How do thermal interface materials help in power electronics cooling?

Thermal interface materials (TIMs) are placed between two mating surfaces, such as a power semiconductor module and a heat sink, to improve heat flow between them. This FAQ will take a few examples to illustrate how TIMs conduct thermal management in power electronics.

Where are TIMs finding their place in power electronics cooling?

Power electronics devices such as EV chargers, voltage regulators in data centers, industrial power supplies, etc. dissipate much heat. As shown in Figure 1, it is necessary to manage the temperature using fans, heat sinks, and TIMs.

Figure 1. An illustration of places where TIMs, such as PCM/grease and gap pads, are used in a CPU-cum-power supply unit. (Image: Wesco)

TIMs can be classified into four categories:

Thermal grease
Polymer-based
Carbon-based
Metal-based

Thermal greases possess a paste-like consistency and low bond line thickness, usually ranging from 0.5 to 1 mm, which aids in reducing the thermal resistance between interfaces. Polymers are commonly used as a matrix for TIMs because of their excellent mechanical properties, ease of handling, and flexibility.

Carbon-based TIMs have seen exciting development due to carbon-based nanomaterials’ ultrahigh theoretical thermal conductivity. Metal-based TIMs such as copper, silver, and aluminum are highly thermal fillers for effective heat dissipation.

How are TIMs helping in the thermal management of power electronics?

An illustration of how TIMs help heat dissipation from a semiconductor chip is shown in Figure 2. The left side of Figure 2 shows a stacked-up arrangement of a semiconductor chip, a metal lid, and then a heat sink on the top. It can be observed that the heat is concentrated in the semiconductor chip, and as we move up, the heat dissipation is minimal, with almost no heat dissipation in the heat sink. The corresponding temperature vs. distance graph shows the maximum thermal resistance at the junction between the chip and the metal lid.

Figure 2. Differences in heat flow in a stack consisting of a semiconductor chip, a metal lid, and a heat sink with and without TIMs. (Image: Wiley Online Library)

The right side of Figure 2 is a stacked-up arrangement of a semiconductor chip, a TIM, a metal lid, another TIM, and then a heat sink on the top. From the heat flow map of this arrangement, it can be observed that heat flows freely through the TIM from the source to the heat sink. The corresponding temperature vs. distance graph shows a reduced thermal resistance as the distance increases from the heat source.

At the National Renewable Energy Laboratory (NREL), an experiment was conducted to study the effect of TIMs with different thermal resistances on a power electronics package. Figure 3 shows the temperature vs. distance graph for four different varieties of TIMs at the IGBT location, the baseplate backside, and the heat sink base.

Figure 3. Variations in temperature at the IGBT location, baseplate backside, and heat sink base when using various thermal resistance values TIMs. (Image: NREL)

The baseline TIM refers to the grease thermal resistance set at 100 mm²K/W. A 5x TIM means that the thermal resistance of the grease is five times lower at 20 mm²K/W, and so is the case with 10x and 20x TIM. As the thermal resistance decreases, the heat dissipation capacity of the TIM improves.

It is evident from the experiment that TIMs help dissipate the heat at each of the junctions. The temperature is maximum at the IGBT location, where the heat is generated. The temperature lowers at the baseplate backside and then at the heat sink base.

The thermal management of a power electronic system using TIMs can also be seen from the perspective of incidence, reflection, and refraction, as shown in Figure 4. Incidence occurs when the heat enters TIM from the chip, and the TIM should allow maximum heat entry with high thermal conductivity. With a low interfacial resistance, TIM can minimize reflection to ensure that heat from the chip does not bounce back. Refraction greatly helps spread the heat in multiple directions, and using anisotropic TIMs, such as graphene, is good for such purposes.

Figure 4. Recent trends in heat sources, TIMs, and heat sink thermal management in power electronics. (Image: Data Centre Dynamics Ltd)

Figure 4 also represents the direction in which power electronics cooling using TIMs is heading. The semiconductor chips are extended in various forms such as CPU, FPGA, GPU, and GaN transistor. TIMs are also catching the speed with a combination of conventional technologies, such as solder, polymer composites, and liquid metals, and emerging ones, such as vertical nanowires and graphene-based materials.

Summary

TIMs may seem a tiny part of a power electronics system, but their importance cannot be underestimated. Their primary role is to fill the small air gaps and voids that naturally exist between even relatively flat surfaces of the power electronics components. Even though TIMs are seen mainly for thermal management, they also help in mechanical stability, such as the thermal pad, which absorbs shocks and vibrations. Thermal management has always been a pain point for electronics engineers, and TIMs will only continue to involve lower thermal resistance, longer durability, and better mechanical properties.