How Do Thermal Compounds Affect Thermal Resistance?

Whenever you apply a thermal compound material to an electronic component (such as a thermal paste or thermal grease), the intent is to speed up the rate at which heat is transferred away from a hot component. In this way, the intent is to lower the device’s temperature rise per unit of power dissipated in the device. This measure of temperature rise per unit of power dissipation is known as thermal resistance, and the goal in applying a thermal compound to a component is to lower that component’s thermal resistance.

If you’re familiar with thermal analysis of electronic components, then you’ll know what the thermal resistance means and how it is quantified in real components. For everyone else, we’ll briefly examine thermal resistance, then we’ll show how the characteristics of a thermal gap filler compound affect thermal resistance.

Thermal Compounds and Thermal Resistance

Thermal compounds are called thermal fillers, or thermal gap fillers, based on the function they perform when applied to a component. When one of these materials is applied to a component, and the heatsink is attached to the component under mechanical pressure, the material fills in microscopic gaps at the component-to-heatsink interface. In other words, any air at the interface is replaced with a high thermal conductivity material that can efficiently transfer heat to a heatsink. By creating a high thermal conductivity path between the component and its heatsink, the heat flux between the component and the heatsink will be maximized.

Because air has low thermal conductivity, removing it through application of a thermal compound under pressure will increase the overall thermal conductivity of the interface. The resulting thermal compound to air volumetric ratio will determine the thermal resistance of the interface. It is this ratio that can be used to predict how a thermal compound will affect thermal resistance of a component (see below).

Thermal Resistance Prediction

In a real component, the thermal resistance depends on several factors, including what ambient temperature is being used to define a temperature rise. In a component datasheet, the thermal conductivity of the component packaging almost always does not appear in the material or packaging data. Instead, a measured thermal resistance value is normally defined between two regions or portions of the component.

For purposes of predicting the change in thermal resistance of a component with a heat sink, the thermal resistance value that matters is the package-to-ambient or die-to-ambient values. The die-to-package thermal resistance can be quite low, meaning heat easily leaves the component die and transfers directly into the package. However, package-to-ambient thermal resistance values, particularly for epoxy-based packages, can be very high. Some typical ranges for different component packages can be found below. 

There is a simple model that can be used to qualify the change in a package’s thermal resistance based on the thermal conductivities of air and the thermal compound. First, we can calculate the package-to-ambient thermal resistance using the thermal conductivities of air and a thermal interface material (TIM):

Package-to-ambient thermal resistance for the air-exposed portion and the TIM-exposed portion.

Next, if we take the difference between these two values, we can see the degree to which the thermal compound (TIM layer) decreases the thermal resistance:

Package-to-ambient thermal resistance for the air-exposed portion and the TIM-exposed portion.

The challenge in using this formula is in determining the filler:air volumetric ratio, which can range anywhere from 10:1 to 1000:1 or even larger. This will also vary based on the type of filler material; some porous thermal pads will have low ratios, while a thermal grease can very efficiently fill microscopic gaps in the component-heatsink interface and can have much higher ratio.

Which Thermal Compound Gives the Best Results?

From the above 1-D model, it should be clear that there are three factors determining which thermal compound might be best suited for thermal resistance reduction:

• Materials in the thermal compound, which determines its thermal conductivity
• The assembly process, which determines the air-filler ratio
• The size of the area where the compound is applied
• The thickness of the compound once applied/fully cured

With this many variables at play in any given system, it can be difficult to make generalized comparative statements about the effectiveness of every thermal compound. Typically, the material selection consideration is not based on the change in thermal resistance, but rather on other engineering aspects like reworkability, cleaning, assembly costs, outgassing, etc. Once a compound is chosen, it will then be validated in a prototype assembly before production is scaled.

There is nothing wrong with this process until you find that the initially chosen thermal compound does not provide enough heat reduction. Instead, a comprehensive set of thermal simulations involving heat transfer and airflow should be performed to determine a target thermal resistance reduction value for important components. This way, you can specify a new criteria the system must meet and use this as a performance target, rather than focusing on cost or assembly. This may be a better strategy for high-reliability designs.

When you need to examine thermal characteristics of your system at every level, make sure you have the complete set of system analysis tools from Cadence. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity.

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