The Electrical and Thermal Characteristics Influencing Thermal Interface Material Selection

Key Takeaways

• Thermal interface materials act as a catalyst for removing heat from electronic components. They are essential to improving thermal coupling between two mechanically attached surfaces.

• Thermal interface materials fill the gaps between uneven surfaces and transfer heat between two or more solid surfaces.

• Types of thermal interface materials include phase-changing materials, gap-fillers, putties, greases, insulators, thermal pastes, gels, adhesive tapes, potting compounds, and liquid adhesives. 

Proper thermal management is critical to ensuring electronics perform reliably. Luckily, there are a variety of thermal management techniques engineers can employ to alleviate heat problems, including heat sinks, cooling fans, and the use of thermal interface materials. 

Thermal interface materials (TIMs) act as a catalyst for heat removal from electronic components and the use of TIMs is a critical part of an efficient thermal management system. This thermal management solution is vital to improving the thermal coupling between two mechanically attached surfaces (for example, a heat sink and a solid-state switch), and thereby multiplying the heat transfer rate compared to surfaces that are interfaced without TIMs. 

In this article, we will discuss some key tips on thermal interface material selection. 

The Thermal Interface Material Selection Process

TIMs fill the gaps between uneven surfaces and transfer heat between two or more solid surfaces

Today, a wide variety of TIMs are available. Irrespective of the type, TIMs fill the gaps between uneven surfaces and transfer heat between two or more solid surfaces. However, the efficiency of this heat transfer varies depending on the material used. Because of this, it is essential that designers choose a TIM suitable for the application they are working on. To select the best TIM, it is important to first understand the different TIMs available. 

Phase Change Materials (PCMs)

Phase change materials change from a solid state to a liquid state

Phase change materials (PCMs) are a type of TIM that changes from a solid state to a liquid state, usually under 50-100℃. Since PCMs are solid at room temperature and liquefy at elevated temperatures, they offer desirable features such as easy handling and wetting. They are free from dry-out issues due to their greasy state and support pre-applications for assembly in the future. They are available in different forms such as rolls, sheets, and pre-cut shapes. 

Gap Fillers

Gap fillers are thermally conductive materials. When used to interface two surfaces, they lower the thermal resistance between them. Gap fillers replace the air gaps between the surfaces and provide a proper thermal path for heat dissipation. 

Putties

Putties are used to fill the gaps between uneven surfaces that are not in firm contact. These materials can be compressed to half of their original thickness and are applied between surfaces to increase the thickness. They optimize thermal performance by allowing proper contacts between surfaces. 

Greases

Thermal greases are applied in thin layers between smooth and flat surfaces to decrease the thermal resistance between them. They offer low bond line thickness between the mating surfaces. 

Insulators

Insulators are thermal products used between two surfaces to lower thermal resistivity. As the name suggests, they are electrically non-conductive.

The list of TIM variants is not limited to those mentioned above. Other TIMs include thermal pastes, gels, adhesive tapes, potting compounds, and liquid adhesives. 

Factors Influencing TIM Selection

Electrical and mechanical factors influence thermal interface material selection. Most TIMs available are electrically conductive. When we concentrate on insulation, we might lack the mechanical properties required for a reliable TIM. So, if any application needs an electrically insulative TIM, it is better to use a combination of TIMs. This combination should include one TIM which is an insulator and another TIM which completely interfaces the surfaces. 

Mica insulators with silicone-based thermal grease, ceramic-filled fiberglass, and various polymers are all examples of potential TIM combinations. 

Thermal Quantities of TIMs

Thermal quantities related to TIM

Thermal Conductivity 

Thermal conductivity is a quantity that is analogous to electrical conductivity. Electrical conductivity describes the ease with which a material allows the flow of electrons through it. Thermal conductivity is expressed in W/m°C.  It is a material property and is not dependent on the size and shape of the TIM. Thermal conductivity is generally used as a figure to compare two TIMs and determine their suitability for a particular application. When selecting TIMs, it is advisable to check the thermal conductivity of the material. The higher the thermal conductivity, the faster the will heat transfer through the material.

Thermal Resistance 

The thermal resistance of a TIM is dependent on its geometry. The unit of thermal resistance is °C/W. Thermal resistance helps designers understand the temperature difference between two surfaces interfaced by TIMs. The thermal resistance is always associated with some dimensions, otherwise, it is meaningless. In such cases, the thermal resistance is expressed in units °C-cm2/W. Thermal resistance can be changed by varying the dimensions. The lower the thermal resistance, the higher the heat transfer between the surface and the higher the performance of the TIM. 

Thermal Impedance 

Thermal impedance is the quantity that defines the temperature gradient, per unit heat flux, passing through the TIM. The unit of thermal impedance is °C-cm2/W. Like thermal resistance, thermal impedance is also not a material property, but it is dependent on TIM geometry. The size of the component and thickness of the TIM are the dimensional factors that influence the value of both thermal resistance and thermal impedance.  

Thermal interface material selection is a significant part of any electronic cooling and thermal management system. Cadence’s software helps designers simulate the heat transfer from electronic components by modeling the solid parts, interfaces, and air dynamics. 

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