The Influence Of the Mechanical Deformation On the Heat Transfer Efficiency in a Semiconductor Power Module

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The Influence Of the Mechanical Deformation

On the Heat Transfer Efficiency in a

Semiconductor Power Module

Jürgen Krome, Hochschule Hamm-Lippstadt, Lippstadt Jiacheng Fan, Kompetenzzentrum Fahrzeug Elektronik, Lippstadt

Andreas Groove, Infineon Technologies AG, Warstein Markus Thoben, Infineon Technologies AG, Warstein

Abstract

As the needs of the near-zero emission energy is nowadays rising, the electrical energy has been considered as the one of the best solution in many areas. Specified electrical properties are required for the different applications as a result. For instance, the current can be over 3000 Amperes in order to gain high power density. An efficient energy conversion will be actualized by using modern power modules, which have been mostly used in the wind energy, solar energy or in the power train applications. The IGBT power module is the predominant power module in electrical and hybrid vehicles applications due to its better conversion effenciency [1].

However, a part of the electrical energy will be converted into heat loss, resulting in an increasing switching loss [2] as well as a higher risk of over heating. To avoid reaching the maximal junction temperature Tj,max, the generated heat inside semiconductors must be transported outwards as much as

possible with an appropriate heat sink. The space between the lower side of the baseplate and the upper side of the heat sink (the space can be described as “gap field”) is filled with thermal grease which behaves as a flexible material. Specified thermal resistance Rth will be defined to estimate the heat

transfer efficiency from the heat source to the lower side of baseplate take in to effect the influences of heat spreading. Besides, a large deformation will be caused by thermal stress due to the different thermal expansion coefficients of the various materials. This mechanical phenomena changes the gap field and can provide a change in heat transfer efficiency performance at the same time. It means that the thermal resistance varies depending on mechanical material properties. Therefore, the motivation of this paper is to investigate how the mechanical deformation affects the calculated thermal resistance in power module. The numerical model will be technically simplified with some assumptions in order to reduce the complexity. By means of setting proper ANSYS configurations in “Workbench”, the mechanical simulation (isotropic thermal expansion distribution) can be well coupled with the following thermal field simulation. An user defined analysis of field-coupled calculation is implemented by inserting APDL. The exported results can be also be used for the further evaluation.

Keywords

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1. Introduction

This project is conducted mainly by Kompetenzzentrum Fahrzeug Elektronik GmbH in Lippstadt, Germany, which is engaged in industrial research on electromobility. Because a great number of power switch devices (inverter, AC/DC converter or DC/DC converter) are required in an electrical vehicle (EV), it is always a considerable issue to achieve a better total performance of electrical vehicles by improving its electronic components. The basic development could start from the power modules or semiconductor themselves. High-power IGBT modules are most widely used for EV applications and hence only IGBT power modules will be taken into account in this work. On this point, Infineon Technology AG in Warstein as the cooperative participator provides technical supports with its great experience in this specific area. One aspect should be permanently focused on is to estimate heat transfer in power module while the thermal dissipation occurs. The temperature of the semiconductors should be controlled below a certain degree, above which the components could be possibly over heated. On the other side, the semiconductor power elements should be activated at high current or high voltage, so that the power module can achieve its best performance. For this purpose, the heat sink is mounted under the baseplate in order to keep the junction temperature in an acceptable range by transporting continually the generated heat away from chips. The baseplate is in fact not planar before and after being mounted onto the heat sink. Some thermal conductive materials, thermal greases (Abbr. TG) for instance, are used to fill up the gaps between the interface of baseplate and heat sink, creating a thermal contact. To estimate the heat transfer efficiency from chips to ambient field adjacent to lower side of the heat sink, the general heat resistance Rth will be calculated. Among all the components in a power module with Direct Copper

Bonding substrate, the TG has an extremely low thermal conductivity. Typical thermal conducting pastes take over 50% of the total thermal resistance in power module.

The mechanical behavior of a power module is another important factor that should be taken into account. It affects the mechanical performance of a power module directly and has an indirect influence on heat transfer performance. Due to the viscos-plasticity or viscos-elasticity of a thermal conducting paste, any tiny mechanical deformations will cause a change in entire TG´s topology, while they affect the other components insignificantly. This irregular gap field topology brings additional modeling work which is not expected for a standardized thermal resistance calculation. The influences on heat transfer efficiency, however, can not be ignored, since the change in deflection and thickness of TG are of the same magnitude.

To solve this problem in ANSYS, it is a common way to couple the static structural analysis with the static thermal computation. Creating a strong coupling is not necessary in this case, as the interaction will be not investigated here. Concerning the aim, it will be efficient if a simplified method is carried out for obtaining the results merely within the ANSYS thermal analysis. All mechanical effects will be as input in a standardized thermal model available, in which the geometry of the TG is independent of the deformation and therefore retains a regular shape. The information transfer operation is executed by compiling a specified ANSYS Parametric Design Language (APDL) in Workbench environment

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2. Technical background

2.1 Power module basics

A power module is an assembly of various power components and semiconductors devices depending on its functionality. It is primarily built to provide regulated power supply, e.g. from high voltage to low voltage or from AC to DC. A typical power module consists generally of semiconductors, substrate, terminals, insulation layouts and load connecting elements, with or without baseplate. The description of some primary components is introduced briefly in following:

Semiconductor: This definition refers to a group of materials that provides an intermediate electric conductivity between that of a conductor and an isolator. Silicon is the absolutely dominant choice for industrial application. The working principle of such kind of unit is that there exist microscopically both free electrons (n-region) and unsaturated bonds (holes, p-region) and the holes will be occupied with free electrons by applying electric field, resulting in appearance of electric current inside the semiconductors. Because of this characteristic, the electric conductivity can be finely controlled by adding impurities (doping). The domain between p-region and n-region is called p-n junction where is the active region while the semiconductor is working.

The power semiconductor device is the product made of semiconductor and used to realize the power switch, from AC to DC, and regulate the output voltage during the “On” and “Off” state switch in electric circuit, preventing the element from the pulsatile damage due to reverse flow or oscillation of current. In general the device family is divided into two-terminal device, namely the diode, and the three-terminal device, e.g. IGBT, power MOSFET, thyristor. As the IGBT covers a better voltage vs. current area concerning the EV application and has less switch losses, the IGBT power module is relatively more appropriate for EV. “TechNavio's analysts forecast the Global IGBT-based Power Module market to grow at a CAGR of 14.07 percent over the period 2012-2016. One of the key factors contributing to this market growth is the increasing demand from renewable energy production. [3]”

Substrate: A substrate is generally a metalized board which is not only used to create the interconnected electric circuit in power electrics, but also helps increasing the heat removal performance. The operative power semiconductor devices will be mounted onto substrate at the final stage of device fabrication. In present project the Direct Copper Bonding substrate is used by reason that it provides a good heat transfer performance. The complete design of a module is schematically exhibited in following graph:

Fig.1: conventional power module topology with DCB substrate before deformation [4] Baseplate: A power module can be designed with or without baseplate. This plate reinforces the structural robustness for the entire system as it sustains the greatest part of the deformation during transporting or mounting. The other elements attached to it will be thereby fixed to some degree. On the other side, it increases the heat resistance at the same time. To study the specified subject, the baseplate should be built herein.

o

x

_𝑄

𝑄

Z0

Thermal

Grease

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3 Thermal Interface Material: This is a group of materials that used to fill up the gap field between baseplate and heat sink to ensure a better thermal conduction than air in between. These materials are in general viscos-plastic within certain range of temperature, so that its shape is usually determined by the topology of the gap filed in three-dimension.

2.2 Heat transfer in power module

The power losses after every switch are dissipated in form of heat which must be transferred away from junction area to keep the semiconductor away from over-heat. There are three different forms of heat transfer according to the thermodynamic basics: heat conduction, convection and thermal radiation. In most cases a complex of all the three ways will occur. Inside the body, the heat can only be transferred in form of heat conduction. The heat is afterwards transported from body to ambient air by means of convection. In present work, the convection can be treated as an ideal case with a constant film coefficient. The thermal radiation and thermoelectric effect (Seebeck effect or Thomson effect) are here neglected due to a case simplification. This thermal motion can be formulated based on Fourier´s Law in Z-direction (primary direction vertical to the chip´s upper side):

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where

is the total heat to be transported per unit time and stands for the heat conductivity coefficient. The heat conductivity is one of the material properties that depends on current temperature and material itself. This coefficient can be isotropic for some materials:

or be anisotropic:

. is defined as the projected area in heat flow direction. The slope describes the temperature differences between upper and lower layer surface with respect to the layer thickness in vertical direction. A negative sign manifests that the heat always flows from hot to cold areas.

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is the power loss that equals to the amount of heat per time unit generated from active elements. The general thermal resistance is defined as the ratio of temperature difference and total power loss. Fig. 2 shows the typical thermal resistance structure with DCB-substrate.

Fig. 2: thermal resistance [5]

The parameter based on steady-state represents the static thermal resistance. (Fig.2) is the thermal resistance in section from junction area to case and represents the part from case to the heat sink.

,

will be as the area-averaged junction temperature of semiconductors, the temperature on lower surface of baseplate vertically below the chip center (case) and the temperature on upper surface of heat sink vertically below the chip center measured, respectively. Furthermore, the thermal impendence denotes the transient thermal resistance with respect to time.

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4 According to the results in Fig. 3, the thermal interface material has a quotient of half of total thermal resistance owning to the low thermal conductivity, commonly around 1W/m∙K. However, the ceramic layer in DCB-substrate is not taken into account, because its thickness will change insignificantly. The thermal resistance of TG is thus the factor describing the heat transfer efficiency that will be analyzed in this work.

Fig. 3: thermal resistance proportion of component in total, data from a 1200V power module with copper baseplate, chip size = 9mm x 9mm, thickness of TG = 100μm (fig. left [6])

2.3 Mechanical influence on resulting thermal analysis

No system can be fabricated without mechanical consequences in practice. Both external loads while mounting the baseplate onto heat sink and thermal stresses because of temperature change could cause the deflection. The deflection for one certain component (here still in z-direction as an example and based on baseplate) can be expressed in two parts:

Mechanical deformation:

(3)

Thermal expansion:

(4)

Total displacement:

(5) When the lower surface of the TG is located on the X-Y-plane Z=0 in coordinate system, in eq.(3) and (4) is the standard TG´s thickness (see also in Fig. 1) before the deformation is taking place and is the Young´s modulus, which is stated for every material under specific conditions. The thermal expansion coefficient in eq. (4) is also a parameter given in material database relating to current temperature. The resultant displacement in one direction

is then obtained by means of adding up both deflections. Due to uneven force and temperature distribution, the results depend strongly on the power module structure

.

As the TG is generally flexible, its thickness will exactly to the gap field topology. The thickness of TG after the module is deformed can be therefore expressed in:

. is according to eq. (2) defined as:

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Fig. 4 exemplifies a possible topology of TG generated after deformation. The other bending behavior is not illustrated in this drawing, as it nearly affects the thermal results. The TG´s thickness is eventually a non-linear function of local x and y position

.

3% 4%

_2%

33%

1%

2%

5%

50%

z=0

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5 Fig. 4: topology after deformation

The case will be solved with ANSYS Workbench 14.5 “mechanical” package which is used to accomplish the structural analysis and thermal analysis (both steady and transient) or a one-way coupled computation of both. For the following reasons, the thermal resistance will be calculated without direct automatic-coupling in ANSYS and with a standard model which is not topologically deformed.

• To standardize the thermal simulation with non-deformed model • To simplify the geometry in thermal simulation

• To control some parameters (e.g. B.C., material properties) at certain load step in thermal calculation without interacting with mechanical calculation

• To individualize the mesh setup for mechanical and thermal simulation when different mesh quality level can be applied

3. Numerical simulation

3.1 Conception to reduce the coupling in multi-physics

The main idea is to transfer the mechanical results to thermal simulation indirectly and the thermal part is therewith topologically independent of the mechanical model, but it will proceed by retrieving important results from mechanical simulation as parameter inputs. It is therefore necessary to create a method that can convert the mechanical information to thermal conditions.

Eq. (6) indicates a linear relationship between thickness of TG

and the thermal resistance , in case the thermal conductivity of TG is assumed to be isotropic

. Since the surface area A is also constant that can be measured, the thermal resistance is a function of location

as a result. This parameter is however directly determined by the final topology of TG after deformation when a direct coupling is applied. The topology change is not represented in standard thermal simulation as the TG´s thickness

should be herein kept constant. Apostrophe “´” denotes the parameters applied with variable thickness of TG to distinguish from that with constant thickness of TG. For obtaining the same results, a conception to fulfill the equation

must be carried out. The complete expressions are as following:

(7)

Thermal

Grease

y

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6 Both and

remain the same if the length and width of the TG are identically defined. and

should be theoretically also the same due to the fact that the selfsame material is provided. Regardless of physical facts, the thermal conductivity

can be also redefined as a variable in numerical simulation. By means of converting

into a corresponding function of location based on

, the eq. (7) will be valid.

This conversion can be realized in ANSYS by using APDL to create the relationship between both. Because the thermal conductivity is an element-related parameter while the Z-position is a magnitude measured from nodes, an averaged for each element is required. When the information must be transferredfrom one element in first model to the corresponding element in the other model, a controllable mapped mesh with same orthogonal element shape and identical mesh structure of both models is demanded. As the temperature is a node-based magnitude as well, a temperature field in standard model can be compared with that in deformed one only if the mesh density is sufficiently fine.

3.2 Modeling and meshing strategy

In order to prove the conception above, it is advisable to create comparable models. In the following table, three variations are given to validate the conception.

variante description Figure

Model 1 • No baseplate deformation • Constant thickness of TG zTG

• Constant thermal conductivity kz

Model 2 • Deformed baseplate • Variable thickness of TG zTG

• Constant thermal conductivity kz

Model 3 • No baseplate deformation • Constant thickness of TG zTG

• Variable thermal conductivity kz

Tab.1: model variation in test

The first model is a standard model which is mostly applied for a quick thermal computation without any topological treatment. In second variation, the actual deformation will take place and thermal conductivity remains merely dependent on current temperature as in actual case. This model provides the most accurate results by using the direct one-way coupling. Geometrically is the last model the same as the first variation but with the introduced input conversion method.

Only a quarter of an entire power module is modeled to reduce the calculation time. The CAD model for thermal simulation is geometrically simplified as some units will nearly affect the results. Mechanical calculation is separately executed for a complete computation in model 2. An indirectly coupled computation is carried out in the third model.

Because of the simplified geometry in thermal simulation, mapped mesh can be used throughout the body mesh. Most important is here meshing the thermal interface material with identical configurations. The mesh is uniformly distributed with a number of elements “m=60” in x-direction and “n=25” in y-direction for all models, but only a single layer of SOLID90 elements is set in z-direction.

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3.3 Implementation in ANSYS “mechanics”

① ③

Fig. 5: implementation procedure in APDL

Mainly three steps need to be done with APDL to convert the mechanical output into thermal input:

① The thickness distribution of TG including the mechanical deflection in second model is written in a text format file with *vwrite. A two dimensional matrix exactly based on the mesh structure must be thus created. As the body mesh is m*n*1 in x,y,z-direction respectively, a m*n matrix will be made to save the output results from mechnical computation by using the *get and *vread.

② To convert the

into

is the mathematical operation *voper required. The data transfer proceed while a matrix with the same dimension in model three is already defined. Regarding the data transfer from element to element in two different models, an element number redistribution in model 3 must be prepared to match those in third model.

x

②

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8 40 60 80 100 120 140 160 -0,005 -0,004 -0,003 -0,002 -0,001 0 0,001 0,002 0,003 0,004 Te m pe ra tur e T/ ° C position z/m

Vertical Temperature Distribution througout the IGBT Center model 2 model 1 model 3 Heat sink TG Baseplate Solder DCB-copper Ceramic Chip solder Chip -80,00% -60,00% -40,00% -20,00% 0,00% 20,00% 40,00% 60,00% 80,00% 100,00% 0 20 40 60 80 100 p er cen ta l d ev ia ti o n o f t h er m al r esist an ce in m o de l 1 ba se d o n re su lt s in m o de l 2 thickness of TG z/μm

thermal resistance R

thch

vs. variable standard thickness of TG

with constant K

IGBT Diode

4. Results estimation

Simulations for IGBTs and diodes run through respectively. It remains still inexplicit at first, how to ascertain a comparable standard thickness of TG when it is able to vary in the first model. The estimation can be carried out by comparing the results from model 1 with that from model 2, while the thickness of TG in model 1 is varying. Final results in diag. 1 help to elucidate the fact that it is impossible to reduce the inaccuracy under an amount of 20%, when the results for both of IGBT and diode must be sufficiently acceptable. The A standard thickness of 50 μm is under this condition a best solution that satisfies the demands of both.

Diag. 1: thermal resistance deviation estimation with respect to variable standard thickness of TG in model 1

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9

View from top of TG

Model 2

Model 3

Model 1

View from bottom of TG

The diag. 2 shows the temperature development starting from the center of an IGBT downwards to the spot vertically below the lower side of heat sink after the IGBT is activated. The curves of model 2 and model 3 match well to each other over the upper surface of heat sink, while the results in the first model can not fit to some extent. To gain a better insight into the temperature distribution on thermal interface, temperature contours for all models are plotted in Fig. 6.

Fig. 6: temperature contour on surfaces of TG (after one IGBT is activated, standard thickness of TG = 50 μm)

Almost no discrepancies are found in second and third model which indicates the pre-processing is correctly configured. Slight difference between model 1 and model 2 can be detected because of the inhomogeneous thickness distribution in third model.

Tab. 2a: Calculated results based on simulation solution for IGBT analysis (standard thickness of thermal grease = 50 μm)

Model Tjm,IGBT

[°C] Tc,IGBT [°C] Th,IGBT [°C] Rth,jc,IGBT [K/W] Rth,ch,IGBT [K/W]

1 131.53 80.47 60.12 0.262 0.103 (dev: -12.8% based on model 2)

2 134.13 83.06 60.08 0.262 0.117

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10 Model Tjm,diode [°C] Tc,diode [°C] Th,diode [°C] Rth,jc,diode [K/W] Rth,ch,diode [K/W]

1 121.41 76.12 61.14 0.217 0.072 (dev: +9.2% based on model 2)

2 121.75 76.12 62.40 0.219 0.066

3 121.51 76.07 62.41 0.218 0.065

Tab. 2b: calculated results based on simulation solution for diode analysis (standard thickness of thermal grease = 50 μm)

All results above are obtained with aid of an APDL snippet, getting the surface averaged temperature. The “junction to case” thermal resistances of all three models are extremely close. In view of few thickness variations above the baseplate, it is in accordance with the assumption. Great gaps appear however between the first and second model in calculating the “case to heat sink” thermal resistance

.

For both IGBT and diode calculations, there will be around 10% inaccuracy compared with the second model. It is not a negligible deviation since it has a major contribution to general heat transfer efficiency. On the other side, the third model offers almost a perfect agreement with the conversion method. This verifies that the method used is feasible as a result.

5. Conclusion

• Influence of mechanical deformation is for estimating thermal resistance significant. At least around 10% inaccuracy for heat transfer calculation of each semiconductor in power module must be counted in if a standard model with certain constant TG thickness is applied.

• With the introduced method, the thermal resistance calculation can be indirectly coupled with the mechanical simulation. It offers under given circumstances a better flexibility in reducing the modeling complexity for thermal simulation. Furthermore, the thermal interface roughness can be also herewith easily reproduced once the roughness distribution with respect to location is available.

• A possible way is proved within this work to solve a multi-physical coupling in an indirect way by mathematically converting input and output parameters if certain conditions are met. Taking great advantage of modifying numeric models contributes to reduce the complexity of physical model if the correlation is known.

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11 References

[1] R. John, R. Bayerer, O. Vermesan: "High temperature power electronics IGBT modules for electrical and hybrid vehicles", HiTEN, 2009, pp. 000199- 000204

[2] R. Bayerer: "Higher Junction Temperature in Power Modules – a demand from hybrid cars, a potential for the next step increase in power density for various Variable Speed Drives", PCIM, 2008, Nuremberg, Germany

[3] New Market Research Report Added in MarketResearchReports.Biz Reports Database Global IGBT based Power Module Market 2012-2016

[4] Application Note AN2012-05 V2.0, 62mm Modules Application and Assembly Notes, pp19, Infineon Technologies AG

[5] Martin Schulz, Inﬁneon Technologies, Thermal Interface A Key Factor in Improving Lifetime in Power Electronics, Infineon Technologies AG

[6] A. Wintrich, U. Nicolai, W. Tursky, T. Reimann, Applikationshandbuch Leistungshalbleiter, SEMIKRON International GmbH

The Influence Of the Mechanical Deformation On the Heat Transfer Efficiency in a Semiconductor Power Module