Tweet

Posted on 04 April 2019

Superior Thermal Stability for Power Modules Achieved by Enhanced Base Plate Design and Optimized Thermal Interface Material

Free Bodo's Power Magazines!

 

 

 

Power semiconductors need to be thermally connected to a heat sink. Besides the mounting process, the thermal interface materials used as well as base plate design have a combined influence on the thermal transfer. In addition to an initial state, the change of the thermal transfer under dynamic load is observed. Micro-movements as a consequence of thermal expansion possibly lead to degradation of the thermal interface due to pump-out effects.

By Martin Schulz, Infineon Technologies AG

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 the design cope with the improvement of the mechanical robustness. Mechanical design also is the key to improve the module’s thermal performance.

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. Thermal aspects however heavily depend on the assembling at the user’s site. The added thermal interface component and the process of application itself have a large impact on the module’s performance. Care has to be taken to thoroughly consider mounting aspects, the interconnection of the module to the according heat sink and subsequent thermo-mechanical effects 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’s base plate to achieve outstanding thermal performance to support the development of highly reliable, long lasting inverter systems.

Thermo Mechanics

Power modules as larger compounds suffer from high temperature swing during operation. The internal structure of a power semiconductor consists of a stack formed from different materials with different coefficients of thermal expansion (CTE). In combination with the macroscopic geometric topologies of the base plate, thermal mechanical movement is induced during operation, leading to a reduction in the volume available between the module and the heat sink as depicted in figure 1.

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

This reduction of volume is the core reason for the pressure that is applied to thermal grease during high temperature operation. Since the structure relaxes during cool-down, the effect reoccurs with every thermal cycle.

From the picture, it can be concluded, that the initial state of the base plate in cold condition has an influence on the thermally induced movement. However, judging the thermal performance of a module by evaluating the shape of the base plate in cold condition will result in misleading interpretations.

The gaps remaining between the base plate and the heat sink at high temperature levels have to be as small as possible to improve the thermal coupling. Optimization has to take the initial shape, preforming and the thermal processes during production into account. The design target is to increase the area of the base plate that is thermally active in the final design. In addition to the methods described in the literature, well-directed, local compacting of metal in certain areas of the base plates is a viable option. During the development phase of the EconoPACK™ + D-Series, different approaches were evaluated. Stamping was done in the areas that later carry the DCB-Material as indicated in figure 2.

Base plate and DCB-Location for stamping

To evaluate the influences of 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.

Test bench and experimental results

The test system consists of a power module mounted to a proper heat sink featuring forced air cooling. Mounting was done according to the recommendations made in the corresponding Application Notes. All IGBT-Chips inside the module are activated and series connected to achieve homogeneous current sharing and homogeneous temperature development respectively. This way, the temperature distribution inside the power module correlates closely to the real application, leading to credible experimental results. As the pump-out effect to be observed becomes more prominent in vertically mounted systems due to gravitational pull, this mounting direction is chosen for the test. Monitoring the experiment includes measuring the voltage across the DUT while the power source is configured to provide a constant current. Thermally, the setup is observed using an IR-Camera to detect even small changes in the temperature distribution. The setup as it was used in the lab is displayed in 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. A cycle of 120 seconds is chosen with an on/off ratio of 1. Finally, two different base plate designs were tested with widely different results, summarized in figure 4.

Stamp A after 45 cycles with massive pump-out. Stamp B after 1200 cycles with reduced pump-out effect

The device under test is a D-Series EconoPACK™ +, FS450R17OE4 consisting of three half bridges and featuring a base plate size of 160mm × 123mm. Due to the vertical mounting, the grease follows the gravitational pull. Therefore, the top part of the module is prone to lose 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 of testing. 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 extent a thermal compound reacts on the pressure applied. Creeping and wetting abilities along with the surface tension in high temperature conditions matter but are difficult to pinpoint in numbers.

Recently, a new Thermal Interface Material (TIM) was developed especially dedicated to power modules. This new material was tested using this same setup, with outstanding results. The graph in figure 5 indicates that after thousands of hours in the test, the chip temperature increase was almost negligible.

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

In this series of tests, two general purpose greases (GPG) that were tested failed without reaching the target set for the qualification. Infineon’s newly designed solution achieved the lowest chip temperature in this test and remained stable for about 4000 hours. No pump-out was observed and the test was discontinued without failure.

Conclusion

Optimizing the base plate of a power module is a necessary step to achieve the desired thermal performance. 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.

 

VN:F [1.9.17_1161]
Rating: 0.0/6 (0 votes cast)

This post was written by:

- who has written 155 posts on PowerGuru - Power Electronics Information Portal.


Contact the author

Leave a Response

You must be logged in to post a comment.