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Investigation of heat transfer and thermal stresses of novel thermal management system integrated with vapour chamber for IGBT power module
Thermal Science and Engineering Progress 10 (2019) 73–81
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Thermal Science and Engineering Progress journal homepage: www.elsevier.com/locate/tsep
Investigation of heat transfer and thermal stresses of novel thermal management system integrated with vapour chamber for IGBT power module
T
Yiyi Chena, Bo Lia, Xin Wanga, Yuying Yana, , Yangang Wangb, Fang Qib ⁎
a b
Faculty of Engineering, University of Nottingham, NG7 2RD Nottingham, UK Dynex Semiconductor Limited, Doddington Road, Lincoln LN6 3LF, UK
ARTICLE INFO
ABSTRACT
Keywords: IGBT power electronics Electric vehicles Phase change cooling Vapour chamber Thermal resistance Thermal stress Thermal fatigue
Thermal stress in IGBT power module can lead to sever thermal reliability problems such as module deformation, performance degradation and even permanent damage. So, it is important to develop innovative and efficient IGBT cooling technologies. In this paper, a novel thermal management system is developed for cooling IGBT power module. The module is integrated with a vapour chamber-based heat sink to reduce thermal resistance and improve temperature uniformity significantly. 3D FEM modelling is conducted to investigate the effect of vapour chamber on temperature distribution, thermal stress, energy strain dissipation density and lifetime under power cycle. The simulation results show that the proposed thermal management system is superior to traditional cooling solution regarding cooling capacity, thermal stress, creep and plastic strain energy dissipation and thermal fatigue life. The study of failure mechanism of solder layer under power cycling suggests that creep causes the main is damage in the power cycling and cracks induced by thermal loading can be expected to initiate at the edge.
1. Introduction Much effort has been made to exploit sustainable and clean energy for mitigating the global crisis of fossil energy and environmental problems. Popularization of hybrid electric vehicles and electric vehicles will become an unstoppable trend in next decades. As a semiconductor switching device, insulated gate bipolar transistor (IGBT) power module is one of the most important components in the system of power conversion and motor control. Recently, IGBT modules are required to be mounted on underhood or directly on an engine. The increased power rating and more compact structure as well as high integration and miniaturization of a vehicle will all require IGBT power to dissipate more heat within a limited space. For example, in next generation of hybrid electric vehicles, heat flux of IGBT module will increase from 100 to 150 W/cm2 to 500 W/cm2, but the temperature of entire module must be kept below the maximum junction temperature of 150 °C or 175 °C [1,2]. Over-high temperature and large temperature gradients can cause a variety of module failure including solder delamination and crack, ceramic and silicon chip crack propagation as well as bonding wire lift out. Therefore, there is an urgent demand for development of advanced thermal management systems for IGBT power
⁎
modules. It is hard to provide sufficient cooling for a high-power and heat flux IGBT module with air cooling, especially when the power dissipation exceeds about 1500 W [3]. Micro, mini-channel and pin–fin liquid cooling solutions can offer a higher cooling capacity typically 120 W/ cm2 in comparison with that of air cooling (typically 50 W/cm2) [4]. Oguntala et al. [5] investigated the effects of particle deposition on the thermal performance of porous fin heat sink of an electronic component. Koo et al. [6] theoretically studied the thermal performance of a microchannel for cooling of three dimensional electronic circuit. However, the flow redistribution results in a non-uniform temperature distribution. In the strategy of jet impingement, an evaporative coolant is sprayed on to a flat surface to be cooled. Jörg et al. [7] designed a direct single impinging jet for cooling a MOSFET power electronic module. Heat transfer rate can be achieved up to 12,000 W/m2K. However, cooling performance rapidly degrades as it departs the central of the jet region, resulting in temperature non-uniformity across the surface being cooled [8]. Therefore, much attention has been paid to array impingement cooling to improve temperature uniformity as well as heat transfer coefficient. When water is used as the coolant, heat flux level of 250 W/cm2 and 1000 W/cm2 can be obtained with single-phase
Corresponding author. E-mail address: [email protected] (Y. Yan).
https://doi.org/10.1016/j.tsep.2019.01.007 Received 31 March 2018; Received in revised form 26 November 2018; Accepted 3 January 2019 2451-9049/ © 2019 Elsevier Ltd. All rights reserved.
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Nomenclature A Ac B C Cp ETkin K L m n Nf q ΔQ r R T t Wf′ σ σshift
ΔT ΔWcr ΔWin ΔWpl :
creep rate coefficient (s−1) cross section area (m2) 1/effective stress (1/MPa) fourth order elasticity tensor heat capacity (J/kg/K) kinematic tangent modulus thermal conductivity (W/mK) thickness (m) fatigue energy exponent Garofalo n parameter fatigue cycles heat transfer rate (W) activation energy (J/mol) thermal resistance (K/W) gas constant (J/mol.K) temperature (oC) time (s) fatigue energy coefficient stress (Pa) kinematic hardening shift stress (Pa)
Temperature difference (oC) creep strain energy density (MJ/m3) inelastic strain energy density (MJ/m3) plastic strain energy density (MJ/m3) double-dot tensor product
Greek letters ε εcr εel εin ρ
strain equivalent uniaxial creep strain elastic strain inelastic strain density (kg/m3)
Subscripts c cr el f in Tkin
and phase change impingement [4,8,9]. Although jet impingement can achieve a very high cooling capacity, the applicability of this strategy is limited due to high cost, complex cooling flow redistribution, cooling loop leakage and channel blockage. As super heat conductive devices with phase change, heat pipe has been applied widely to improve the cooling capacity of high power electronic module [10]. Avenas et al. [11] developed a power chip module with vapour chamber heat spread. Ivanova et al. [12] proposed that heat pipe can be directly integrated in DBC for cooling of power electronics packaging. Fig. 1 illustrates the structures of three different thermal management for IGBT module. Case A represents conventional indirect cooling. The DBC layer is soldered on a copper baseplate first and then bolt to a heat sink with grease-based thermal interface material (TIM). TIM accounts for a great part of total thermal resistance so it becomes a dominating obstacle in improving cooling capacity. Case B shows a cross section of an assembly using direct attach method. DBC layer is directly soldered on a copper baseplate. Currently, this direct cooling method is most used due to its compact structure and avoiding using any kind of thermal interface material. Fig. 1 (c) shows the novel design in this study. The DBC layer is directly soldered on a vapour chamber-based heat sink. Also, no thermal interfacial material is used and the structure becomes more compact. In this paper, finite element method (FEM) simulation is used to investigate thermal-mechanical performance and reliability under power cycling. It allows us to estimate fatigue life which is a major
cross section creep elestic fatigue inplastic kinematic tangent
challenge for the electronic area. In electronic power module system, near-eutectic Sn-Ag-Cu and Sn-Ag materials are widely used to joint semiconductor on substrate for providing thermal, mechanical and electrical connection. It is well known that solder layer is the weakest part and break of which leads to failure of electronic power module at high operation temperature. This is because the solders are prone to creep and the accumulation of plastic strain triggers crack and propagation [13]. Moreover, Sn-3.5Ag solder has lower melting temperature and yield stress and higher coefficient of thermal expansion compared with other packaging layers, which will deteriorate mechanical performance of solder subjected to power cycling. In finite element simulation, a coupled thermo-mechanical model was applied in transient condition. Strain range and energy density are used as the failure parameters. An energy-based model is used to correlate crack initiation and propagation through solder from fracture mechanics approach. The simulation results reveal temperature, elastic-plastic and creep behaviour, deformation and lifetime prediction of Sn-3.5Ag solder joints exposed to power cycling condition. 2. IGBT module description 2.1. Module components A schematic of the IGBT power module is given by Fig. 2. The module is composed of six pairs of IGBT chip and diode, Direct Bond
Fig. 1. Structure of IGBT (a) with traditional indirect cooling (b) with direct cooling attached with copper baseplate IGBT module (c) with direct cooling attached with vapour chamber baseplate. 74
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Fig. 2. Schematic of the analysed power module.
Copper (DBC), solder, copper or vapour chamber. The DBC layer is made of two copper layers bonded on an aluminium oxidation (Al2 O3 ) layer. The Al2 O3 functions as heat conductor and electrical insulator due to its good thermal conductivity and electrical insulation. The chip solder has the same size with that of IGBT chips and diodes. Table 1 summarizes sizes of main components and Table 2 shows the material physical properties of each layer.
is assumed to be elastic-plastic because the stress within DCB substrate exceeds its yield stress easily. The Sn-3.5Ag solder layer is assume to viscoplastic with properties of hardening plasticity and implicit creep as it is widely agreed that rate of dependent plasticity creep occurs in the solder over time[17]. To achieve such effect, Garofalo hyperbolic sine law is applied to the solder layer.
d =A(sinhB ) ne dt
2.2. IGBT parameters
2.3. Initial design of IGBT integreted with thermal management Figs. 1c and 4 show the novel design of IGBT module integrated with a vapour chamber. Different from traditional thermal management system, baseplate and thermal paste are not used. The Direct Copper Bond (DCB) layer is directly soldered on lid of vapour chamber. A water cooling plate is bolted to vapour chamber together with IGBT power module. Fig. 1c shows traditional IGBT module (case A) thermal resistance stack in IGBT module. Thermal resistance of each layer is descripted by:
total
Shift
3.1. Finite element method (FEM) model Firstly, a coupled thermo-mechanical model is applied in transient condition. The governing equations are given as,
0
= C: (
0
)
+
p
+
(5)
cr
q s
(6)
=
E Tkin 1
ETkin E
2 3
p
(7)
E Tkin is kinematic tangent modulus. Both kinematic and mixed hardening result in a so-called back stress or shift stress, which is a new stress level that is equally far from yielding in tension and compression. Mechanical properties of packaging material and chip solder material are listed in Table 3. The thermal fatigue lifetime modules for prediction of lifetime of solder include stress, strain, energy and damaged based models. Energy based model are the most convenient and accurate method due to the ability to obtain test conditions more precisely [19,20]. Therefore, energy based model and Morrow model [21] are used to assess the lifetime of Sn-3.5Ag solder layer under power cycle in this study. Fatigue damage parameters such as inelastic strain and inelastic strain energy density and energy-based model are expressed as follow.
3. Methodology
(k T) = Q + Qted
el
Kinematics hardening plastics model is given as,
(1)
T + Cp u T t
=
=
p
where L is the thickness of material layer, K is the thermal conductivity and Ac is the cross-sectional area. As shown in Fig. 5, thermal paste and cast cold plate contribute the major thermal resistance, which account for 29% and 24% of overall module respectively. Therefore, the removal of those should bring a significant decrease in total resistance.
Cp
(4)
The cr is the equivalent uniaxial creep strain, is the equivalent tensile stress, T is absolute temperature, R is the gas constant; A, B, n, ΔQ are creep rate coefficient, 1/effective stress, Garofalo n parameter and activation energy respectively[18]. Also, the creep analysis uses an implicit time integration method, which includes temperature - dependent constants, together with modelling of creep and kinematics hardening plasticity. In this case, the total strain can be expressed as,
Power applied to the IGBT chips and diodes is 97.83 W (high state) and 25.17 W (low state) per cycle. As suggested by Xu [14], IGBT cycle is six times longer than diode power cycle in hybrid electrical vehicle application. Fig. 3 shows the power cycle applied to the IGBT module.
L r= KA c
Q RT
in
(2)
=
pl
+
(8)
cr
Table 1 Component sizes.
(3)
Eq. (2) is energy equation, where T is temperature and Q is heat generation. Eq. (3) is the Hooke’s law relating the stress-stress of material, where is stress tensor, is strain tensor, C is the fourth order elasticity tensor and “:” stands for the double-dot tensor product [16]. In solid mechanics model, semiconductor silicon and aluminium oxide in DCB substrate are assumed to be a linear-elastic model. Copper 75
Component
Dimensions
Silicon chips Silicon diodes DCB copper layer DCB Aluminium oxidation
9.718 mm*3.342 mm*0.02 mm 11.146 mm*7.746 mm*0.02 mm 47.14 mm*44.35 mm*0.3 mm 48.74 mm*46.48 mm*0.635 mm
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Table 2 Material property.
Al2O3 Cu Vapour chamber Al Silicon Sn-3.5Ag Sn-Pb
Table 3 Viscoplastic properties of Sn-Ag solder. Thermal capacity J/ (kg·K)
Thermal conductivity W/ (m·K)
Coefficient of thermal expansion (10−6/K)
730 320 385 904 678 240 242
35 413 5000 237 71 52 57
6.5 17 17 23.1 2.1 25 19
A (1/s)
B (Mpa)
n
ΔQ (J/mol)
R (J/molK)
ETkin (MPa)
44,100
0.005
4.2
44,995
8.314
4.9 × 109
Fig. 6. Mesh detail of the module.
Win = Wpl + Wcr Win = W f (2Nf ) m
(9) (10)
ΔWin, ΔWpl and ΔWc are inelastic, plastic and creep strain energy density respectively. εin, εpl and εcr are inelastic, plastic and creep strain respectively. Wf′ is fatigue energy coefficient, Nf is fatigue cycles and m is fatigue energy exponent with value of −0.69. 3.2. Boundary condition The simulation is conducted by Comsol Multiphysics software [22]. It is found that the simulation results of temperature distribution keep almost unchanged after the density exceeds 222,646. Therefore, mesh elements of 222,646 is selected in this study. Fig. 6 shows the detail of final mesh of module. The size of mesh ranged between 2 mm and 0.5 mm. Thin and weak components such as solder layers, chips and DBC are fined meshed whereas baseplate are coarse meshed. In this thermo-mechanical coupling model, heat sources are generated from IGBTs and diodes, which is estimated by power loss of these two components. Power loss is mainly caused by conduction losses from chip and collector current and switching loss. Total power losses are applied to each IGBT and diode are 96.83 W and 26.17 W respectively. Also, convection heat transfer is added to this model. The ambient temperature is initially set to 20 °C and heat transfer coefficient of air is 5 W/m2K. To analyse thermal performance of each module, the heat transfer coefficient of cooling solutions is set to 1000 W/m2K according to the experiment. Table 4 summarizes basic thermal parameters used in modelling. Solid mechanics is applied to evaluate displacement
Fig. 3. IGBT power loss cycle.
Fig. 4. Explosion view of thermal management system.
Fig. 5. Approximate thermal resistances stack in traditional IGBT module [15]. 76
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Table 4 Boundary condition in IGBT modelling. Boundary condition
Value
Heat source from diode Heat source from IGBT Ambient air temperature Coolant temperature Heat transfer coefficient of air Heat transfer coefficient of coolant
26.17 W/per diode 96.83 W/per IGBT 20 °C 75 °C 5 W/m2K 1000 W/m2K
Table 5 Descriptions of IGBT integrated thermal management system models. CU VC
Case B: IGBT is integrated with copper substrate Case C: IGBT power module is integrated with vapour chamber
caused by thermal expansion and thermal stress. Only the copper baseplate of IGBT module and four bolt holes are fixed in all direction and four bolt holes are fixed while others displace freely. Two models are built. Table 5 gives the details of each model.
Fig. 8. Comparison of junction temperature for validation.
4. Discussion and results
3.3. Validation of the simulation model
4.1. Temperature distribution
In this work, the model is validated by comparing the simulation results of the junction temperature of IGBT chips to those obtained from experiment at stage of IGBT chips power loss. Fig. 7 shows the entire schematic diagram of experiment apparatus. Heat is directly given to each diode and IGBT chip by a copper block which contains six cartridge heaters. Thermal grease, TIG780-38 (thermal conductivity 3.8 W/mK) is applied to improve thermal contact the thermal contact between the heater and the IGBT chips and diodes. The heat block is covered with polyether ether ketone to minimize the heat loss. The heat is powered with six DC power supplies (30–05:HSPY). The baseplate of the vapour chamber is kept being cooled with a water-cooling plate connected to a chiller. Three T-type thermocouples are mounted and used to measure junction temperature. Data of the junction temperature is collected by data acquisition system (DataTaker DT 800). It is impractical to directly measure junction temperature of semiconductor module [23]. In this study, thermocouples are mounted as close as possible to IGBT chip to measure the junction temperature and positions are shown in the insert picture of Fig. 8. Fig. 8 shows that the simulated junction temperature is lower than the experimental results at all three positions, most likely due to variation in material properties. In general, the simulation is in good agreement with the experiment and the average error of junction temperature is 8.27%.
Chip temperature is a prerequisite for precursor extraction as it is related to many IGBT temperature dependent parameters and failure mechanisms [24]. Figs. 9 and 10 show temperature distribution of IGBT module with copper and VC substrate respectively. At the stage of IGBT chip power loss, hot spots mainly locate on IGBT chips; at the other stage the hot spots move to the diodes. The maximum temperature at the chip is reduced from 101 °C to 98.2 °C at the stage of chip power loss stage after the copper substrate is replaced by vapour chamber. It’s because that by means of phase change, vapour chamber transfers heat much more efficiently than the copper substrate. It is also found that the temperature variance in one cycle of IGBT with copper substrate is greater than that of vapour chamber. At the stage of IGBT chip power loss, the temperature difference of IGBT with copper baseplate is 13.3 °C, but the temperature variance is only 7.6 °C when the vapour chamber is used. This wider temperature range can cause higher thermal stress, which may lead to deformation within each layer of IGBT module and eventually damage the thermal reliability of module. High temperature uniformity is another important requirement for thermal management of power module. If a cooling device generates a low-uniformity cooling performance, large temperature variation in packaging components with different properties and size cause forces at the interfaces between adjacent materials [25]. Thus, high temperature
Fig. 7. Schematic diagram of experiment apparatus. 77
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Fig. 9. Temperature distribution in IGBT with copper ba seplate (a) at stage of IGBT chip power loss (b) at stage of diode power loss.
Fig. 10. Temperature distribution in IGBT with VC baseplate (a) at stage of IGBT chip power loss (b) at stage of diode power loss.
Fig. 11. Temperature difference T against time within one power loss cycle.
Fig. 12. Von mises stress distribution. Bottom bar: IGBT with copper substrate. Top bar: IGBT with vapour chamber substrate.
nonuniformity within material layers is also responsible for the high stresses experienced by the packaging. This will impair the reliability and efficiency of the IGBT module. Therefore, thermal management system is required to supply a uniform cooling so that the difference between the highest and lowest temperature of a chip, T is within a few degrees. Fig. 11 shows the change of T of the hottest IGBT chip as a function of time within a power loss cycle. It is obvious that ambient
temperature has less effect on temperature uniformity. The T is reduced from 4.6 °C to 3.9 °C after copper substrate is replaced by vapour chamber, which demonstrates vapour chamber can improve temperature uniformity of IGBT power module. This is because that with phase change, heat spreads from evaporation surface to the entire chamber rapidly and also thermal conductivity of vapour chamber is more than 78
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Fig. 15. Energy dissipation within two thermal cycles.
Fig. 13. Peek peel (SY) and first principal (S1) stresses of Sn-3.5Ag.
and this is attributed to mismatch in length of DBC layer and chips and higher temperature at the central. Also, the stress is higher at the interface between DBC and IGBT chips than at the interface between diodes and DBC because of higher temperature gradient across IGBT chips. In particular, the highest thermal stress occurs at the edge of upper ceramic/copper interface. This indicates that the crack or peeling starts from the edge of ceramic layer and grow along the interface finally bifurcate and break ceramic layer, which is in agreement with other studies [2,26–28] Sn-3.5Ag solder has lower melting temperature, thus yielding larger stress and higher coefficient of thermal expansion compared with other packaging layers, therefore it is more easily damaged. Fig. 13 shows peaks of component stress in Y direction (SY, also referred as peeling stress) and first principal stress (S1) are quite close to chip solder layer. This indicates that the peeling stress is the dominant factor in inducing crack initiation or failure at solder interface [29]. There is an apparent increase in peeling stress and first principal stress at 70 min because of temperature variation. For IGBT with vapour chamber, the maximum principal stress is 47 MPa which are not larger than the fracture strength of solder layer, which is 57.6 MPa reported by Hwang and Vargas [30]. Under the thermal stresses, each component of IGBT power module with copper or vapour chamber substrate have a specific amount of
10 times higher than copper. 4.2. Thermal stress The mismatch between two adjacent layers of module is not uniform due to the coefficient of thermal expansions difference and mismatch of length of each layer. These generate thermal stress which leads to deformation, solder delamination and bond wire lift off. In this case, to identify the region with highest mechanical load, von mises stress is used for evaluating the thermal stress distribution of IGBT module. Fig. 12 shows the von mises stress distribution of IGBT power module at heat transfer coefficient of 1000 W/m2K. In order to have a clear sight of thermal stress distribution in DBC, the left switch IGBT chips, diodes and solder layer are hided. DBC layer is subjected to highest von mises stress especially for ceramic layer as shown in Fig. 10. This is because the thermal expansion coefficient of copper is around 3 times higher than that of Al2O3, which is at the origin of thermal fatigue. IGBT with copper and vapour chamber substrates experience maximal stress of 298 MPa and 233 MPa respectively, which both occurs at the edge of ceramic layer due to the distance to neutral point (DNP) effect and geometrical singularity. This shows that there is a 20.1% reduction in thermal stress. The central region of DBC layers also suffer high stress
(a) First solder layer
(b) Second solder layer
(c) Third solder layer
Fig. 14. Total displacement of solder layer [µm] 79
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Fig. 16. Cycle to failure of solder layer.
deformation and displacement relative to their original shape and position. Fig. 14 shows total displacement and deformation of chip solder layer. As shown in Fig. 14b, c and d, the Sn-3.5Ag solder undergoes a displacement toward the edge of the IGBT power module and bend into a convex shape under tensile force generated by thermal stress. This is attributed to the fact that top surface of solder layer is hotter than bottom surface, which forced the chip solder layers to deform in that way. The maximum displacement of 8.21 µm and 10.6 µm occur at the solder which locates at the central for vapour chamber case and copper case respectively.
Therefore, in this simulation, inelastic energy dissipation including creep and plastic energy is used to predict the life time. Fig. 16 shows the cycles to failure of solder layer. The minimum simulated number of cycles to failure of vapour chamber case is 2640 which is 9% longer than the lifetime of copper case with value of 2402. 5. Conclusions A detailed 3-D FEM simulation of thermo-mechanical performance in high power IGBT modules is presented. This work also proposed a novel structure of IGBT power module integrated with vapour chamber and thermal management. The Direct Copper Bond (DCB) layer is directly soldered on lid of a vapour chamber. To evaluate the performance of whole IGBT module integrated with vapour chamber, a thermo-mechanical coupling cyclic model including creep and plastic constitutive equation is established. Based on simulation results, the junction temperature can maintain at a lower value and decrease from 101 °C to 98.2 °C at the stage of chip power loss stage comparing with that of IGBT with copper baseplate. IGBT module integrated with vapour chamber has a higher temperature uniformity in a single chip. The temperature difference in a chip decreases from 4.6 °C to 3.9 °C. In this study, thermo-mechanical performance is also studied. It helps us to have a good understanding of failure mechanism in such operating mode. The maximum thermal stress is decreased 20.1% compared with copper baseplate case. It is also found that the outermost solder layer always suffered the highest thermal stress due to its distance from neutral point effect and mismatch in length between solder and DBC substrate. Cracks induced by thermal loading are expected to initiate at the edge. The failure mechanism is also investigated by comparing with creep and plastic strain energy dissipation density. Creep is the principle damage in the thermal cycling. The lifetime of the solder joint is calculated by the Morrow’s life prediction model. The minimum simulated number of cycles to failure of the vapour chamber case is 9% longer than the lifetime of the copper case.
4.3. Energy disspation density Generally, with thermo-mechanical fatigue, Sn-3.5ag chip solder layers are subjected to creep and plastic which play significant roles in deformation behaviour of materials. As shown in Fig. 15, ΔWcr continuiously increases from 0 MJ/m3 to 0.194 MJ/m3 and from 0 MJ/m3 to 0.171 MJ/m3 for copper case and vapour chamber case respectively. It indicates that creep dissipation density drops around 11.9% when vapour chamber is integrated. As predicted by Morrow’s life prediction model described in Eq. (10), the fatigue lifetime reduces when inelastic dissipation energy density increase. The ΔWcr is about 2.6 and 3.7 times larger than ΔWpl for both vapour chamber and copper case respectively. It is found that creep is principal damage in power cycling. 4.4. Fatigue In consequence, the weakest part of the module, Sn-3.5Ag solder layer, can be predicted. There are several fatigue life prediction models such as strain, stress and energy-based fatigue models. Strain distribution is more complex because each location has different strain and energy dissipation. It is difficult to obtain a represent position [31]. Moreover, the stress or strain is not sufficient to identify the fatigue properties for some cases. To improve the accuracy, the cycle to failure is estimated by using energy-based model combining the effect of stress and strain into energy. In some researches [16,32–34], only creep or plastic alone was considered. Based on the analysis of energy dissipation density, plastic and creep play significant in energy dissipation.
Conflict of interest None. 80
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Contents lists available at ScienceDirect
Thermal Science and Engineering Progress journal homepage: www.elsevier.com/locate/tsep
Investigation of heat transfer and thermal stresses of novel thermal management system integrated with vapour chamber for IGBT power module
T
Yiyi Chena, Bo Lia, Xin Wanga, Yuying Yana, , Yangang Wangb, Fang Qib ⁎
a b
Faculty of Engineering, University of Nottingham, NG7 2RD Nottingham, UK Dynex Semiconductor Limited, Doddington Road, Lincoln LN6 3LF, UK
ARTICLE INFO
ABSTRACT
Keywords: IGBT power electronics Electric vehicles Phase change cooling Vapour chamber Thermal resistance Thermal stress Thermal fatigue
Thermal stress in IGBT power module can lead to sever thermal reliability problems such as module deformation, performance degradation and even permanent damage. So, it is important to develop innovative and efficient IGBT cooling technologies. In this paper, a novel thermal management system is developed for cooling IGBT power module. The module is integrated with a vapour chamber-based heat sink to reduce thermal resistance and improve temperature uniformity significantly. 3D FEM modelling is conducted to investigate the effect of vapour chamber on temperature distribution, thermal stress, energy strain dissipation density and lifetime under power cycle. The simulation results show that the proposed thermal management system is superior to traditional cooling solution regarding cooling capacity, thermal stress, creep and plastic strain energy dissipation and thermal fatigue life. The study of failure mechanism of solder layer under power cycling suggests that creep causes the main is damage in the power cycling and cracks induced by thermal loading can be expected to initiate at the edge.
1. Introduction Much effort has been made to exploit sustainable and clean energy for mitigating the global crisis of fossil energy and environmental problems. Popularization of hybrid electric vehicles and electric vehicles will become an unstoppable trend in next decades. As a semiconductor switching device, insulated gate bipolar transistor (IGBT) power module is one of the most important components in the system of power conversion and motor control. Recently, IGBT modules are required to be mounted on underhood or directly on an engine. The increased power rating and more compact structure as well as high integration and miniaturization of a vehicle will all require IGBT power to dissipate more heat within a limited space. For example, in next generation of hybrid electric vehicles, heat flux of IGBT module will increase from 100 to 150 W/cm2 to 500 W/cm2, but the temperature of entire module must be kept below the maximum junction temperature of 150 °C or 175 °C [1,2]. Over-high temperature and large temperature gradients can cause a variety of module failure including solder delamination and crack, ceramic and silicon chip crack propagation as well as bonding wire lift out. Therefore, there is an urgent demand for development of advanced thermal management systems for IGBT power
⁎
modules. It is hard to provide sufficient cooling for a high-power and heat flux IGBT module with air cooling, especially when the power dissipation exceeds about 1500 W [3]. Micro, mini-channel and pin–fin liquid cooling solutions can offer a higher cooling capacity typically 120 W/ cm2 in comparison with that of air cooling (typically 50 W/cm2) [4]. Oguntala et al. [5] investigated the effects of particle deposition on the thermal performance of porous fin heat sink of an electronic component. Koo et al. [6] theoretically studied the thermal performance of a microchannel for cooling of three dimensional electronic circuit. However, the flow redistribution results in a non-uniform temperature distribution. In the strategy of jet impingement, an evaporative coolant is sprayed on to a flat surface to be cooled. Jörg et al. [7] designed a direct single impinging jet for cooling a MOSFET power electronic module. Heat transfer rate can be achieved up to 12,000 W/m2K. However, cooling performance rapidly degrades as it departs the central of the jet region, resulting in temperature non-uniformity across the surface being cooled [8]. Therefore, much attention has been paid to array impingement cooling to improve temperature uniformity as well as heat transfer coefficient. When water is used as the coolant, heat flux level of 250 W/cm2 and 1000 W/cm2 can be obtained with single-phase
Corresponding author. E-mail address: [email protected] (Y. Yan).
https://doi.org/10.1016/j.tsep.2019.01.007 Received 31 March 2018; Received in revised form 26 November 2018; Accepted 3 January 2019 2451-9049/ © 2019 Elsevier Ltd. All rights reserved.
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Nomenclature A Ac B C Cp ETkin K L m n Nf q ΔQ r R T t Wf′ σ σshift
ΔT ΔWcr ΔWin ΔWpl :
creep rate coefficient (s−1) cross section area (m2) 1/effective stress (1/MPa) fourth order elasticity tensor heat capacity (J/kg/K) kinematic tangent modulus thermal conductivity (W/mK) thickness (m) fatigue energy exponent Garofalo n parameter fatigue cycles heat transfer rate (W) activation energy (J/mol) thermal resistance (K/W) gas constant (J/mol.K) temperature (oC) time (s) fatigue energy coefficient stress (Pa) kinematic hardening shift stress (Pa)
Temperature difference (oC) creep strain energy density (MJ/m3) inelastic strain energy density (MJ/m3) plastic strain energy density (MJ/m3) double-dot tensor product
Greek letters ε εcr εel εin ρ
strain equivalent uniaxial creep strain elastic strain inelastic strain density (kg/m3)
Subscripts c cr el f in Tkin
and phase change impingement [4,8,9]. Although jet impingement can achieve a very high cooling capacity, the applicability of this strategy is limited due to high cost, complex cooling flow redistribution, cooling loop leakage and channel blockage. As super heat conductive devices with phase change, heat pipe has been applied widely to improve the cooling capacity of high power electronic module [10]. Avenas et al. [11] developed a power chip module with vapour chamber heat spread. Ivanova et al. [12] proposed that heat pipe can be directly integrated in DBC for cooling of power electronics packaging. Fig. 1 illustrates the structures of three different thermal management for IGBT module. Case A represents conventional indirect cooling. The DBC layer is soldered on a copper baseplate first and then bolt to a heat sink with grease-based thermal interface material (TIM). TIM accounts for a great part of total thermal resistance so it becomes a dominating obstacle in improving cooling capacity. Case B shows a cross section of an assembly using direct attach method. DBC layer is directly soldered on a copper baseplate. Currently, this direct cooling method is most used due to its compact structure and avoiding using any kind of thermal interface material. Fig. 1 (c) shows the novel design in this study. The DBC layer is directly soldered on a vapour chamber-based heat sink. Also, no thermal interfacial material is used and the structure becomes more compact. In this paper, finite element method (FEM) simulation is used to investigate thermal-mechanical performance and reliability under power cycling. It allows us to estimate fatigue life which is a major
cross section creep elestic fatigue inplastic kinematic tangent
challenge for the electronic area. In electronic power module system, near-eutectic Sn-Ag-Cu and Sn-Ag materials are widely used to joint semiconductor on substrate for providing thermal, mechanical and electrical connection. It is well known that solder layer is the weakest part and break of which leads to failure of electronic power module at high operation temperature. This is because the solders are prone to creep and the accumulation of plastic strain triggers crack and propagation [13]. Moreover, Sn-3.5Ag solder has lower melting temperature and yield stress and higher coefficient of thermal expansion compared with other packaging layers, which will deteriorate mechanical performance of solder subjected to power cycling. In finite element simulation, a coupled thermo-mechanical model was applied in transient condition. Strain range and energy density are used as the failure parameters. An energy-based model is used to correlate crack initiation and propagation through solder from fracture mechanics approach. The simulation results reveal temperature, elastic-plastic and creep behaviour, deformation and lifetime prediction of Sn-3.5Ag solder joints exposed to power cycling condition. 2. IGBT module description 2.1. Module components A schematic of the IGBT power module is given by Fig. 2. The module is composed of six pairs of IGBT chip and diode, Direct Bond
Fig. 1. Structure of IGBT (a) with traditional indirect cooling (b) with direct cooling attached with copper baseplate IGBT module (c) with direct cooling attached with vapour chamber baseplate. 74
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Fig. 2. Schematic of the analysed power module.
Copper (DBC), solder, copper or vapour chamber. The DBC layer is made of two copper layers bonded on an aluminium oxidation (Al2 O3 ) layer. The Al2 O3 functions as heat conductor and electrical insulator due to its good thermal conductivity and electrical insulation. The chip solder has the same size with that of IGBT chips and diodes. Table 1 summarizes sizes of main components and Table 2 shows the material physical properties of each layer.
is assumed to be elastic-plastic because the stress within DCB substrate exceeds its yield stress easily. The Sn-3.5Ag solder layer is assume to viscoplastic with properties of hardening plasticity and implicit creep as it is widely agreed that rate of dependent plasticity creep occurs in the solder over time[17]. To achieve such effect, Garofalo hyperbolic sine law is applied to the solder layer.
d =A(sinhB ) ne dt
2.2. IGBT parameters
2.3. Initial design of IGBT integreted with thermal management Figs. 1c and 4 show the novel design of IGBT module integrated with a vapour chamber. Different from traditional thermal management system, baseplate and thermal paste are not used. The Direct Copper Bond (DCB) layer is directly soldered on lid of vapour chamber. A water cooling plate is bolted to vapour chamber together with IGBT power module. Fig. 1c shows traditional IGBT module (case A) thermal resistance stack in IGBT module. Thermal resistance of each layer is descripted by:
total
Shift
3.1. Finite element method (FEM) model Firstly, a coupled thermo-mechanical model is applied in transient condition. The governing equations are given as,
0
= C: (
0
)
+
p
+
(5)
cr
q s
(6)
=
E Tkin 1
ETkin E
2 3
p
(7)
E Tkin is kinematic tangent modulus. Both kinematic and mixed hardening result in a so-called back stress or shift stress, which is a new stress level that is equally far from yielding in tension and compression. Mechanical properties of packaging material and chip solder material are listed in Table 3. The thermal fatigue lifetime modules for prediction of lifetime of solder include stress, strain, energy and damaged based models. Energy based model are the most convenient and accurate method due to the ability to obtain test conditions more precisely [19,20]. Therefore, energy based model and Morrow model [21] are used to assess the lifetime of Sn-3.5Ag solder layer under power cycle in this study. Fatigue damage parameters such as inelastic strain and inelastic strain energy density and energy-based model are expressed as follow.
3. Methodology
(k T) = Q + Qted
el
Kinematics hardening plastics model is given as,
(1)
T + Cp u T t
=
=
p
where L is the thickness of material layer, K is the thermal conductivity and Ac is the cross-sectional area. As shown in Fig. 5, thermal paste and cast cold plate contribute the major thermal resistance, which account for 29% and 24% of overall module respectively. Therefore, the removal of those should bring a significant decrease in total resistance.
Cp
(4)
The cr is the equivalent uniaxial creep strain, is the equivalent tensile stress, T is absolute temperature, R is the gas constant; A, B, n, ΔQ are creep rate coefficient, 1/effective stress, Garofalo n parameter and activation energy respectively[18]. Also, the creep analysis uses an implicit time integration method, which includes temperature - dependent constants, together with modelling of creep and kinematics hardening plasticity. In this case, the total strain can be expressed as,
Power applied to the IGBT chips and diodes is 97.83 W (high state) and 25.17 W (low state) per cycle. As suggested by Xu [14], IGBT cycle is six times longer than diode power cycle in hybrid electrical vehicle application. Fig. 3 shows the power cycle applied to the IGBT module.
L r= KA c
Q RT
in
(2)
=
pl
+
(8)
cr
Table 1 Component sizes.
(3)
Eq. (2) is energy equation, where T is temperature and Q is heat generation. Eq. (3) is the Hooke’s law relating the stress-stress of material, where is stress tensor, is strain tensor, C is the fourth order elasticity tensor and “:” stands for the double-dot tensor product [16]. In solid mechanics model, semiconductor silicon and aluminium oxide in DCB substrate are assumed to be a linear-elastic model. Copper 75
Component
Dimensions
Silicon chips Silicon diodes DCB copper layer DCB Aluminium oxidation
9.718 mm*3.342 mm*0.02 mm 11.146 mm*7.746 mm*0.02 mm 47.14 mm*44.35 mm*0.3 mm 48.74 mm*46.48 mm*0.635 mm
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Table 2 Material property.
Al2O3 Cu Vapour chamber Al Silicon Sn-3.5Ag Sn-Pb
Table 3 Viscoplastic properties of Sn-Ag solder. Thermal capacity J/ (kg·K)
Thermal conductivity W/ (m·K)
Coefficient of thermal expansion (10−6/K)
730 320 385 904 678 240 242
35 413 5000 237 71 52 57
6.5 17 17 23.1 2.1 25 19
A (1/s)
B (Mpa)
n
ΔQ (J/mol)
R (J/molK)
ETkin (MPa)
44,100
0.005
4.2
44,995
8.314
4.9 × 109
Fig. 6. Mesh detail of the module.
Win = Wpl + Wcr Win = W f (2Nf ) m
(9) (10)
ΔWin, ΔWpl and ΔWc are inelastic, plastic and creep strain energy density respectively. εin, εpl and εcr are inelastic, plastic and creep strain respectively. Wf′ is fatigue energy coefficient, Nf is fatigue cycles and m is fatigue energy exponent with value of −0.69. 3.2. Boundary condition The simulation is conducted by Comsol Multiphysics software [22]. It is found that the simulation results of temperature distribution keep almost unchanged after the density exceeds 222,646. Therefore, mesh elements of 222,646 is selected in this study. Fig. 6 shows the detail of final mesh of module. The size of mesh ranged between 2 mm and 0.5 mm. Thin and weak components such as solder layers, chips and DBC are fined meshed whereas baseplate are coarse meshed. In this thermo-mechanical coupling model, heat sources are generated from IGBTs and diodes, which is estimated by power loss of these two components. Power loss is mainly caused by conduction losses from chip and collector current and switching loss. Total power losses are applied to each IGBT and diode are 96.83 W and 26.17 W respectively. Also, convection heat transfer is added to this model. The ambient temperature is initially set to 20 °C and heat transfer coefficient of air is 5 W/m2K. To analyse thermal performance of each module, the heat transfer coefficient of cooling solutions is set to 1000 W/m2K according to the experiment. Table 4 summarizes basic thermal parameters used in modelling. Solid mechanics is applied to evaluate displacement
Fig. 3. IGBT power loss cycle.
Fig. 4. Explosion view of thermal management system.
Fig. 5. Approximate thermal resistances stack in traditional IGBT module [15]. 76
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Table 4 Boundary condition in IGBT modelling. Boundary condition
Value
Heat source from diode Heat source from IGBT Ambient air temperature Coolant temperature Heat transfer coefficient of air Heat transfer coefficient of coolant
26.17 W/per diode 96.83 W/per IGBT 20 °C 75 °C 5 W/m2K 1000 W/m2K
Table 5 Descriptions of IGBT integrated thermal management system models. CU VC
Case B: IGBT is integrated with copper substrate Case C: IGBT power module is integrated with vapour chamber
caused by thermal expansion and thermal stress. Only the copper baseplate of IGBT module and four bolt holes are fixed in all direction and four bolt holes are fixed while others displace freely. Two models are built. Table 5 gives the details of each model.
Fig. 8. Comparison of junction temperature for validation.
4. Discussion and results
3.3. Validation of the simulation model
4.1. Temperature distribution
In this work, the model is validated by comparing the simulation results of the junction temperature of IGBT chips to those obtained from experiment at stage of IGBT chips power loss. Fig. 7 shows the entire schematic diagram of experiment apparatus. Heat is directly given to each diode and IGBT chip by a copper block which contains six cartridge heaters. Thermal grease, TIG780-38 (thermal conductivity 3.8 W/mK) is applied to improve thermal contact the thermal contact between the heater and the IGBT chips and diodes. The heat block is covered with polyether ether ketone to minimize the heat loss. The heat is powered with six DC power supplies (30–05:HSPY). The baseplate of the vapour chamber is kept being cooled with a water-cooling plate connected to a chiller. Three T-type thermocouples are mounted and used to measure junction temperature. Data of the junction temperature is collected by data acquisition system (DataTaker DT 800). It is impractical to directly measure junction temperature of semiconductor module [23]. In this study, thermocouples are mounted as close as possible to IGBT chip to measure the junction temperature and positions are shown in the insert picture of Fig. 8. Fig. 8 shows that the simulated junction temperature is lower than the experimental results at all three positions, most likely due to variation in material properties. In general, the simulation is in good agreement with the experiment and the average error of junction temperature is 8.27%.
Chip temperature is a prerequisite for precursor extraction as it is related to many IGBT temperature dependent parameters and failure mechanisms [24]. Figs. 9 and 10 show temperature distribution of IGBT module with copper and VC substrate respectively. At the stage of IGBT chip power loss, hot spots mainly locate on IGBT chips; at the other stage the hot spots move to the diodes. The maximum temperature at the chip is reduced from 101 °C to 98.2 °C at the stage of chip power loss stage after the copper substrate is replaced by vapour chamber. It’s because that by means of phase change, vapour chamber transfers heat much more efficiently than the copper substrate. It is also found that the temperature variance in one cycle of IGBT with copper substrate is greater than that of vapour chamber. At the stage of IGBT chip power loss, the temperature difference of IGBT with copper baseplate is 13.3 °C, but the temperature variance is only 7.6 °C when the vapour chamber is used. This wider temperature range can cause higher thermal stress, which may lead to deformation within each layer of IGBT module and eventually damage the thermal reliability of module. High temperature uniformity is another important requirement for thermal management of power module. If a cooling device generates a low-uniformity cooling performance, large temperature variation in packaging components with different properties and size cause forces at the interfaces between adjacent materials [25]. Thus, high temperature
Fig. 7. Schematic diagram of experiment apparatus. 77
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Fig. 9. Temperature distribution in IGBT with copper ba seplate (a) at stage of IGBT chip power loss (b) at stage of diode power loss.
Fig. 10. Temperature distribution in IGBT with VC baseplate (a) at stage of IGBT chip power loss (b) at stage of diode power loss.
Fig. 11. Temperature difference T against time within one power loss cycle.
Fig. 12. Von mises stress distribution. Bottom bar: IGBT with copper substrate. Top bar: IGBT with vapour chamber substrate.
nonuniformity within material layers is also responsible for the high stresses experienced by the packaging. This will impair the reliability and efficiency of the IGBT module. Therefore, thermal management system is required to supply a uniform cooling so that the difference between the highest and lowest temperature of a chip, T is within a few degrees. Fig. 11 shows the change of T of the hottest IGBT chip as a function of time within a power loss cycle. It is obvious that ambient
temperature has less effect on temperature uniformity. The T is reduced from 4.6 °C to 3.9 °C after copper substrate is replaced by vapour chamber, which demonstrates vapour chamber can improve temperature uniformity of IGBT power module. This is because that with phase change, heat spreads from evaporation surface to the entire chamber rapidly and also thermal conductivity of vapour chamber is more than 78
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Fig. 15. Energy dissipation within two thermal cycles.
Fig. 13. Peek peel (SY) and first principal (S1) stresses of Sn-3.5Ag.
and this is attributed to mismatch in length of DBC layer and chips and higher temperature at the central. Also, the stress is higher at the interface between DBC and IGBT chips than at the interface between diodes and DBC because of higher temperature gradient across IGBT chips. In particular, the highest thermal stress occurs at the edge of upper ceramic/copper interface. This indicates that the crack or peeling starts from the edge of ceramic layer and grow along the interface finally bifurcate and break ceramic layer, which is in agreement with other studies [2,26–28] Sn-3.5Ag solder has lower melting temperature, thus yielding larger stress and higher coefficient of thermal expansion compared with other packaging layers, therefore it is more easily damaged. Fig. 13 shows peaks of component stress in Y direction (SY, also referred as peeling stress) and first principal stress (S1) are quite close to chip solder layer. This indicates that the peeling stress is the dominant factor in inducing crack initiation or failure at solder interface [29]. There is an apparent increase in peeling stress and first principal stress at 70 min because of temperature variation. For IGBT with vapour chamber, the maximum principal stress is 47 MPa which are not larger than the fracture strength of solder layer, which is 57.6 MPa reported by Hwang and Vargas [30]. Under the thermal stresses, each component of IGBT power module with copper or vapour chamber substrate have a specific amount of
10 times higher than copper. 4.2. Thermal stress The mismatch between two adjacent layers of module is not uniform due to the coefficient of thermal expansions difference and mismatch of length of each layer. These generate thermal stress which leads to deformation, solder delamination and bond wire lift off. In this case, to identify the region with highest mechanical load, von mises stress is used for evaluating the thermal stress distribution of IGBT module. Fig. 12 shows the von mises stress distribution of IGBT power module at heat transfer coefficient of 1000 W/m2K. In order to have a clear sight of thermal stress distribution in DBC, the left switch IGBT chips, diodes and solder layer are hided. DBC layer is subjected to highest von mises stress especially for ceramic layer as shown in Fig. 10. This is because the thermal expansion coefficient of copper is around 3 times higher than that of Al2O3, which is at the origin of thermal fatigue. IGBT with copper and vapour chamber substrates experience maximal stress of 298 MPa and 233 MPa respectively, which both occurs at the edge of ceramic layer due to the distance to neutral point (DNP) effect and geometrical singularity. This shows that there is a 20.1% reduction in thermal stress. The central region of DBC layers also suffer high stress
(a) First solder layer
(b) Second solder layer
(c) Third solder layer
Fig. 14. Total displacement of solder layer [µm] 79
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Fig. 16. Cycle to failure of solder layer.
deformation and displacement relative to their original shape and position. Fig. 14 shows total displacement and deformation of chip solder layer. As shown in Fig. 14b, c and d, the Sn-3.5Ag solder undergoes a displacement toward the edge of the IGBT power module and bend into a convex shape under tensile force generated by thermal stress. This is attributed to the fact that top surface of solder layer is hotter than bottom surface, which forced the chip solder layers to deform in that way. The maximum displacement of 8.21 µm and 10.6 µm occur at the solder which locates at the central for vapour chamber case and copper case respectively.
Therefore, in this simulation, inelastic energy dissipation including creep and plastic energy is used to predict the life time. Fig. 16 shows the cycles to failure of solder layer. The minimum simulated number of cycles to failure of vapour chamber case is 2640 which is 9% longer than the lifetime of copper case with value of 2402. 5. Conclusions A detailed 3-D FEM simulation of thermo-mechanical performance in high power IGBT modules is presented. This work also proposed a novel structure of IGBT power module integrated with vapour chamber and thermal management. The Direct Copper Bond (DCB) layer is directly soldered on lid of a vapour chamber. To evaluate the performance of whole IGBT module integrated with vapour chamber, a thermo-mechanical coupling cyclic model including creep and plastic constitutive equation is established. Based on simulation results, the junction temperature can maintain at a lower value and decrease from 101 °C to 98.2 °C at the stage of chip power loss stage comparing with that of IGBT with copper baseplate. IGBT module integrated with vapour chamber has a higher temperature uniformity in a single chip. The temperature difference in a chip decreases from 4.6 °C to 3.9 °C. In this study, thermo-mechanical performance is also studied. It helps us to have a good understanding of failure mechanism in such operating mode. The maximum thermal stress is decreased 20.1% compared with copper baseplate case. It is also found that the outermost solder layer always suffered the highest thermal stress due to its distance from neutral point effect and mismatch in length between solder and DBC substrate. Cracks induced by thermal loading are expected to initiate at the edge. The failure mechanism is also investigated by comparing with creep and plastic strain energy dissipation density. Creep is the principle damage in the thermal cycling. The lifetime of the solder joint is calculated by the Morrow’s life prediction model. The minimum simulated number of cycles to failure of the vapour chamber case is 9% longer than the lifetime of the copper case.
4.3. Energy disspation density Generally, with thermo-mechanical fatigue, Sn-3.5ag chip solder layers are subjected to creep and plastic which play significant roles in deformation behaviour of materials. As shown in Fig. 15, ΔWcr continuiously increases from 0 MJ/m3 to 0.194 MJ/m3 and from 0 MJ/m3 to 0.171 MJ/m3 for copper case and vapour chamber case respectively. It indicates that creep dissipation density drops around 11.9% when vapour chamber is integrated. As predicted by Morrow’s life prediction model described in Eq. (10), the fatigue lifetime reduces when inelastic dissipation energy density increase. The ΔWcr is about 2.6 and 3.7 times larger than ΔWpl for both vapour chamber and copper case respectively. It is found that creep is principal damage in power cycling. 4.4. Fatigue In consequence, the weakest part of the module, Sn-3.5Ag solder layer, can be predicted. There are several fatigue life prediction models such as strain, stress and energy-based fatigue models. Strain distribution is more complex because each location has different strain and energy dissipation. It is difficult to obtain a represent position [31]. Moreover, the stress or strain is not sufficient to identify the fatigue properties for some cases. To improve the accuracy, the cycle to failure is estimated by using energy-based model combining the effect of stress and strain into energy. In some researches [16,32–34], only creep or plastic alone was considered. Based on the analysis of energy dissipation density, plastic and creep play significant in energy dissipation.
Conflict of interest None. 80
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