- Email: [email protected]
Implementation of a new thermal path within the structure of inorganic encapsulated power modules
Microelectronics Reliability xxx (xxxx) xxxx
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Implementation of a new thermal path within the structure of inorganic encapsulated power modules ⁎
S. Behrendta, , R. Eiselea, M.G. Scheibelb, S. Kaessnerc a
FuE-Zentrum Fachhochschule Kiel GmbH, 24149 Kiel, Germany Heraeus Deutschland GmbH & Co. KG, 63450 Hanau, Germany c Robert Bosch GmbH, 71272 Renningen, Germany b
A B S T R A C T
The demands on power modules regarding power density and reliability are continuously rising. Conventional encapsulation materials are limited in their thermal performance. In this study, a completely new material class is used within a power module. This new class is the group of inorganic materials, specifically ceramic encapsulation materials. The use of ceramic encapsulation materials generates new possibilities for optimizing the heat dissipation within power modules. In comparison to conventional encapsulation materials, ceramic composites can dissipate the semiconductor's heat loss much more efficient due to its enhanced thermal conductivity. This improves the transient thermal resistance of the module (Zth). Furthermore, an additional copper layer (TMC - Thermal mass circuit), which is connected to the heatsink below, is applied to the top of the encapsulation. This opens up a new thermal path in the module without changing its footprint or overall structure. In particular, the geometrically difficult area above the on-chip contacts can be used completely for cooling for the first time. Experiments show that the combination of a ceramic encapsulation and the TMC structure can reduce the semiconductor's temperature by more than 12 K. This enables a higher lifetime, efficiency and reliability of power modules with high energy density.
1. Introduction The constantly rising demands with regard to power density and compactness make the packaging of power electronic modules an increasingly challenging task. New die attach technologies like silver sintering extend the semiconductor's lifetime, thus enhancing the reliability of power modules [1,2]. Another example of a new packaging technology is copper ribbon bonding in combination with the Heraeus Die Top System (DTS®). This top side technology extends the semiconductors lifetime by a factor 16.6 in comparison to conventional aluminium ribbon bonding. In addition, the failure mechanism shifts from a top-side contact failure to a die metallization failure [3]. However, the currently used encapsulation materials are still a limiting factor when it comes to carrying the high amount of power density that is demanded in today's power applications. High currents are accompanied by high power losses, which are converted into a high amount of heat. This heat loss needs to be dissipated as efficient as possible to ensure a high level of reliability. Heat dissipation depends on the thermal structure (geometric design and arrangement) as well as the materials used and their properties. The encapsulation materials used today, which mainly consist of polymers, are strongly limited with regard to their thermal performance, resp. thermal expansion and thermal conductivity. ⁎
Conventional encapsulation materials can be divided into two categories dependent on the module structure they are used in. On one hand, there are the silicone softgels, which are used in conventional frame modules. On the other hand, there are the epoxy based molding compounds (EMC), which are used in mold modules. Both types of encapsulation material have a very low thermal conductivity (TC) and a low continuous operating temperature. Especially EMCs can only be used at temperatures up to about 175 °C. At higher temperatures the material starts to deteriorate [4]. Furthermore, the coefficient of thermal expansion (CTE) is very high and leads to a CTEmismatch between the materials within the module. Consequently, a high amount of thermally induced mechanical tension is the result, which leads to failures within the module structure. Another disadvantage is the thermal performance of those materials. While silicone softgels have almost no thermal conductivity (~0.2 W/(m·K)), EMCs reach thermal conductivities of about 2 to 3 W/ (m·K) [5–7]. Such high values can only be achieved by a high level (90 wt%) of inorganic fillers within the epoxy matrix such as SiO2 or Si3N4 [8]. Due to the high filler ratio, the viscosity of the EMC compound is increased and therefore very difficult to process. High temperature and pressure are necessary to achieve a successful molding process. Thus the material, tools and process are cost intensive. To overcome these limitations the use of novel encapsulation materials is necessary. Ceramic encapsulants offer a high TC and an
Corresponding author. E-mail address: [email protected] (S. Behrendt).
https://doi.org/10.1016/j.microrel.2019.113430 Received 14 May 2019; Accepted 12 July 2019 0026-2714/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: S. Behrendt, et al., Microelectronics Reliability, https://doi.org/10.1016/j.microrel.2019.113430
Microelectronics Reliability xxx (xxxx) xxxx
S. Behrendt, et al.
intrinsic low CTE. Ceramic encapsulants have a high thermal conductivity due to their inorganic structure. The CTE can be adjusted within a certain range to fit the assembly the encapsulant shall encase. This adjustment is possible without compromising the processability of the material. The feasibility of the materials used in this investigation has already been proved in other studies [9,10]. Reliability tests such as HTRB (high temperature reverse bias) and active power cycling showed that the ceramic encapsulants are capable of enlarging the semiconductor's lifetime by a factor of 3.5 compared to softgel encapsulations. In this work, the influence of the enhanced thermal conductivity of ceramic encapsulants is investigated via thermal simulations. Based on these results a new thermal path within the conventional module structure is presented. This path is realized without compromising the module's footprint or peripherals. Furthermore, this attempt to enhance the power modules cooling efficiency is experimentally evaluated. Fig. 1. Model of an E3 frame module for transient thermal simulation; encapsulant and lid are not displayed.
2. Materials and methods 2.1. Inorganic encapsulations
FloEFD (Mentor Graphics Corp., Wilsonville, OR, USA) is used. The model for investigating the influence of different encapsulation materials with variable thermal conductivities consist of an E3 type frame module with three DBCs and a 3 mm copper baseplate that has been established for years in the industry (Fig. 1). While the DBC's layout is arbitrary, the amount of dies and all bonding and joining technologies suit a standard industrial module layout. Each IGBT is set to have a power loss density of 2 W/mm2, while the cooling is simulated via a thermal heat transfer coefficient (HTC) of 5000 W/(m2·K) at the bottom of the copper baseplate. This kind of HTC is equivalent to a direct liquid cooling in resemblance of a Danfoss ShowerPower® cooler. All other surfaces are configured to have an HTC of 5 W/(m2·K). This approach prevents adiabatic conditions and represents natural convection [12]. The transient thermal resistance (Zth,ja) of several hypothetical encapsulation materials with variable thermal conductivities are simulated. Analyzed TCs are 3, 5, 8, and 10 W/(m·K). The reference is the thermal conductivity of a standard silicone softgel of 0.17 W/(m·K) [6].
The novel material class of inorganic materials in this application is described in the following chapter. Two types of inorganic encapsulation materials are under investigation (Table 1) 2.1.1. Phosphate cement (PC) based encapsulants Conventional cement systems such as Portland cement hydrate under strong basic conditions with pH values beyond 12. All alumina compounds assembled in power modules, such as bonding wires and chip metallization are dissolved under formation of dihydrogen within such basic conditions. To circumvent such degradation mechanism, PC provides a system that hydrates under pH milieus of 5 to 7. Within the acid-base reaction given by a hydrogen phosphate salt (acid) with a metal oxide (base), a metal phosphate hydrate is formed that provides the insoluble cement phase. The main advantage of PCs besides its tunable material properties is the ambient pH-value in combination with simple application procedures, including established methods to control the cement reaction rate. Vacuum potting is used for pore free encapsulations with high bending strength and sufficient thermal conductivity.
2.3. Thermal impedance (Zth) measurement The transient thermal resistance (Zth,ja) of different specimen and encapsulation materials is the subject of this investigation. In this chapter, the methodology and equipment of the thermal impedance measurement is presented. The experiment is realized with a power cycling test bench TLW23 (Schuster Elektronik GmbH, Herzogenaurach, Germany). During a power cycling test, a periodically switched load current flows through the test specimens (semiconductor). At the end of every load pulse the semiconductor's junction temperature is measured. This means that every semiconductor experiences a static load pulse width. Consequently, the semiconductors reach the same junction temperature with every load pulse. However, to capture the Zth,ja of a semiconductor, the junction temperature has to be measured at different points in time after the load current is applied. This is achieved by modulating the load pulse width to measure the semiconductor's temperature at different time constants. The accumulation and interpolation of these specific time dependent temperatures delivers an approximation of a Zth,ja curve. Therefore, it is ensured that the heating-up characteristics are measured rather than the cooling-down characteristics. The specific semiconductor temperatures are captured via VCE method. The load pule width is modulated between 0.2 s and 1000 s. Test specimen is a 600 V IGBT (Infineon Technologies AG, Neubiberg, Germany) within a E1 type frame module (Danfoss Silicon Power GmbH, Flensburg, Germany). The modules used are
2.1.2. Alumina based ceramic encapsulants (CE) Ceramic encapsulation materials with a hydratable alumina matrix and alumina filler particles (CE) are described in detail by [9,11]. The advantages of CE consist in a high thermal conductivity above 6 W/ (m∙K) and a temperature stability up to 300 °C. Therefore, CE is capable to realize novel module concepts with efficient heat dissipation above 200 °C. With a bending strength of about 10 MPa, the adjustable CTE of 7 ppm/K and 6 W/(m∙K) thermal conductivity, the recent CE formulation already shows promising results in terms of reliability [9]. 2.2. Model for thermal simulation The influence of the encapsulant's thermal conductivity as well as a new thermal path is evaluated via transient thermal simulations. For this purpose the computational fluid dynamics (CFD) analysis tool Table 1 Properties of inorganic encapsulation materials under investigation. Property
PC
CE
CTE TC Bending strength
4–21 ppm/K 1–3 W/(m·K) 10–17 MPa
7 ppm/K 6 W/(m·K) 10 MPa
2
Microelectronics Reliability xxx (xxxx) xxxx
S. Behrendt, et al.
3.2. Thermal mass circuit - TMC
0.6 0,17 W/m*K
0.5
0.451 K/W
To improve the static behaviour and utilize the ceramic encapsulant's thermal capabilities, it is important to understand the thermal structure of the module. The current state of the art does not offer any possibility to dissipate heat energy out of the encapsulation material other than the baseplate. One way to dissipate the heat to the top side of the encapsulation would be to apply a second heatsink on top of the encapsulation to drain the accumulated heat energy. Cooling strategies similar to this, where a second heat sink is contacted to the top side of the semiconductor were subject of several other studies [13,14]. Nevertheless, this approach drastically changes the structure and peripherals of the power module. The goal is to utilize the low thermal resistivity of the ceramic encapsulation while maintaining the overall structure and footprint of a standard module. The thermal mass circuit (TMC) enables the accumulated heat to be dissipated via the standard cooling, which is already applied at the bottom of the module. An additional copper heat spreader is applied to the top of the ceramic encapsulation (Fig. 3). The additional thermal mass anticipates the vertical heat flow and removes the thermal energy from the encapsulation. Furthermore, the heat spreader is mechanically and thermally connected to the copper base plate of the module, thus interconnecting the newly developed thermal path through the ceramic encapsulation with the standard heat sink below. This increases cooling efficiency by utilizing the ceramic encapsulant's thermal potential while maintaining the assembly of a standard module with all its peripherals. Fig. 4 presents the influence of the TMC in comparison to the influence of the thermal conductivity without TMC. The simulated Zth,ja curves show a significant improvement within the dynamic as well as the static behavior of the module. In comparison to a the state of the art silicone softgel the TMC enables an Rth,ja reduction of 8.5% in the static range beyond 100 s. This is an additional reduction of 5.7% to solely an encapsulation with 10 W/ (m·K). The dynamic range is also significantly improved. With the TMC an Rth,ja reduction of 17.6%. in comparison to a silicone softgel is possible at a time constant of 7 s. Another study of CFD simulations show that the efficiency of the TMC is dependent on the encapsulant's thermal conductivity. Fig. 5 shows that the Rth,ja reduction is below 1% when using the TMC with a standard softgel (0.17 W/(m·K)). With increasing TC, more thermal energy reaches the TMC through the encapsulation, thus leaving the system. The TMC is only functional when a thermal conductive encapsulation material is used.
Rth,ja [K/W]
5 W/m*K
0.4
10 W/m*K
0.3
0.388 K/W
0.2
Zth
Rth
0.1 0 0.001
0.01
0.1
1 time [s]
10
100
1000
Fig. 2. Results from simulation of encapsulants with different thermal conductivities.
encapsulated with silicone softgel and a CE material from Bosch (Robert Bosch GmbH, Renningen, Germany), which has a TC of 6 W/(m·K). In addition to that, modules with CE material and the TMC are analyzed. The modules are mounted onto a standard cold plate cooler as a part of a temperature controlled liquid coolant loop of water-glycol. 3. Results and discussion In this chapter, the results of the thermal simulations as well as the Zth,ja measurement is presented. 3.1. Simulative influence of the encapsulation's thermal conductivity The simulations were conducted with the model described in Section 2.2. For the sake of clarity Fig. 2, only shows the reference as well as a TC of 5 and 10 W/(m·K). The Figure can be divided into two segments. The dynamic behavior (Zth) is shown in the portion up to a time constant of 100 s. Beyond this point the steady state is achieved and the static behavior of the thermal resistance (Rth) is displayed. By increasing the encapsulants thermal conductivity the dynamic behavior of the simulated module is significantly improved. With a thermal conductivity of 10 W/(m·K) a maximum Rth reduction of about 14% is achieved at a time constant of 4 s. However, the static behavior of the module is only marginally improved. Although a thermal conductivity of 10 W/(m·K) is about 60× higher than the reference, the Rth,ja is reduced by only 2.8%. The reason for this phenomenon is the absence of an opportunity for the thermal energy to be drained from the system through the top side. The enhanced thermal conductivity reduces the thermal resistance of the encapsulation and enables the heat energy to penetrate the encapsulant quicker. Consequently, the encapsulant's thermal capacity is filled quicker and the dynamic behavior is improved significantly. Once all thermal capacities are filled the heat energy has no possibility to be dissipated. The sole way of dissipation is the surface of the encapsulant, which has a very low heat transfer coefficient of natural convection. As a result, the temperature within the encapsulation equilibrates and the static behavior of the module is not improved significantly.
frame
TMC
3.3. Thermal impedance (Zth) measurement The measurement of the transient thermal resistance experimentally evaluates the influence of the TMC. The results displayed in Fig. 6 show the significant influence of the ceramic encapsulation, in particular the TMC. The state of the art softgel produces a steady state junction temperature of 128.4 °C. The CE material delivers a temperature reduction of 5.3 K in comparison with the softgel. Furthermore, the CE material in combination with the TMC
DBC with semiconductors ceramic encapsulant
screw
baseplate
cooling/heat sink
3
Fig. 3. Schematic of a conventional power module with TMC.
Microelectronics Reliability xxx (xxxx) xxxx
S. Behrendt, et al.
0.6
0.529 K/W
0,17 W/m*K
Rth,ja [K/W]
0.5
5 W/m*K 10 W/m*K
0.4
0.485 K/W
10 W/m*K + TMC
0.3 0.2
Zth
Rth
0.1 0 0.001
0.01
0.1
1 time [s]
10
100
1000
Fig. 4. Results from simulation of encapsulants with different thermal conductivities.
100 99 98 97 96 95 94 93 92 91 90
pong + TMC
on the results a new thermal path was constructively implemented in the modules structure. This so called TMC was focus of further thermal simulations (Fig. 7). The results of all simulations were evaluated experimentally via a thermal impedance measurement. The thermal simulation showed that increasing the thermal conductivity of an encapsulation improves the dynamic behavior of the Rth,ja. However the static behavior is only improved marginally. By the addition of the TMC on top of the encapsulants more thermal mass is added to the thermal stack, which anticipates vertical heat flow. The dynamic behavior of the modules Rth,ja is significantly improved by 17.6% according to the simulation. Furthermore, the thermal coupling of the TMC to the baseplate of the module enables a steady heat flow towards the cooling, reducing the module's static Rth,ja. The simulation was experimentally verified via a thermal impedance measurement of several modules with different encapsulations and TMC. A module with CE material and TMC achieved a junction temperature reduction of 12.9 K in comparison to the state of the art assembly with silicone softgel. This enhanced cooling efficiency is realized without compromising the modules footprint of general structure. The TMC concept is applicable for every standard industrial frame module. Industrial applications with high dynamic demands like motordrives for elevators, cranes and servo-drives esp. with energy recovery will strongly benefit from CE materials with TMC assemblies. The significantly reduced semiconductor temperatures result in a longer module lifetime, thus gaining a high level of reliability. This development leads to cost efficient re-fits of power modules with improved properties without design changes in the system built-up.
relative Rth,ja [%]
pong
Fig. 7. Test specimens of thermal impedance measurement; left: E1 module filled with CE material, right: E1 module filled with CE material and assembled TMC.
0,17 W/m*K
3 W/m*K
5 W/m*K
10 W/m*K
Fig. 5. Relative Rth,ja in dependency of the encapsulant's thermal conductivity and used thermal paths.
junction temperature [°C]
140 128.4
130
123.1
120
115.5
110 100
softgel CE
90
CE + TMC
80
softgel sim CE sim
70
CE + TMC sim
60 0.1
1
10 time [s]
100
1000
Fig. 6. Results of thermal impedance measurement and simulation; time constants from 0.2 s to 1000 s.
Acknowledgements The work presented received financial support by the Federal Ministry of Education and Research BMBF, Germany (Contract: 16EMO0226).
reduces the semiconductor's temperature by additional 7.6 K in comparison with solely CE. With a junction temperature of 115.5 °C, the TMC reduces the semiconductor's temperature by 12.9 K in comparison with the state of the art. These results correlate with the results gathered by the thermal simulations with an average deviation of about 2 K.
References [1] R. Eisele, J. Rudzki, M. Kock, Pressure sintering for thermal stack assembly, PCIM 2007, Nuremberg, Germany, 2007. [2] P. Beckedahl, M. Hermann, M. Kind, M. Knebel, J. Nascimento, A. Wintrich, Performance comparison of traditional packaging technologies to a novel bond wire less all sintered power module, PCIM 2011, Nuremberg, Germany, 2011. [3] S. Behrendt, R. Eisele, K. Andersen, T. Xu, C. Luechinger, M. Becker, R. Woehl, D. Benning, F. Osterwald, A. Abibe, Feasibility of copper-based ribbon bonding as an assembly method for advanced power modules, PCIM 2018, Nuremberg, Germany, 2018. [4] A. Mavinkurve, L. Goumans, J. Martens, Epoxy molding compounds for high temperature applications, EMPC 2013, Grenoble, France, 2013.
4. Conclusions This study investigates thermal conductivity's influence of encapsulation materials for power electronic modules. Several materials with different thermal conductivities have been compared with regard to the semiconductors temperature in the dynamic as well as the static range. This was achieved by thorough thermal CFD simulations. Based 4
Microelectronics Reliability xxx (xxxx) xxxx
S. Behrendt, et al.
[5] A.G. Wacker Chemie, Silicone Gel Solutions, (2019). [6] Momentive Performance Materials, Inc., TSE 3062 – Technical Data Sheet, (2007). [7] Zeng J, Fu R, Shen Y, He H, Song X. High thermal conductive epoxy molding compound with thermal conductive pathway. Journal of Applied Polymer Science. J APPL POLYM SCI. 113 https://doi.org/10.1002/app.30045. 2117–2125. [8] L. Nguyen, M. Shibuya, High thermal conductivity Mold compounds for advanced packaging applications, ECTC 2017, Lake Buena Vista, FL, 2017. [9] S. Kaessner, M.G. Scheibel, S. Behrendt, B. Boettge, C. Berthold, K.G. Nickel, Reliability of novel ceramic encapsulation materials for electronic packaging, J. Microelectron. Electron. Packag. 15 (H. 3) (2018) 132–139. [10] B. Boettge, F. Naumann, S. Behrendt, M.G. Scheibel, S. Kaessner, S. Klengel, M. Petzold, K.G. Nickel, G. Hejtmann, A.Z. Miric, R. Eisele, Material
[11] [12] [13]
[14]
5
characterization of advanced cement-based encapsulation systems for efficient power electronics with increased power density, ECTC 2018, San Diego, CA, 2018. S. Kaessner, Thermisch effiziente Keramik-Komposite für die Verkapselung elektronischer Bauelemente, submitted PhD-thesis University of Tuebingen, 2019. Poech M H. Thermisches Management auf Bauelemente Level. Workshop documents, Fraunhofer ISiT, (2008). J. Favre, J.-M. Reynes, J.-P. Fradin, C. Cadile, S. Sanchez, D. Elzo, E. Marcault, A double side cooled electronic power module, PCIM 2017, 2017 Nuremberg, Germany. M. Schneider-Ramelow, T. Baumann, E. Hoene, Double-Side Cooled Semiconductors for Automotive Applications, (2019).
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Implementation of a new thermal path within the structure of inorganic encapsulated power modules ⁎
S. Behrendta, , R. Eiselea, M.G. Scheibelb, S. Kaessnerc a
FuE-Zentrum Fachhochschule Kiel GmbH, 24149 Kiel, Germany Heraeus Deutschland GmbH & Co. KG, 63450 Hanau, Germany c Robert Bosch GmbH, 71272 Renningen, Germany b
A B S T R A C T
The demands on power modules regarding power density and reliability are continuously rising. Conventional encapsulation materials are limited in their thermal performance. In this study, a completely new material class is used within a power module. This new class is the group of inorganic materials, specifically ceramic encapsulation materials. The use of ceramic encapsulation materials generates new possibilities for optimizing the heat dissipation within power modules. In comparison to conventional encapsulation materials, ceramic composites can dissipate the semiconductor's heat loss much more efficient due to its enhanced thermal conductivity. This improves the transient thermal resistance of the module (Zth). Furthermore, an additional copper layer (TMC - Thermal mass circuit), which is connected to the heatsink below, is applied to the top of the encapsulation. This opens up a new thermal path in the module without changing its footprint or overall structure. In particular, the geometrically difficult area above the on-chip contacts can be used completely for cooling for the first time. Experiments show that the combination of a ceramic encapsulation and the TMC structure can reduce the semiconductor's temperature by more than 12 K. This enables a higher lifetime, efficiency and reliability of power modules with high energy density.
1. Introduction The constantly rising demands with regard to power density and compactness make the packaging of power electronic modules an increasingly challenging task. New die attach technologies like silver sintering extend the semiconductor's lifetime, thus enhancing the reliability of power modules [1,2]. Another example of a new packaging technology is copper ribbon bonding in combination with the Heraeus Die Top System (DTS®). This top side technology extends the semiconductors lifetime by a factor 16.6 in comparison to conventional aluminium ribbon bonding. In addition, the failure mechanism shifts from a top-side contact failure to a die metallization failure [3]. However, the currently used encapsulation materials are still a limiting factor when it comes to carrying the high amount of power density that is demanded in today's power applications. High currents are accompanied by high power losses, which are converted into a high amount of heat. This heat loss needs to be dissipated as efficient as possible to ensure a high level of reliability. Heat dissipation depends on the thermal structure (geometric design and arrangement) as well as the materials used and their properties. The encapsulation materials used today, which mainly consist of polymers, are strongly limited with regard to their thermal performance, resp. thermal expansion and thermal conductivity. ⁎
Conventional encapsulation materials can be divided into two categories dependent on the module structure they are used in. On one hand, there are the silicone softgels, which are used in conventional frame modules. On the other hand, there are the epoxy based molding compounds (EMC), which are used in mold modules. Both types of encapsulation material have a very low thermal conductivity (TC) and a low continuous operating temperature. Especially EMCs can only be used at temperatures up to about 175 °C. At higher temperatures the material starts to deteriorate [4]. Furthermore, the coefficient of thermal expansion (CTE) is very high and leads to a CTEmismatch between the materials within the module. Consequently, a high amount of thermally induced mechanical tension is the result, which leads to failures within the module structure. Another disadvantage is the thermal performance of those materials. While silicone softgels have almost no thermal conductivity (~0.2 W/(m·K)), EMCs reach thermal conductivities of about 2 to 3 W/ (m·K) [5–7]. Such high values can only be achieved by a high level (90 wt%) of inorganic fillers within the epoxy matrix such as SiO2 or Si3N4 [8]. Due to the high filler ratio, the viscosity of the EMC compound is increased and therefore very difficult to process. High temperature and pressure are necessary to achieve a successful molding process. Thus the material, tools and process are cost intensive. To overcome these limitations the use of novel encapsulation materials is necessary. Ceramic encapsulants offer a high TC and an
Corresponding author. E-mail address: [email protected] (S. Behrendt).
https://doi.org/10.1016/j.microrel.2019.113430 Received 14 May 2019; Accepted 12 July 2019 0026-2714/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: S. Behrendt, et al., Microelectronics Reliability, https://doi.org/10.1016/j.microrel.2019.113430
Microelectronics Reliability xxx (xxxx) xxxx
S. Behrendt, et al.
intrinsic low CTE. Ceramic encapsulants have a high thermal conductivity due to their inorganic structure. The CTE can be adjusted within a certain range to fit the assembly the encapsulant shall encase. This adjustment is possible without compromising the processability of the material. The feasibility of the materials used in this investigation has already been proved in other studies [9,10]. Reliability tests such as HTRB (high temperature reverse bias) and active power cycling showed that the ceramic encapsulants are capable of enlarging the semiconductor's lifetime by a factor of 3.5 compared to softgel encapsulations. In this work, the influence of the enhanced thermal conductivity of ceramic encapsulants is investigated via thermal simulations. Based on these results a new thermal path within the conventional module structure is presented. This path is realized without compromising the module's footprint or peripherals. Furthermore, this attempt to enhance the power modules cooling efficiency is experimentally evaluated. Fig. 1. Model of an E3 frame module for transient thermal simulation; encapsulant and lid are not displayed.
2. Materials and methods 2.1. Inorganic encapsulations
FloEFD (Mentor Graphics Corp., Wilsonville, OR, USA) is used. The model for investigating the influence of different encapsulation materials with variable thermal conductivities consist of an E3 type frame module with three DBCs and a 3 mm copper baseplate that has been established for years in the industry (Fig. 1). While the DBC's layout is arbitrary, the amount of dies and all bonding and joining technologies suit a standard industrial module layout. Each IGBT is set to have a power loss density of 2 W/mm2, while the cooling is simulated via a thermal heat transfer coefficient (HTC) of 5000 W/(m2·K) at the bottom of the copper baseplate. This kind of HTC is equivalent to a direct liquid cooling in resemblance of a Danfoss ShowerPower® cooler. All other surfaces are configured to have an HTC of 5 W/(m2·K). This approach prevents adiabatic conditions and represents natural convection [12]. The transient thermal resistance (Zth,ja) of several hypothetical encapsulation materials with variable thermal conductivities are simulated. Analyzed TCs are 3, 5, 8, and 10 W/(m·K). The reference is the thermal conductivity of a standard silicone softgel of 0.17 W/(m·K) [6].
The novel material class of inorganic materials in this application is described in the following chapter. Two types of inorganic encapsulation materials are under investigation (Table 1) 2.1.1. Phosphate cement (PC) based encapsulants Conventional cement systems such as Portland cement hydrate under strong basic conditions with pH values beyond 12. All alumina compounds assembled in power modules, such as bonding wires and chip metallization are dissolved under formation of dihydrogen within such basic conditions. To circumvent such degradation mechanism, PC provides a system that hydrates under pH milieus of 5 to 7. Within the acid-base reaction given by a hydrogen phosphate salt (acid) with a metal oxide (base), a metal phosphate hydrate is formed that provides the insoluble cement phase. The main advantage of PCs besides its tunable material properties is the ambient pH-value in combination with simple application procedures, including established methods to control the cement reaction rate. Vacuum potting is used for pore free encapsulations with high bending strength and sufficient thermal conductivity.
2.3. Thermal impedance (Zth) measurement The transient thermal resistance (Zth,ja) of different specimen and encapsulation materials is the subject of this investigation. In this chapter, the methodology and equipment of the thermal impedance measurement is presented. The experiment is realized with a power cycling test bench TLW23 (Schuster Elektronik GmbH, Herzogenaurach, Germany). During a power cycling test, a periodically switched load current flows through the test specimens (semiconductor). At the end of every load pulse the semiconductor's junction temperature is measured. This means that every semiconductor experiences a static load pulse width. Consequently, the semiconductors reach the same junction temperature with every load pulse. However, to capture the Zth,ja of a semiconductor, the junction temperature has to be measured at different points in time after the load current is applied. This is achieved by modulating the load pulse width to measure the semiconductor's temperature at different time constants. The accumulation and interpolation of these specific time dependent temperatures delivers an approximation of a Zth,ja curve. Therefore, it is ensured that the heating-up characteristics are measured rather than the cooling-down characteristics. The specific semiconductor temperatures are captured via VCE method. The load pule width is modulated between 0.2 s and 1000 s. Test specimen is a 600 V IGBT (Infineon Technologies AG, Neubiberg, Germany) within a E1 type frame module (Danfoss Silicon Power GmbH, Flensburg, Germany). The modules used are
2.1.2. Alumina based ceramic encapsulants (CE) Ceramic encapsulation materials with a hydratable alumina matrix and alumina filler particles (CE) are described in detail by [9,11]. The advantages of CE consist in a high thermal conductivity above 6 W/ (m∙K) and a temperature stability up to 300 °C. Therefore, CE is capable to realize novel module concepts with efficient heat dissipation above 200 °C. With a bending strength of about 10 MPa, the adjustable CTE of 7 ppm/K and 6 W/(m∙K) thermal conductivity, the recent CE formulation already shows promising results in terms of reliability [9]. 2.2. Model for thermal simulation The influence of the encapsulant's thermal conductivity as well as a new thermal path is evaluated via transient thermal simulations. For this purpose the computational fluid dynamics (CFD) analysis tool Table 1 Properties of inorganic encapsulation materials under investigation. Property
PC
CE
CTE TC Bending strength
4–21 ppm/K 1–3 W/(m·K) 10–17 MPa
7 ppm/K 6 W/(m·K) 10 MPa
2
Microelectronics Reliability xxx (xxxx) xxxx
S. Behrendt, et al.
3.2. Thermal mass circuit - TMC
0.6 0,17 W/m*K
0.5
0.451 K/W
To improve the static behaviour and utilize the ceramic encapsulant's thermal capabilities, it is important to understand the thermal structure of the module. The current state of the art does not offer any possibility to dissipate heat energy out of the encapsulation material other than the baseplate. One way to dissipate the heat to the top side of the encapsulation would be to apply a second heatsink on top of the encapsulation to drain the accumulated heat energy. Cooling strategies similar to this, where a second heat sink is contacted to the top side of the semiconductor were subject of several other studies [13,14]. Nevertheless, this approach drastically changes the structure and peripherals of the power module. The goal is to utilize the low thermal resistivity of the ceramic encapsulation while maintaining the overall structure and footprint of a standard module. The thermal mass circuit (TMC) enables the accumulated heat to be dissipated via the standard cooling, which is already applied at the bottom of the module. An additional copper heat spreader is applied to the top of the ceramic encapsulation (Fig. 3). The additional thermal mass anticipates the vertical heat flow and removes the thermal energy from the encapsulation. Furthermore, the heat spreader is mechanically and thermally connected to the copper base plate of the module, thus interconnecting the newly developed thermal path through the ceramic encapsulation with the standard heat sink below. This increases cooling efficiency by utilizing the ceramic encapsulant's thermal potential while maintaining the assembly of a standard module with all its peripherals. Fig. 4 presents the influence of the TMC in comparison to the influence of the thermal conductivity without TMC. The simulated Zth,ja curves show a significant improvement within the dynamic as well as the static behavior of the module. In comparison to a the state of the art silicone softgel the TMC enables an Rth,ja reduction of 8.5% in the static range beyond 100 s. This is an additional reduction of 5.7% to solely an encapsulation with 10 W/ (m·K). The dynamic range is also significantly improved. With the TMC an Rth,ja reduction of 17.6%. in comparison to a silicone softgel is possible at a time constant of 7 s. Another study of CFD simulations show that the efficiency of the TMC is dependent on the encapsulant's thermal conductivity. Fig. 5 shows that the Rth,ja reduction is below 1% when using the TMC with a standard softgel (0.17 W/(m·K)). With increasing TC, more thermal energy reaches the TMC through the encapsulation, thus leaving the system. The TMC is only functional when a thermal conductive encapsulation material is used.
Rth,ja [K/W]
5 W/m*K
0.4
10 W/m*K
0.3
0.388 K/W
0.2
Zth
Rth
0.1 0 0.001
0.01
0.1
1 time [s]
10
100
1000
Fig. 2. Results from simulation of encapsulants with different thermal conductivities.
encapsulated with silicone softgel and a CE material from Bosch (Robert Bosch GmbH, Renningen, Germany), which has a TC of 6 W/(m·K). In addition to that, modules with CE material and the TMC are analyzed. The modules are mounted onto a standard cold plate cooler as a part of a temperature controlled liquid coolant loop of water-glycol. 3. Results and discussion In this chapter, the results of the thermal simulations as well as the Zth,ja measurement is presented. 3.1. Simulative influence of the encapsulation's thermal conductivity The simulations were conducted with the model described in Section 2.2. For the sake of clarity Fig. 2, only shows the reference as well as a TC of 5 and 10 W/(m·K). The Figure can be divided into two segments. The dynamic behavior (Zth) is shown in the portion up to a time constant of 100 s. Beyond this point the steady state is achieved and the static behavior of the thermal resistance (Rth) is displayed. By increasing the encapsulants thermal conductivity the dynamic behavior of the simulated module is significantly improved. With a thermal conductivity of 10 W/(m·K) a maximum Rth reduction of about 14% is achieved at a time constant of 4 s. However, the static behavior of the module is only marginally improved. Although a thermal conductivity of 10 W/(m·K) is about 60× higher than the reference, the Rth,ja is reduced by only 2.8%. The reason for this phenomenon is the absence of an opportunity for the thermal energy to be drained from the system through the top side. The enhanced thermal conductivity reduces the thermal resistance of the encapsulation and enables the heat energy to penetrate the encapsulant quicker. Consequently, the encapsulant's thermal capacity is filled quicker and the dynamic behavior is improved significantly. Once all thermal capacities are filled the heat energy has no possibility to be dissipated. The sole way of dissipation is the surface of the encapsulant, which has a very low heat transfer coefficient of natural convection. As a result, the temperature within the encapsulation equilibrates and the static behavior of the module is not improved significantly.
frame
TMC
3.3. Thermal impedance (Zth) measurement The measurement of the transient thermal resistance experimentally evaluates the influence of the TMC. The results displayed in Fig. 6 show the significant influence of the ceramic encapsulation, in particular the TMC. The state of the art softgel produces a steady state junction temperature of 128.4 °C. The CE material delivers a temperature reduction of 5.3 K in comparison with the softgel. Furthermore, the CE material in combination with the TMC
DBC with semiconductors ceramic encapsulant
screw
baseplate
cooling/heat sink
3
Fig. 3. Schematic of a conventional power module with TMC.
Microelectronics Reliability xxx (xxxx) xxxx
S. Behrendt, et al.
0.6
0.529 K/W
0,17 W/m*K
Rth,ja [K/W]
0.5
5 W/m*K 10 W/m*K
0.4
0.485 K/W
10 W/m*K + TMC
0.3 0.2
Zth
Rth
0.1 0 0.001
0.01
0.1
1 time [s]
10
100
1000
Fig. 4. Results from simulation of encapsulants with different thermal conductivities.
100 99 98 97 96 95 94 93 92 91 90
pong + TMC
on the results a new thermal path was constructively implemented in the modules structure. This so called TMC was focus of further thermal simulations (Fig. 7). The results of all simulations were evaluated experimentally via a thermal impedance measurement. The thermal simulation showed that increasing the thermal conductivity of an encapsulation improves the dynamic behavior of the Rth,ja. However the static behavior is only improved marginally. By the addition of the TMC on top of the encapsulants more thermal mass is added to the thermal stack, which anticipates vertical heat flow. The dynamic behavior of the modules Rth,ja is significantly improved by 17.6% according to the simulation. Furthermore, the thermal coupling of the TMC to the baseplate of the module enables a steady heat flow towards the cooling, reducing the module's static Rth,ja. The simulation was experimentally verified via a thermal impedance measurement of several modules with different encapsulations and TMC. A module with CE material and TMC achieved a junction temperature reduction of 12.9 K in comparison to the state of the art assembly with silicone softgel. This enhanced cooling efficiency is realized without compromising the modules footprint of general structure. The TMC concept is applicable for every standard industrial frame module. Industrial applications with high dynamic demands like motordrives for elevators, cranes and servo-drives esp. with energy recovery will strongly benefit from CE materials with TMC assemblies. The significantly reduced semiconductor temperatures result in a longer module lifetime, thus gaining a high level of reliability. This development leads to cost efficient re-fits of power modules with improved properties without design changes in the system built-up.
relative Rth,ja [%]
pong
Fig. 7. Test specimens of thermal impedance measurement; left: E1 module filled with CE material, right: E1 module filled with CE material and assembled TMC.
0,17 W/m*K
3 W/m*K
5 W/m*K
10 W/m*K
Fig. 5. Relative Rth,ja in dependency of the encapsulant's thermal conductivity and used thermal paths.
junction temperature [°C]
140 128.4
130
123.1
120
115.5
110 100
softgel CE
90
CE + TMC
80
softgel sim CE sim
70
CE + TMC sim
60 0.1
1
10 time [s]
100
1000
Fig. 6. Results of thermal impedance measurement and simulation; time constants from 0.2 s to 1000 s.
Acknowledgements The work presented received financial support by the Federal Ministry of Education and Research BMBF, Germany (Contract: 16EMO0226).
reduces the semiconductor's temperature by additional 7.6 K in comparison with solely CE. With a junction temperature of 115.5 °C, the TMC reduces the semiconductor's temperature by 12.9 K in comparison with the state of the art. These results correlate with the results gathered by the thermal simulations with an average deviation of about 2 K.
References [1] R. Eisele, J. Rudzki, M. Kock, Pressure sintering for thermal stack assembly, PCIM 2007, Nuremberg, Germany, 2007. [2] P. Beckedahl, M. Hermann, M. Kind, M. Knebel, J. Nascimento, A. Wintrich, Performance comparison of traditional packaging technologies to a novel bond wire less all sintered power module, PCIM 2011, Nuremberg, Germany, 2011. [3] S. Behrendt, R. Eisele, K. Andersen, T. Xu, C. Luechinger, M. Becker, R. Woehl, D. Benning, F. Osterwald, A. Abibe, Feasibility of copper-based ribbon bonding as an assembly method for advanced power modules, PCIM 2018, Nuremberg, Germany, 2018. [4] A. Mavinkurve, L. Goumans, J. Martens, Epoxy molding compounds for high temperature applications, EMPC 2013, Grenoble, France, 2013.
4. Conclusions This study investigates thermal conductivity's influence of encapsulation materials for power electronic modules. Several materials with different thermal conductivities have been compared with regard to the semiconductors temperature in the dynamic as well as the static range. This was achieved by thorough thermal CFD simulations. Based 4
Microelectronics Reliability xxx (xxxx) xxxx
S. Behrendt, et al.
[5] A.G. Wacker Chemie, Silicone Gel Solutions, (2019). [6] Momentive Performance Materials, Inc., TSE 3062 – Technical Data Sheet, (2007). [7] Zeng J, Fu R, Shen Y, He H, Song X. High thermal conductive epoxy molding compound with thermal conductive pathway. Journal of Applied Polymer Science. J APPL POLYM SCI. 113 https://doi.org/10.1002/app.30045. 2117–2125. [8] L. Nguyen, M. Shibuya, High thermal conductivity Mold compounds for advanced packaging applications, ECTC 2017, Lake Buena Vista, FL, 2017. [9] S. Kaessner, M.G. Scheibel, S. Behrendt, B. Boettge, C. Berthold, K.G. Nickel, Reliability of novel ceramic encapsulation materials for electronic packaging, J. Microelectron. Electron. Packag. 15 (H. 3) (2018) 132–139. [10] B. Boettge, F. Naumann, S. Behrendt, M.G. Scheibel, S. Kaessner, S. Klengel, M. Petzold, K.G. Nickel, G. Hejtmann, A.Z. Miric, R. Eisele, Material
[11] [12] [13]
[14]
5
characterization of advanced cement-based encapsulation systems for efficient power electronics with increased power density, ECTC 2018, San Diego, CA, 2018. S. Kaessner, Thermisch effiziente Keramik-Komposite für die Verkapselung elektronischer Bauelemente, submitted PhD-thesis University of Tuebingen, 2019. Poech M H. Thermisches Management auf Bauelemente Level. Workshop documents, Fraunhofer ISiT, (2008). J. Favre, J.-M. Reynes, J.-P. Fradin, C. Cadile, S. Sanchez, D. Elzo, E. Marcault, A double side cooled electronic power module, PCIM 2017, 2017 Nuremberg, Germany. M. Schneider-Ramelow, T. Baumann, E. Hoene, Double-Side Cooled Semiconductors for Automotive Applications, (2019).