Thermal characterization of a 500W motor driver module ... · Thermal characterization of a 500W motor driver module with embedded power transistors Johann Nicolics1), Michael Unger1),

Thermal characterization of a 500W motor driver module with embedded power transistors

Johann Nicolics1), Michael Unger1), Stephanie Groß2), Mike Morianz3), and Hannes Stahr3)

1) Institute of Sensor and Actuator Systems, TU Wien, Vienna, Austria 2) Continental, Nürnberg, Germany

3) AT & S Austria Technologie & Systemtechnik AG, Leoben, Austria [email protected]

Abstract: An entirely novel multilayer printed circuit board technology called EmPower allows embedding of chip components like power diodes, MOSFETs (Metal-oxide Semiconductor Field Effect Transistors) or IGBTs (Insulated Gate Bi-polar Transistors) at a significantly reduced module thickness into a glass fiber reinforced epoxy-resin layer built-up. One major concern is to handle the power loss within the given small module volume and to investigate the thermomechanical stress conditions in the module after the embedding process. In this contribution the authors also show the full thermal characterization procedure of a B6-converter bridge of a 500W motor driver demonstrator module for a Pedelec (Pedal Electric Cycle). Results are compared to those obtained from a module fabricated with state-of-the-art SMT MOSFETs. Advancements like superior thermal performance, increase in efficiency, and reduction of weight and space requirement obtained with this embedding technology and the built-up concept for mass fabrication are also discussed.

1. INTRODUCTION

Following the insight and understanding of decades of research of reasons of the climate changes, one of which still is the CO2 emission by combustion of fossil fuels [1, 2], the global automotive industry moves continuously towards environmentally friendly electrically driven bicycles and cars. Though, hybrid electric vehicles are currently the most fuel-efficient ones, the trend towards fully electrically driven vehicles will continue to grow in the future [3]. Miniaturization and reduction of weight are the main drivers in the high power application segment for electrically driven applications. A further increase of fuel economy and improved driving dynamics are considered as key strengths. Moreover, scalable and modular system for product solutions and the electrification of the powertrain from 48 V to high-voltage hybrid and electric vehicles are targeted.

A key parameter to achieve a noticeable reduction of space and weight is the switching frequency: an increase of switching frequency allows shrinking the volume of the inductors in the driver circuits. However, this takes reduced switching losses for granted, which is only realizable with a new wire

bond-free interconnection technology that allows re-ducing interconnection lengths and therewith parasitic impedances – the embedding technology [4].

2. MULTILAYER SET-UP CONCEPT

The innovative character of this entirely new packaging concept called EmPower technology lies on the idea of embedding the power drive components (IGBTs, MOSFETs, diodes) as thinned chips into a glass fiber reinforced epoxy-resin layer built-up comparable to a multilayer printed circuit board and thereby to realize large-area interconnections on both sides of the chips by direct copper plating. In this way the EmPower concept also sets aside thick wire bonding of the power devices on Direct Copper Bonded (DCB) substrates and (i) a conductor structure with lowest possible electrical impedance, (ii) an optimum heat removal, and (iii) essentially reduced volume and weight is achieved.

The inner part of this multilayer is a small-sized power core. Such power cores are manufactured entirely in a state-of-the-art embedding manufacturing environment. The power core fabrication starts on an adhesive tape on which prepregs and core laminates

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with cut-outs for the embedded devices, inlays, and semiconductor devices are assembled. After that a further prepreg without cut-out covers devices and inlays. At this stage a core with the devices as close as possible to the surface is fabricated. After finishing the core the interconnections to the chip are formed with a laser ablation process. Then holes are filled with copper by electroplating. An additional advantage of this concept is an adhesive-free integration of components into a printed circuit board. Depending on the thermal needs copper inlays with a wide variety in size and shape can be placed close to the components or/and arrays of copper filled microvias or slots can be placed to increase the perpendicular heat flow.

The power core needs an electrical isolation to the heat sinks for cooling which could be double-sided. The electrically insulating material requires high thermal conductivity and high breakdown voltage with low thickness. A good trade-off has been achieved with an IMS (Insulated Metal Substrate) consisting of a dielectric material with a thermal conductivity of up to 8 W/mK and a thickness of 100 µm, and a thick copper layer allowing the IMS boards to act as massive heat spreaders. The signal layer of the IMS is structured with conventional wet-etching technique. The structured IMS boards are placed on both sides of the power core (Fig. 1). The large-area interconnections between the core and the IMS boards are made with a silver sinter paste. A reliable metallic interconnection between the copper layers and the silver particles is formed by low-temperature sintering during a lamination process at a temperature between 200 and 250°C and a pressure of below 5 MPa.

Fig. 1. Process steps for Power Module fabrication.

The silver sinter paste has several functions: It connects mechanically and electrically the elements of the module while it acts as a stress releasing zone between silicon chip and the copper of the power traces. It equalizes the topography between power core and IMS board. In Fig. 2 a microsection of a motor driver demonstrator module fabricated using the EmPower technology [4] is depicted showing the layer structure in the region of an embedded high-power MOSFET. A convincing impression how much embedding can contribute to miniaturization is gained by comparing Fig. 3 a) showing a B6 bridge with the MOSFETS assembled in conventional SMT and b) demonstrating an EmPower demonstrator module with equivalent electrical data and embedded B6 bridge.

Fig. 2. Microsection of a motor driver demonstrator module

showing a part of an embedded high-power MOSFET.

Fig. 3. 500 W-Pedelec power module (a) in surface mount

technology (benchmark module), (b) EmPower Demonstrator module.

Thermal simulations of different design concepts were made for finding the optimum combination of geometries, materials, and process-steps. This is shown in parts in the next section.


3. THERMAL SIMULATION-BASED CONCEPT

DESIGN OPTIMIZATION

In order to find the optimum copper structure of the 500 W Demonstrator power module different 14-layer models were created. One model with copper-filled slots for to-side MOSFET connections (variant a) and one with a filled via array (variant b) are depicted in Fig. 6. For model discretization geometry data of the copper structure were converted from GERBER format into an orthogonal grid with 20 µm resolution using our high-resolution thermal simula-tion tool TRESCOM [5]. A steady-state thermal simu-lation of an operation case at full-power (24 W) with bottom-side cooling at an ambient temperature of 25°C reveals following (Fig. 6): In the lateral direc-tion the temperature variation within the MOSFETs is rather small and high in the immediate vicinity of the silicon; the differences between the respective temper-ature maxima in x-direction are rather insignificant (Fig. 7). By contrast, the profile in y-direction (Fig. 8) shows higher temperature maxima of the MOSFETs closer to the boarder of the PCB (Q2, Q4, Q6), since given by the layout they have less heat spreading area, whereas the lower MOSFETs (Q1, Q3, Q5) have ca. 1.5 K less high temperature due to heat spreading over the remaining PCB.

The simulations revealed that variant a) with copper-filled slots thermally perform almost as well as those with copper-filled microvia array design (variant b) and that both designs will meet the heat dissipation requirements (Table 1).

Fig. 4. 3D view of thermal model; a) general view with parts of copper wiring structure and B6-bridge part; b) cross

section with details of vertical model layer set-up.

Fig. 5. B6 schematic with used nomenclature.

Fig. 6. Temperature distribution in upper MOSFET/Cu-interface layer; a) model with copper-filled slots, b) with µvia connections.

Fig. 7. Temperature profile in x-direction at the semi-conductor/Cu interface layer (location shown in Fig. 6 a).


Fig. 8. Temperature profile in y-direction at the semi-

conductor/Cu interface layer (location shown in Fig. 6 b).

Table 1. Calculated maximum MOSFET temperature (Q6).

Var. Contact geometry ACu on MOSFET TMAX

1 Slots 2 x 1.2 x 0.2 mm2 cross section

0.271 mm² 33.4°C

2 μvias 39 x 0.1 mm diameter

0.306 mm² 33.2°C

4. MEASUREMENT-BASED THERMAL

CHARACTERIZATION

Two different sets of thermal data were assessed:

Firstly, each MOSFET was operated at full power while the following defined boundary conditions were applied to the module:

a) Double-sided cooling b) Bottom side cooling c) Topside cooling

In case a) the heat is removed from two coolers (attached to the upper and the lower IMS) in such a way that their temperature differences during operation remain equal. In this way only one heat sink temperature THS exists. This is important in order to use the common thermal resistance definition:

J HSth,J-HS

F F

T TTR

P U I

, (1)

where TJ is the respective junction temperature and UF and IF denote the voltage drop and forward current of the integrated diode. In case b) and c) only one heat sink at the time was used while on the opposite IMS adiabatic boundary conditions were realized.

Secondly, in a further experiment the temporal thermal behavior was assessed under extreme conditions: the module, mounted in a housing, was operated at full power at still air. The warming-up until reaching steady-state conditions and cooling process was observed by IR thermography.

4.1 Thermal characterization based on single-MOSFET operation

An accurate thermal performance compare of different module variants is possible by assessing the junction–to–heat sink thermal resistances of each MOSFET at single-transistor operation while the respective boundary condition was applied to the module. For this purpose the nominal power was applied to the integrated diode until steady-state conditions were reached, then the current was switched from the load value to a measuring value of 50 mA and the forward voltage change was recorded. As an example, a set of such forward voltage recordings of all MOSFETs of the B6 bridge in case of double-sided cooling is depicted in Fig. 9.

In a separate measurement under isothermal condition (not shown here) the temperature dependence of the forward voltage of the diodes was assessed using a forward current of 50 mA so that they can be applied as temperature sensors allowing converting the recorded forward voltage curves into temperature curves (Fig. 10).

Fig. 9. Forward voltage measurement, condition double-

sided cooling.

It should be noted that in the moment of switching and some few milliseconds later, the temperature value obtained in this way is not correct because the limited switching speed of the load current. However, by extrapolating the voltage curve to the moment of switching (t = 0) as shown exemplarily in Fig. 10 the


respective junction temperature Tj(0) of the steady-state case can be assessed [6, 7]. By applying these values together with the measured heat sink tempera-tures and applied power (P = UF·IF) into equation (1) the thermal resistance values are obtained as listed in Table 2 on the example of sample module #4. The same values are compared in the graph in Fig. 11 with values of a B6 bridge assembled with MOSFETs in conventional SMD package as a benchmark module. On a first glance one can see that the thermal resistances of the EmPower module even in the case of single-sided cooling are less than 50% of the SMT benchmark module. In case of double-side cooling the performance is even slightly better. This improvement can be explained by the superior heat spreading ability of the EmPower concept by the large-area copper connection and the short thermal paths between the MOSFET chips and the upper and lower IMS (Fig. 1).

Fig. 10. Exemplary results of junction temperature

measurement derived from forward voltage measurement shown in Fig. 9 (Sample #3, condition: double-sided

cooling).

Fig. 11. Comparison of junction-to-heat sink thermal resistances of different cooling cases with those of a

benchmark module fabricated in SMT.

Table 2. Thermal junction-to-heat sink resistances of sample #4 (for transistor nomenclature refer to Fig. 5).

Transistor connection

Q2A/L1

Q4A/L2

Q6 A/L3

Q1 B/L1

Q3 B/L2

Q5B/L3

Average

SMT module(benchmark) 4.40 4.35 4.57 4.39 5.79 5.49 4.83

EmPower Top-side 2.42 2.09 2.45 2.17 2.49 2.74 2.39

EmPower Bottom-side 1.50 1.45 1.61 1.59 2.01 2.15 1.72

EmPower Double-side 1.22 1.28 1.33 1.29 1.78 1.99 1.48

4.2 Temporal thermal module behavior

Special interest is devoted to the question how the module behaves under extreme conditions, i.e. full load operation and still-air condition. For this purpose a demonstrator housing of aluminum was developed with a design as close as possible to a final housing design which could be fabricated in mass production. The modules to be tested were fixed in this housing using thermal grease at the interfaces between the respective outer IMS outer copper layer and the alu-minum housing part. The temperature of a test point in the vicinity of the transistor Q3 was recorded. For

Fig. 12. Location of power supply connections and thermocouple assessing the test point temperature.

Fig. 13. Test point temperature of a copper track in the vicinity of the transistor Q3 (see Fig. 6) of module #6

versus time during three hours of heating at nominal power and three hours of cooling, the module horizontally aligned,

boundary condition: “still air” (natural convection).


this purpose a thin-wire k-type thermocouple was soldered on a copper track (Fig. 12).

In Fig. 13 a comparison of the test point temperatures in the cases of single- and double-sided cooling is depicted. After about 3 hours the final temperature distribution is reached and the test point temperature in case of single-sided cooling is about 5 K higher than in case of double-sided cooling (The recordings were made at slightly different room temperatures). In both cases the maximum temperature remained far below a critical value proving thermal robustness of the packaging concept.

The spreading of the heat over the housing surface has been observed by IR thermography. For this purpose the housing was provided uniformly with an emission coating. A temperature distribution at the surface of the housing is shown in Fig. 14.

Fig. 14. IR thermography image of housing 33 minutes

after power-on, case double-sided cooling.

5. CONCLUSIONS

A new packaging concept called EmPower technology has been introduced allowing embedding of power semiconductor devices (IGBTs, MOSFETs, diodes) neighbored to each other at small distances as thinned chips into a polymer based multilayer printed circuit board. On the example of a 500 W motor driver module a significant miniaturization compared to current SMT solutions has been proved.

The thermal significance of this concept has been studied and the module’s design optimized by thermal simulation. Moreover, for establishing the thermal performance of a demonstrator module two kinds of thermal measurements were made:

(1) Single-transistor operation based thermal characterization measurements under well-defined

laboratory conditions allowed precise and direct quantitative comparison of different modules and

(2) investigations of the temporal thermal behavior of final demonstrator modules mounted in a prototype housing under harsh operation conditions at full nominal power loss.

Type (1) results can be summarized as follows: In the case of top-sided cooling the thermal resistances of demonstrators are roughly half of the one of the SMT benchmark module. In case of double-sided cooling the improvement is even better.

Type (2) results prove that the maximum temperatures remain uncritical even under worst case operation of still air after reaching steady-state conditions and differences between the cooling cases “single-side” and double-side” are not significant.

ACKNOWLEDGEMENT

This work was done in the CATRENE Project “EmPower” (Embedded power components for electric vehicle applications). Authors are grateful for financial support by the Austrian Research Promotion Agency (FFG), project number 840449 and the German Ministry of Education and Research (BMBF), FKZ 16EMO0014.

REFERENCES

[1] John Cook et 15 al. "Consensus on consensus: a synthesis of consensus estimates on human-caused global warming", Environmental Research Letters, Vol. 11, No. 4, April 2016.

[2] James Hansen, Makiko Sato, Reto Ruedy, Andrew Lacis, Valdar Oinas: “Global warming in the twenty-first century: An alternative scenario”; Proceedings of the National Academy of Sciences, PNAS, Aug. 29, 2000, vol. 97, no. 18, pp. 9875-9880.

[3] S. Groß, W. Grübl, B. Schuch, H. Stahr, M. Morianz, L. Böttcher, D. Manessis, J. Nicolics, M. Unger: “EmPower – Embedded power components for electric vehicle applications”; APE // Automotive Power Electronics 2017, 26-27 April 2017, Paris.

[4] http://catrene-empower.ats.net/, April, 2017. [5] http://www.agta.at/index-referenzen.htm, April, 2017. [6] M. Unger, H. Stahr, M. Morianz, J. Nicolics, F.

Dosseul: “Miniaturized Power-Diode Package with Superior Thermal Performance using Embedding Technology”; Proc. of 39th ISSE 2016, Pilsen, Czech Republic, 18-22 May, 2016, pp. 97-105.

[7] P. Fulmek, G. Langer, F.-P. Wenzl, W. Nemitz, S. Schweitzer, H. Hoschopf , J. Nicolics: "Direct Junction Temperature Measurement in High-Power LEDs"; IEEE Proc. of 37th ISSE, Dresden Germany, May 7-11, 2014, pp. 58-63.


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Thermal characterization of a 500W motor driver module ... · Thermal characterization of a 500W motor driver module with embedded power transistors Johann Nicolics1), Michael Unger1),