Superior thermal stability by enhanced base plate design combined withdedicated Thermal Interface Material

Martin Schulz Marc Essert Wilhelm Rusche

Infineon Technologies AG

Germany

martin.schulz@infineon.com marc.essert@infineon.com wilhelm.rusche@infineon.com

Abstract

Power semiconductors need to be thermally connected to a heat sink. Besides the mounting process,thermal interface materials in use as well as base plate design have a combined influence on the thermal

transfer. In addition to an initial state, the thermal transfer in dynamic load is observed. Micro-movements

as a consequence of thermal expansion lead to degradation of the interface due to pump-out effects.The present paper describes the development of a new module base plate and the possible adaptations

to reduce this influence. Measurements of different types in combination with various thermal interface

materials are presented.

1 Introduction

New developments in power electronic components focus on three major aspects. Electric improvements

target the reduction of switching- and static losses along with EMI behavior. Mechanical changes in thedesign cope with the improvement of the mechanical robustness as well as thermal aspects.

There is a noteworthy difference within these three subjects. Electrical tuning and internal improvements

of the overall construction of the power module are done by the semiconductor manufacturer. Thermalaspects however depend on the assembling at the user’s site. The added thermal interface component

and the application itself have a large impact on the module’s performance. Care has to be taken tothoroughly consider mounting aspects, the interconnection of the module to the according heat sink

and the thermo dynamics throughout the predicted lifetime of the final inverter. Using the example of

a module with a larger base plate reveals, how various approaches can help to optimize the module’sbase plate to achieve outstanding thermal performance and therefore support the development of highly

reliable, long lasting inverter systems.

2 Thermo Dynamics

Power modules as larger entities suffer from high temperature swing during operation. In combinationwith the macroscopic geometric topologies of the base plate, this leads to a change in volume between

the module and the heat sink as depicted in figure 1.

Figure 1: Change in shape of a base plate due to temperature difference, Arrows denote screw forces

It can be concluded, that the initial state of the base plate in cold condition has an influence of thethermally induced movement. The gap between base plate and heat sink at high temperature levels has

to be as small as possible to improve the thermal coupling [4]. Optimization has to take the initial shape,preforming and the thermal processes during production into account to increase the area of the base

plate that is thermally active in the final design. In addition to the methods described in [1] well-directed,

local compacting in certain areas of base plates is a viable option. During the development phase of theEconoPACK+ D-Series, different approaches were evaluated. Stamping was done in the areas that later

carry the DCB-Material as hinted out in figure 2.

Figure 2: Base plate and DCB-Location for stamping

To evaluate the influence regarding thermal performance and the magnitude of the pump-out effect, a

test bench was set up to achieve reproducible and comparable results in an active thermal stress test.

3 Test bench and experimental results

The system to test the setup consists of a module mounted to a proper heat sink featuring forced air

cooling. All IGBT-Chips inside the module are active and series connected to achieve symmetric current

sharing. Thermal measurement is done using an IR camera. As pump-out in vertically mounted systemsgrows, this mounting direction is chosen for the test, the setup is displayed in figure 3.

Figure 3: Test bench to evaluate thermally induced pump-out effects

The current through the module is controlled by an external power source; the turn-on timing is done by

a microcontroller that is also used to count the cycles done in this test.

The two different base plate designs were tested with widely different results, summarized in figure 4:

Figure 4: Left side - Stamp A after 45 cycles with massive pump-out. Right side - Stamp B after 1200cycles with reduced pump-out effect

The device under test is an EconoPACK+, 450A consisting of three half bridges. Due to the verticalmounting, the top part of the module looses the thermal transfer path first. It can be observed, that the

temperature in this part of the module increases rapidly. For the chosen solution of grease, base plate

with Stamp A and forced air cooled heat sink, the temperature rise was 20K in less than 35 hours oftesting. Though the pumping itself is a consequence of the changing volumes below the module’s base

plate, the thermal interface component in use plays a major part as well. It depends on several physical

parameters to what extend a thermal compound reacts on the pressure applied. Creeping and wettingabilities along with the surface tension in high temperature conditions matter but are difficult to pinpoint in

numbers. Lately, newly developed Thermal Interface Material (TIM) was developed especially dedicatedto power modules [2, 3]. This new material was tested with the setup described above with outstanding

results. The graph in figure 5 hints out, that after thousands of hours in the test the chip temperature

increase was almost negligible.

Figure 5: Comparing different thermal interface materials regarding degradation due to pump-out effect

Two general purpose greases (GPG) were tested but failed without reaching the target set for the qualifi-

cation. Infineon’s newly designed solution achieved the lowest chip temperature in this test and remianedstable for about 4000 hours. The test was discontinued without failure. No pump-out was observed.

4 Conclusion

Optimizing the base plate of a power module is a necessary step to achieve the desired thermal perfor-

mance. However, the base plate, the thermal interface material and the application requirements have

to be considered as a complete system that has to work properly under specified thermal conditions.Optimizing has to take every part involved into consideration to achieve the best possible result.

References

[1] David Dalleau, Indrajit Paul and Frank BroermannFirst principle optimization of power module baseplate

PCIM 2011 Nuremberg, Germany in May 2011

[2] Scott T. Allen, Martin Schulz, Wilhelm PohlOptimizing Thermal Interface Material for the Specific Needs of Power Electronics

PCIM 2012 Nuremberg, Germany in May 2012

[3] Martin Schulz

Thermal Interface A Key Factor in Improving Lifetime in Power ElectronicsPCIM 2012 Nuremberg, Germany in May 2012

[4] Mark Essert et. al

EconoPACK+, A package with enhanced characteristicsPCIM 2011 Nuremberg, Germany in May 2011