Impact of the Pump-Out-Effect on the thermal long-term behaviour of power electronic modules

Impact of the Pump-Out-Effect on the thermal long-term behaviour of power electronic modules

Microelectronics Reliability xxx (xxxx) xxxx Contents lists available at ScienceDirect Microelectronics Reliability journal homepage: www.elsevier.c...

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Microelectronics Reliability xxx (xxxx) xxxx

Contents lists available at ScienceDirect

Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel

Impact of the Pump-Out-Effect on the thermal long-term behaviour of power electronic modules ⁎

S. Söhl , R. Eisele University of applied sciences Kiel, Institute for Mechatronics, Germany

A B S T R A C T

One objective in the field of power electronics is to increase the lifetime of the components. An important factor in this context is the long-term thermal behaviour of the semiconductors. This behaviour can be negatively influenced by different effects and failure mechanisms. One of these failure mechanisms is the result of the displacement of the thermal interface material and is called the Pump-Out-Effect. This report includes an experimental measurement of the Pump-Out-Effect, the evaluation of the influence related to thermal performance and the presentation of an alternative module concept to minimize the mentioned failure mechanism. The alternative module concept includes the use of multi-layer heat spreader in an asymmetrical layer combination. With help of the multi-layer heat spreader the deformation behaviour can be adjusted under the influence of an alternating thermal load. This leads to a reduction of the Pump-Out-Effect and to an improvement of the thermal long-term behaviour.

1. Introduction The used frame modules basically consist of a ceramic circuit board (dbc) which is connected to a copper heat spreader by solder technology. This design has been a proven thermal solution in the field of power electronics for years. Such modules are usually used in combination with a heat sink to improve the heat dissipation. For further optimization of the heat transfer, there is a heat-conducting paste (TIM layer = Thermal Interface Material) between the module and the heat sink. This layer is necessary to compensate unevenness between the components and to avoid air inclusions [1]. However, this type of module structure carries the risk of displacing the TIM layer under the influence of an alternating thermal load. This effect is due to the different coefficients of thermal expansion (CTE) of the components. The CTE-mismatch causes a deformation of the stack with every temperature change. In combination with the fixation of the module to the cooler the change leads to a membrane-like pump movement [2]. As a result of this movement, the TIM layer is displaced and air inclusions are formed. These voids form a barrier in the thermal path. The heat from the semiconductor can no longer be dissipated properly. This leads to an overheating of the semiconductors and to early failure of the module. The use of CTE reduced materials minimizes the mismatch, thereby also the deformation and thus also the Pump-Out-Effect. One possibility to achieve the CTE reduction is the use of layered heat spreaders. Laminated heat spreaders are already used in many technical areas. Many times they consist of a symmetrically constructed material



composite of copper/molybdenum/copper [3]. The objective of this study is to use a layered heat spreader in an asymmetrical material ratio (in the following also referred as hybridmetal). Due to the asymmetry, the heat spreader behaves similarly to a bi-metal strip. A defined deformation occurs. This deformation is designed in such a way that the hybrid baseplate compensates the resulting crowning of the entire power module. This leads to a reduction of the deformation under a thermal load and to a minimization of the Pump-Out-Effect. 2. Measurement of the Pump-Out-Effect 2.1. Samples description Two kinds of samples are considered in the context of this investigation. For the reference samples a Danfoss ECO3 module [4] is used (Fig. 1). The module is designed as a frame module and contains two dbc's. Each dbc is designed as a half bridge. The copper heat spreading plate has a dimension of 120x60x3[mm]. The module design of the reference sample was converted to a FEM analysis and the 3 mm thick copper heat spreader plate was replaced by a multi-layer heat spreader consisting of a copper/molybdenum/copper composite. Molybdenum has a low coefficient of thermal expansion (5.35 10–6 K–1 at room temperature [5]) and a high module of elasticity (330 GPa at room temperature [5]). With these properties the Mo-Layer is able to influence the deformation behaviour of the assembly, depending on the layer thickness and the position in the stack. The

Corresponding author. E-mail address: [email protected] (S. Söhl).

https://doi.org/10.1016/j.microrel.2019.113479 Received 15 May 2019; Received in revised form 8 July 2019; Accepted 31 July 2019 0026-2714/ © 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: S. Söhl and R. Eisele, Microelectronics Reliability, https://doi.org/10.1016/j.microrel.2019.113479

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Fig. 1. Danfoss ECO3 module.

objective of the FEM analysis was to find an optimal material composition, which leads to a nearly flat module at room temperature and which shows low changes in curvature with an increasing process temperature. In addition, another goal was to keep the molybdenum amount as low as possible. Molybdenum has a thermal conductivity of 138 W/m ∗ K [5]. This result in a reduction of thermal performance (compared to pure copper). Based on the simulation results samples with the material composition, shown in Fig. 2, were prepared. The multi-layer heat spreader contains a 0,3 mm thick molybdenum layer. The upper copper layer has a thickness of 2,5 mm and the lower layer a thickness of 0,2 mm. Due to the asymmetry, a thermo-mechanical pre-crowning is generated. The pre-crowning compensates the deformation after joining the dbc's and heat spreader. The joining of the components is carried out using the silver sintering technology. This process is used in power electronics for the connection between dbc and semiconductor. The sinter layer has a high thermal conductivity and good mechanical resistance [6,7]. A sinter paste of the ASP 338 series [8] from Heraeus is used. The paste is applied by screen printing. A quasi-hydrostatic system is used for the sintering process. Process parameters [7]:

Fig. 3. Schematic structure of the test bench.

2.2. Experimental measurement For the investigation of the Pump-Out-Effect a test bench is developed. The Pump-Out test bench is used to visualize the displacement of the thermal interface material. A schematic drawing of the test bench is shown in Fig. 3. The Pump-Out test bench basically consists of an upper and a lower frame. The lower frame contains a glass bottom, which allows the observation of the TIM layer over the entire surface. The upper frame is used to fix the power module on the glass bottom with a torque of 5 Nm. There is a camera under the glass bottom to document the TIM layer. The thermal interface material TC-5026 from Dow Corning [9] is used for the investigation. This paste is designed for applications in power electronics. To ensure that the initial conditions for all samples are identical, the TIM layer is applied over the entire surface using stencil printing. The TIM layer has a thickness of 70 μm. During the test the power module is cyclically loaded. In the first step, the module is heated to a temperature of 130 °C. After reaching the temperature the module is cooled down to 40 °C. When a temperature limit (40 °C and 130 °C) is reached, an image of the TIM layer is automatically captured. An image of the TIM layer after 200 cycles is shown in Fig. 4. Gray and black areas are visible. In order to ensure that the black areas are voids, the TIM layer is observed with the help of 3D microscopy (Fig. 5). In the 3D image, the TIM layer is visible as a red area. The heat spreader is colored according to the crowning. A maximum height difference of 66.3 μm can be measured in the center area between the TIM layer and the heat spreader. This shows that the black areas in the 2D image are voids in the thermal interface material.

- Process time: 300 s - Process pressure: 20 MPa - Process temperature: 265 °C This joining technology has the additional benefit of a process temperature which is almost identical to the solder temperature. If the joining temperature is reached, the thermo-mechanical pre-crowning returns into its initial state. This allows the system soldering to be carried out on a flat base plate. The simulations as well as the construction of the samples and the following investigations were carried out at the University of Applied Sciences in Kiel.

2.2.1. Measurement of the defect formation To determine the formation of the Pump-Out-Effect, the samples are operated up to a cycle number of 200. An excerpt from the optical 2D images is shown in Fig. 6.

Fig. 2. Three layer hybrid heat spreader.

Fig. 4. 2D image of the TIM layer after x cycles. 2

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Fig. 5. 3D image of the TIM layer after x cycles.

Fig. 8. Test bench for measuring the thermal resistance and the junction temperature.

standard module is about 36% and under the use of the hybrid variant about 16%. 2.2.2. Measurement of thermal performance In Section 2.2.1 it was shown that the defect formation can be reduced by using hybrid heat spreading plates. However, the defect growth does not give any information on the influence of the PumpOut-Effect in relation to the thermal behaviour of the power modules. To evaluate the thermal influence, the thermal resistance is measured and the maximum junction temperature is determined. The used test bench is shown in Fig. 8. In this study the module is mounted on a cool plate. A torque of 5 Nm is used for fixation. The module is operated at a current of 180 A. The surface of the module is colored black to ensure an accurate measurement of the junction temperature. The temperature is measured by using a thermal imaging camera, located directly above the module. The first measurement takes place in the initial state (0 cycles). After the measurement, the mounted module is loaded with the same boundary conditions as the modules from the Pump-Out test. The measurement of the junction temperature is repeated after 50 cycles. This process is repeated up to a final value of 200 cycles. Fig. 9 shows the junction temperature as a function of the number of cycles. At the beginning (0 cycles) the module with hybrid base plate shows a higher junction temperature. This is due to the additional molybdenum layer and the two silver sinter layers. However, the increase in junction temperature by using the copper baseplate is significantly higher after a few cycles. After 200 cycles the module with copper heat

Fig. 6. TIM layers of the modules with an increasing number of cycles.

The upper part of the figure shows the defect growth for the power module with a copper heat spreader and the lower part for a module with hybrid base plate. The single images are arranged from left to right with an increasing number of cycles. It can be seen that an increase in the number of cycles leads to an increase in defect formation. It can also be seen that the use of the hybrid heat spreaders leads to a reduction of the defect formation. To evaluate the increase of defect formation, the growth of the black areas is determined with the help of image processing software. For this purpose, the images are compared after x-cycles with the images in the initial state to calculate a relative difference. The results for a standard module and for a module with hybrid base plate are shown in Fig. 7. The figure shows the relative defect formation as a function of the number of cycles. Both module types were evaluated at 40 °C and 130 °C. After 200 cycles the defect formation under the use of a

Fig. 9. Junction temperatures of the samples as a function of the numbers of cycles.

Fig. 7. Defect formations as a function of the number of cycles. 3

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Fig. 11. Comparison between measured and simulated junction temperatures.

3. Conclusion and discussion The objective of this study was to analyze the influence of the PumpOut-Effect on the long-term thermal behaviour of power electronic modules. Furthermore, the behaviour was improved by the use of multilayer heat spreaders. It was shown that the use of asymmetrically designed heat spreaders leads to a reduction of deformation under the influence of a thermal load. This reduction has a positive influence on the defect formation within the TIM layer and thus also a positive effect on the long-term thermal behaviour of the semiconductors. Early failures, due to an unacceptable semiconductor temperature, can be prevented. This leads to an increase in the lifetime of power electronic assemblies. In the context of this study the positive effect of the hybrid heat spreader was shown. However, only one design of the hybrid heat spreader was investigated. The thermo-mechanical behaviour can be improved by further optimization of the asymmetric material composite. With help of the results of the investigations, the simulation models are optimized and the material compositions are improved. In addition to the heat spreader, the other module components also have an influence of the thermo-mechanical behaviour of the power module. This influence can be analyzed and evaluated, in relation to the PumpOut-Effect by using the presented test bench. Furthermore, it is also possible to investigate different thermal interface materials and their long-term behaviour. The previous investigations were limited to a number of cycles of 200. Figs. 7 and 9 show that a steady state has not been reached. Therefore this will be considered in the next investigations and the number of cycles will be increased. In addition, the modules on cool plate will be examined in the next investigations using ultrasound microscopy. The results will be compared with the defect formation from the Pump-Out test to determine the influence of the glass bottom. The problem of thermo-mechanical failures does not only occur with the use of frame modules. This is a known problem in many power electronic applications. Therefore, the technology of the hybrid metal could be used in other module concepts to prevent early failures and increase the lifetime of the components.

Fig. 10. Conversion of 2D images into a 3D model.

spreader shows an increase in junction temperature by 8.9 K and the module with hybrid heat spreader by 2.3 K. This results in a ΔT of 6.6 K. By comparing the curves in Figs. 7 and 9, the relationship between the defect formation and the increase in the junction temperature can be seen.

2.3. FEM modelling For a correlation between the calculated defect formation within the TIM-layer and the measured increase of the junction temperature, the 2D images from Fig. 6 are transferred into a FEM model. The 2D images are modelled in a way that they can be used as a 3D model in the thermal simulation (see Fig. 10). The values from the junction temperature measurement are used to parameterize the simulation model: -

Volumetric flow: 4.927 e–5 m3/s. Power output: 684 W. Fluid temperature: 31.2 °C. Ambient temperature: 23.3 °C.

The thermal interface material adapts to the module deformation, therefore the layer does not have a constant thickness. With the help of 3D microscopy, an average value for the thickness over the entire layer is determined. The average value is 42 μm and is used for the TIM layer thickness in the simulation model. A value of 2.87 W/m ∗ K is used for the thermal conductivity of the thermal interface material [9]. The results of the simulations are shown in Fig. 11. In the figure the measured temperatures are compared to the simulated temperatures. The temperatures are plotted as a function of the numbers of cycles. It can be seen that the simulated results deviate only slightly from the results of the measurements. The small deviation between the measured values and the simulation suggest that the PumpOut-Effect occur under real conditions in a similar form as shown in Section 2.2.1. The difference can be explained by several reasons. One reason is that during a real power cycle a degradation of the joints, in the thermal path from the chip to the cooler, can be occur.

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement This work was funded by Bundesministerium für Wirtschaft und Energie (Grant ID: 020E-41V7712). 4

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References

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