °C is applied. The thermal ramp rate is 1 °C/min and the dwell time at temperature extremes is 10 min. The thermo-mechanical strain energy of the die attach and base plate solder layers is calculated using the Anand model. The results (Fig. 10) show that plastic strain energy is very high in the base plate solder at the interface between solder and the DBC substrate. This may explain the warping observed on all the modules. Plastic strain energy in the periphery of the die attach solder is about half the energy calculated at the boundary of the base plate solder.
4.3. Possible improvements Several causes may explain the observed increase of junction temperature caused by thermal stress: base plate solder delamination, die attach delamination and degradation of the thermal properties of the TIM. The simulation results of the finite element model show that the solder plastic strain energy is high in the base plate solder layer. New materials now exist [6] and can be used to improve the resistance of this layer. The plastic strain energy in the die attach layer is significantly lower and thus should not cause an important increase of thermal
Fig. 8. Module A warping (arrows) at the base plate solder- DBC interface.
Fig. 9. Module C base plate solder- DBC interface a) before test b) after 1038 cycles.
resistance. Experimental data displayed in Fig. 6 shows that the junction temperature increase is significant 1.5 s after the start of the power current. This suggests that an important part of the junction temperature increase is due to ageing of the TIM. This possible cause should be carefully studied by improving the test equipment to measure Tj at the start of the heating period and repeating this Thermal Cycling Test on more samples and various power module designs.
5. Conclusion The industrial development of the electric car relies on the availability of compact, low cost and robust power modules. Power module technologies evolve rapidly. Before launching mass production of converters using innovative power modules, accelerating testing should be applied to evaluate their lifetime in the specific mounting and cooling use conditions. The thermal cycling tester presented in this paper applies thermal cycles corresponding to worst case use conditions. Three power modules are tested. Results show that thermal cycling stress has an impact on the power module thermal resistance and on the equivalent junction temperature at the end of a power cycle. Acoustic microscopy characterizations performed before and after thermal stress cycling reveals warping and delaminating in the base plate and die attach solder layers. A Finite Element model is developed to study the effect of thermal cycles on the materials of the power
Fig. 10. Distribution of cumulated plastic energy in die attach and in base plate solder after 6 thermal cycles.
Please cite this article as: P. Pougnet, et al., Power module thermal cycling tester for in-situ ageing detection, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.06.042
P. Pougnet et al. / Microelectronics Reliability xxx (2017) xxx–xxx
module assembly. This model confirms that the plastic strain energy is high in the periphery of the base plate and die attach solder layers. The results obtained by this Thermal Cycling approach are however strongly related to the design, process and materials of the power modules under study. This approach will be used shortly to evaluate the effect of thermal cycling on double side cooling power modules. Acknowledgements The authors acknowledge MOVEO (Ile-De-France Region Automotive competitiveness cluster) and the French Government for technical and financial support of the French automotive research projects “AUDACE”, “SOFRACI” and “FIRST MFP”, which led to this study.
5
References [1] G. Coquery, S. Carubelli, J.P. Ousten, R. Lallemand, F. Lecoq, D. Lhotellier, V. De Viry, Ph. Dupuy, Power Module Lifetime Estimation from Chip Temperature Direct Measurement in an Automotive Traction Inverter, ESREF 2001, pp, 1695–1700. [2] J. Lutz, H. Schlangenotto, U. Scheuermann, R. De Doncker, “Semiconductor Power Devices” , ISBN 978364211242, Springer-Verlag, Berlin, Heidelberg, 2011. [3] G. Coquery, M. Piton, R. Lallemand, S. Pagiusco, A. Jeunesse, Thermal stresses on railways traction inverter IGBT modules concept, methodology, results on sub-urban mass transit. Application to predictive maintenance, EPE (2003) 1160. [4] Ling Xu, Yong Liu, Sheng Liu, Modeling and simulation of power electronic modules with microchannel coolers for thermo-mechanical performance, Microelectron. Reliab. 54 (2014) 2824–2835. [5] Pushparajah Rajaguru, Hua Lu, Chris Bailey, Application of kriging and radial basis function in power electronic module wire bond structure reliability under various amplitude loading, Int. J. Fatigue 45 (2012) 61–70. [6] M. Thoben, F. Sauerland, K. Mainka, S. Edenharter, L. Beaurenaut, Lifetime Modeling and Simulation of Power Modules for Hybrid Electrical/Electrical Vehicles, ESREF, 2014.
Please cite this article as: P. Pougnet, et al., Power module thermal cycling tester for in-situ ageing detection, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.06.042
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Power module thermal cycling tester for in-situ ageing detection Ph. Pougnet a,b,⁎, G. Coquery b,c, R. Lallemand c, A. Makhloufi d a
Valeo Siemens eAutomotive, Cergy, France VEDECOM Institute, Versailles, France c IFSTTAR, Versailles, France d INSA, Rouen, France b
a r t i c l e
i n f o
Article history: Received 20 May 2017 Received in revised form 13 June 2017 Accepted 20 June 2017 Available online xxxx
a b s t r a c t The success of the electric car depends heavily on battery technology and on compact, reliable and efficient power converters in which power modules are a key element. Power Module technology is evolving rapidly. New semiconductor chips are developed to improve electrical performance and reduce power losses. Innovative design architectures are introduced to improve thermal management. Before mass production launch, the performance and lifetime of these technological innovations needs to be evaluated to respect the longer warranty requirements. This is usually achieved by Accelerated Testing. In this case, the effect of thermal cycles on the lifetime of power modules is usually studied by placing unpowered power modules in a thermal chamber and checking that their performance matches the specification after a given number of thermal cycles. However this method provides no information on the power module performance degradation due to cyclic thermal stresses. The Thermal Cycling Tester presented in this paper reproduces the stresses created in use conditions by environmental thermal cycles. This approach makes it possible to monitor ageing by applying power cycles periodically and measuring thermal and electric parameters. These results are then combined with mechanical and acoustic microscopy characterizations and finite element modeling results to provide a better understanding of the failure mechanisms. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction High Power modules are used in electric cars to drive the electric motor. In use, these power modules are submitted to harsh thermal loads and the resulting thermo-mechanical stresses impact their lifetime. As these stresses depend on application specific mounting and cooling conditions, it is important to verify that power modules function correctly in the planned use conditions during the warranty period which lasts in some cases 15 years. To evaluate the effect of thermal cycles on lifetime, successive thermal cycles which can be found in worst case use conditions are applied on a sample of power modules. This Thermal Cycling Test reveals significant failures. To monitor the degradation caused by these thermal cycles, power cycles are periodically applied and electrical and thermal measurements are performed. These data provide information on the degradation of the thermal properties and performance of the power modules under test. Combining these data with characterisations performed at the end of TCT and 3D finite element modelling a good
⁎ Corresponding author. E-mail address: [email protected] (P. Pougnet).
understanding of the failure mechanism caused by thermal cycling is obtained. 2. Mission profile and use conditions 2.1. Power module technology Compact power modules make it possible to optimize the cost of power convertors while guaranteeing their performance in terms of mass, volume and energy conversion yield. The electronic circuitry of power modules is obtained by attaching electronic chips on an electrical insulating and thermal conductive substrate. The design architecture of power modules evolves rapidly. According to application requirements in terms of voltage and current, the electronic chips used are either Metal Oxide Semiconductor Field Effect Transistors (MOSFET) or Insulated Gate Bipolar Transistors (IGBT) combined with power diodes. Since 2011 appeared new semiconductor chips based on Silicon Carbide (SiC) and Gallium Nitride (GaN) materials. Power Module substrates often consist in a stack of insulating ceramic bonded to thick copper layers (Direct Bond Copper or DBC). There is a variety of die attach materials: SnAgCu, SnAg and silver sintered layers. Power chips and circuit interconnects are done by bonding aluminum wires (wedge bonding).
http://dx.doi.org/10.1016/j.microrel.2017.06.042 0026-2714/© 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: P. Pougnet, et al., Power module thermal cycling tester for in-situ ageing detection, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.06.042
2
P. Pougnet et al. / Microelectronics Reliability xxx (2017) xxx–xxx
end of a given number of thermal cycles that their performances match the specification. However, experience and simulation show that the thermal management of a power module depends strongly on the thermal interface material used (nature, thickness) and on the mechanical mounting conditions on the heat sink. The test approach presented in this paper considers power modules mounted and cooled in the same way as in the final automotive application. The measurement system provides information on the ageing of power modules under thermal cycling loads throughout the whole test duration and this without dismounting the devices.
3. Thermal cycling tester 3.1. Test protocol Fig. 1. Single side cooled power module.
However new technologies are developed: copper wire bonding, aluminum ribbon bonding, sintering layer interconnects. The thermal design of a power module uses often a water or air cooled base plate (Fig. 1). Usually in single side cooled power modules power chips are encapsulated in a silicon gel and the module is enclosed in a box. However new two side cooling architectures using cooling through the base plate and the top copper layer are proposed (Fig. 2). Before launching an electric vehicle into production it is recommended to check that power modules function correctly in the planned use conditions. The effect of use load on power module is usually assessed by Accelerated Testing. 2.2. Mission profile loads
The effect of thermal loads on power modules lifetime is studied by applying thermal cycles to the modules under test [2]. The test protocol is displayed in Fig. 3. Before test, the voltage drop as a function of temperature is measured on IGBT and diode chips. To obtain this calibration function, power modules are mounted on a water plate which makes it possible to set the modules at temperatures ranging between 20 °C and 150 °C. The calibration procedure consists in applying a low amplitude measuring current (100 mA) and measuring voltage drop at various temperature steps and once a thermal equilibrium is reached. This calibration function provides the equivalent junction temperature when the voltage drop is known. Thermal cycles are applied on the modules under test. The low and high temperature levels (Fig. 4) correspond to the mission profile worst case conditions: −20 °C/125 °C. Dwell time is 10 min.
In use, power modules are submitted to passive and active thermal cycles. Active thermal cycles are fast varying cycles caused by power losses [1]. Passive thermal cycles are slow varying cycles due to variation of temperature in the vicinity of the power module. Various materials are used in the assembly of power modules. Usually these materials are selected so that their thermal expansion coefficients match the thermal coefficient of the silicon power chips. However as thermal expansion mismatch of these materials remains, thermal cycle loads lead to thermo-mechanical stresses. These stress cycles damage the power modules and have an effect on their lifetime. 2.3. Accelerated tests The effect of passive thermal cycles is usually studied by placing unpowered power modules in a thermal chamber and checking at the
Fig. 2. Double side cooled power module.
Fig. 3. Thermal cycling test protocol.
Please cite this article as: P. Pougnet, et al., Power module thermal cycling tester for in-situ ageing detection, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.06.042
P. Pougnet et al. / Microelectronics Reliability xxx (2017) xxx–xxx
3
Equivalent junct ion t emperat ure ( ° C)
160 140 120 100 80 60 Variable initial * time (s) 350 * time 717 * time_2 1039 * time_3
40 20 0 0
1
2
3.2. Case study of a single sided cooled power module 3.2.1. Power modules under test Three power modules are tested simultaneously. They are labelled A, B and C. Their architecture is a single side cooled base plate (see Fig. 1). The chips under test (two chips in parallel) are IGBT chips (labelled (14 and 42) and power diodes (23). To improve cooling, a Thermal Interface Material (TIM) is coated between the base plate and the tester cooling plate This TIM is coated applying the production assembly procedures. The power modules are mounted on the cooling plate by tightening screws at the torque recommended by the power module supplier. Thermal cycles are applied varying the coolant temperature. The power modules under test are enclosed in a box filled with thermal insulation foam to protect them from humidity condensation. The voltage drop measuring current is set at 100 mA. 3.2.2. Results Characterization of thermal parameters and junction temperature is carried out before applying thermal cycles load, after 350 cycles, 717 cycles and 1038 cycles. Junction equivalent chip temperature (Tj) at the end of the 5 s power pulse increases with the number of applied thermal cycles (see Fig. 5). Before applying thermal stress, Tj for chips 14 and 42 of module A is 138.9 °C. After 1038 thermal cycles, Junction temperature at 4.8 s is 158.4 °C, which means thermal cycle loads provoke an 19.5 °C increase of the equivalent junction temperature The relative variation of junction temperature T j is obtained from measurement data on IGBT chips 14 and 42 of modules A, B and C and power diodes of modules A and C (sample size: 8). Anderson Darling normality test shows that
4
5
Fig. 5. Junction equivalent temperature variation of module A chip 14 during power cycle at various applied thermal cycles (initial, 350, 717, 1038).
these data come from a normal distribution. The 95% confidence for the mean of this relative T j increase is 8.1%–14.6% and its standard deviation is 3.9%. This means that the increase of the equivalent junction temperature due to thermal stress is statistically significant. Moreover, results obtained on IGBT chip 14 of module A before applying thermal cycles load, after 350 cycles, 717 cycles and 1038 cycles (see Fig. 6) show that thermal stress increases the thermal impedance of chips under study. The thermal resistance (Rth) is defined by the value of thermal impedance at the of the power cycle (4.8 s). Fig. 7 displays the thermal resistance as a function of the applied number of thermal cycles. An important variation of the thermal resistance (Rth) is observed between 0 and 350 cycles.
4. Discussion 4.1. Acoustic microscopy characterization Acoustic microscopy of the interfaces of the layers of modules A, B and C is carried out before applying thermal stress and after 1038 cycles. These characterizations reveal warping and delamination at the base plate solder – Direct Bond Copper (DBC) interface. This is observed on module A (see Fig. 8) and on module C (see Fig. 9). Further study of solder die attach does not provide clear results as warping makes it difficult to focus on the surface of the chips. To get a sound status of the die attach solder layer, physical analysis by microsection remains to be done.
0,7 0,6
Thermal impedance ( ° C/ W)
The slow ramp rate (1 °C/min) is close to the temperature variation rate in the vicinity of the power convertor in use conditions. In order to monitor degradation, the gate emitter leak current (Iges), the emitter-collector voltage (Vce) of IGBT chips and the forward voltage drop (Vd) of power diodes are measured constantly and logged. Periodically, in order to measure the thermal impedance (Zth) and the thermal resistance (Rth), power pulses are applied [3]. The current amplitude (Ice) is set at the worst case (250 A). The overall power current duration is 5 s. However every 0.5 s, this power pulse is switched off. Applying a low amplitude and constant current in short time intervals at 0, 5, 1, 1, 5, 2, 2.5, 3, 3.5, 4, 4.5 and 5 s from the start of the power current, Vce, Vd, Ice, and Iges are measured. These data are used to obtain the equivalent junction temperature as a function of time and thus the thermal impedance (Zth) of the IGBT and diode chips. When the thermal flow from the semiconductor junction to the cool plate is stabilized at the end of the power pulse the thermal resistance between junction and coolant water is measured. In this protocol, the total number of power cycles is low and does not contribute to ageing. Only thermal cycles loads damage the modules under test. As it is a common practice, end of life is detected when thermal resistance Rth or voltage drop Vce increase more than 20% and 5% respectively.
3 Time ( s)
Fig. 4. Applied temperature cycles as a function of time.
0,5 0,4 0,3 a) : Zth initial b) Zth after 1038 cy cles
0,2 0,1 0,0 0
1
2
3
4
5
Time ( s)
Fig. 6. Zth variation of module A IGBT chip 14 a) before thermal cycles, b) after 1038 thermal cycles.
Please cite this article as: P. Pougnet, et al., Power module thermal cycling tester for in-situ ageing detection, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.06.042
4
P. Pougnet et al. / Microelectronics Reliability xxx (2017) xxx–xxx
Thermal resistance (K/W)
0,67 0,66 0,65 0,64
0,63 0,62 0,61 0,6 0,59 0
200
400
600
800
1000
1200
Number of thermal cycles Fig. 7. Thermal resistance of module A IGBT chip 14 as a function of applied thermal cycles.
4.2. Thermo-mechanical plastic strain distribution A finite element model of the power module under study is developed. The module base plate is coated with a thermal interface material layer and screwed to the heat sink. The contact boundary condition is modeled using GLUE command in ANSYS APDL software. The physical properties of most materials of the module assembly are assumed linear. However the die attach and base plate solder layers have visco-plastic properties and are modeled using the Anand approach and parameters of references [4,5]. The thermal load simulation consists in heating up the power module to the liquidus temperature of the solder (220 °C) and it is assumed a rigid connection is formed at this liquidus temperature. This makes it possible to take into account built in stress. Then a series of 6 consecutive thermal cycles ranging from − 20 °C to 125 °C is applied. The thermal ramp rate is 1 °C/min and the dwell time at temperature extremes is 10 min. The thermo-mechanical strain energy of the die attach and base plate solder layers is calculated using the Anand model. The results (Fig. 10) show that plastic strain energy is very high in the base plate solder at the interface between solder and the DBC substrate. This may explain the warping observed on all the modules. Plastic strain energy in the periphery of the die attach solder is about half the energy calculated at the boundary of the base plate solder.
4.3. Possible improvements Several causes may explain the observed increase of junction temperature caused by thermal stress: base plate solder delamination, die attach delamination and degradation of the thermal properties of the TIM. The simulation results of the finite element model show that the solder plastic strain energy is high in the base plate solder layer. New materials now exist [6] and can be used to improve the resistance of this layer. The plastic strain energy in the die attach layer is significantly lower and thus should not cause an important increase of thermal
Fig. 8. Module A warping (arrows) at the base plate solder- DBC interface.
Fig. 9. Module C base plate solder- DBC interface a) before test b) after 1038 cycles.
resistance. Experimental data displayed in Fig. 6 shows that the junction temperature increase is significant 1.5 s after the start of the power current. This suggests that an important part of the junction temperature increase is due to ageing of the TIM. This possible cause should be carefully studied by improving the test equipment to measure Tj at the start of the heating period and repeating this Thermal Cycling Test on more samples and various power module designs.
5. Conclusion The industrial development of the electric car relies on the availability of compact, low cost and robust power modules. Power module technologies evolve rapidly. Before launching mass production of converters using innovative power modules, accelerating testing should be applied to evaluate their lifetime in the specific mounting and cooling use conditions. The thermal cycling tester presented in this paper applies thermal cycles corresponding to worst case use conditions. Three power modules are tested. Results show that thermal cycling stress has an impact on the power module thermal resistance and on the equivalent junction temperature at the end of a power cycle. Acoustic microscopy characterizations performed before and after thermal stress cycling reveals warping and delaminating in the base plate and die attach solder layers. A Finite Element model is developed to study the effect of thermal cycles on the materials of the power
Fig. 10. Distribution of cumulated plastic energy in die attach and in base plate solder after 6 thermal cycles.
Please cite this article as: P. Pougnet, et al., Power module thermal cycling tester for in-situ ageing detection, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.06.042
P. Pougnet et al. / Microelectronics Reliability xxx (2017) xxx–xxx
module assembly. This model confirms that the plastic strain energy is high in the periphery of the base plate and die attach solder layers. The results obtained by this Thermal Cycling approach are however strongly related to the design, process and materials of the power modules under study. This approach will be used shortly to evaluate the effect of thermal cycling on double side cooling power modules. Acknowledgements The authors acknowledge MOVEO (Ile-De-France Region Automotive competitiveness cluster) and the French Government for technical and financial support of the French automotive research projects “AUDACE”, “SOFRACI” and “FIRST MFP”, which led to this study.
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Please cite this article as: P. Pougnet, et al., Power module thermal cycling tester for in-situ ageing detection, Microelectronics Reliability (2017), http://dx.doi.org/10.1016/j.microrel.2017.06.042