Thermal Interface Materials for Heatsinkable Devices

Modern electronics design comes with many challenges, with reductions in size, weight, and cost being huge drivers in development of new technology. Many devices that run at higher frequencies, higher power, or more compact form factor require a well-conceived thermal management strategy, where multiple are used to keep devices cool. In the past, cooling fans were often the first line of defense against high temperatures. Today, advanced thermal interface materials, smaller heat sinks, and unique enclosure designs are playing a greater role in thermal management for advanced electronics.

Selecting a thermal interface material for a heat sink or direct attachment to an enclosure requires considering many material options. In terms of the physics, there are only two parameters to consider: thermal conductivity, or the equivalent thermal resistance. Practically, the goal is to select a material with reliable thermal contact between components, a heat sink, and/or the enclosure that does not impact electrical performance or assembly. Different thermal interface materials have their advantages and disadvantages in both areas, and all aspects of the basic physics and practical application need to be considered when selecting thermal interface materials.

WHAT DRIVES HEAT TRANSFER?

At the molecular level, the sensation that we perceive as temperature is entirely due to microscopic motion of the atoms and molecules that make up any object. When a temperature difference exists between two objects, the result is heat transfer between those objects. In terms of molecular motion, hot particles give up some of their energy to cooler particles through three possible heat transfer mechanisms:

• Conduction, which refers to heat transfer through direct contact between two objects and is governed by the thermal conductivity of both
• Convection, which refers to heat transfer as assisted by circulation or motion of a heated gas or Convective heat transfer is aided by forced airflow and mixing.
• Radiation, which is heat transfer through emission of electromagnetic waves from hot objects and subsequent absorption by cool

In electronics, the main quantity of interest is thermal conductivity, which describes the rate at which heat flows through the body of an object towards a cooler region. A related quantity, thermal resistance, is more often used instead of thermal conductivity to describe how an electronic component’s power consumption increases its temperature. This is a summative metric engineers can use to determine the heat management approach they should take in their designs.

TAKING ADVANTAGE OF HEAT TRANSFER MECHANISMS IN ELECTRONICS

Thermal management in electronics is all about manipulating and directing heat flow away from critical components. Electronics designers can take two strategies to remove heat from important components and dissipate it into the surrounding environment. Active thermal management approaches typically involve forced airflow, such as with a cooling fan mounted on a CPU. Although modern fan designs have become smaller, quieter, and more powerful, they are still not useful in many compact electronic devices. In addition, fans can create electrical noise that might interfere with low-level measurements or power circuitry.

Passive thermal management is simpler; it typically involves allowing heat to naturally conduct into a heat sink, or possibly directly into an enclosure. Heat dissipation into the heat sink is driven by conduction, and heat can then dissipate into the surrounding air via convective heat transfer. Natural convection can carry some of this heat away from the device. However, passive heat dissipation can be combined with forced airflow, which will aid convective heat transfer away from hot components and the heat sink. Forcing high heat flux away from a component and into a heat sink starts with conduction and occurs directly across the interface between a component and its heat sink. This is where a thermal interface material is needed to increase heat dissipation away from the target component.

FUNCTIONS OF THERMAL INTERFACE MATERIALS

Thermal interface materials provide three main functions as part of heat conduction: 

1. They act as an adhesive for the heat sink and can eliminate the need for bulky clips or screw attachments
2. They fill in microscopic gaps between components and a heat sink element, which ensures higher heat dissipation away from a hot component
3. They provide a path for heat dissipation with high thermal conductivity, which will increase convective heat transfer from the heat sink element into the surrounding environment

There are many options for thermal interface materials, each with their particular advantages in terms of thermal conductivity, cost, and ease of application. In terms of high volume manufacturing and range of use, certain materials have broader applicability than others and are an excellent choice for attaching heat sinks.

COMPONENT FORM FACTOR MATTERS

Not all high power components are candidates for attaching a heat sink with a thermal interface material. The best candidates are planar integrated circuits, high speed digital processors, power FETs, and some power management components in standard packages. For example, CPUs can accommodate a range of thermal interface materials, including pastes or tapes across the top surface of the component package. Thermal interface materials that can be applied to components in standard packaging have an advantage in that they can be easily applied in a high volume production environment and without manual cleanup.

TYPES OF THERMAL INTERFACE MATERIALS

There are many types of thermal interface materials, with designations often being used interchangeably. These materials can be liquid-dispensed, solid materials, or flexible adhesives.

THERMAL ADHESIVES (PASTES, GREASES, AND GAP FILLERS)

Adhesive thermal interface materials are sometimes referred to interchangeably, but they have a range of possible thermal conductivity values and application ranges. These materials are liquid dispensable during assembly, and they are best used on larger planar components like CPUs, or on battery packs. In the latter application area, thermal paste materials can be used to direct heat directly into an enclosure, but this is less common in other areas of electronics. Generally, these are only used for heatsinks on larger components as they do provide strong adhesion. For very large heat sinks, or when targeting smaller electronic devices with standard packaging, an alternative is needed to remove heat from the system.

THERMAL EPOXY

One class of thermal interface material with strong adhesion for larger heat sinks is thermal epoxy. These two-part materials need to be mixed and dispensed onto the component, and they will begin to adhere to the surface as they harden. Because they are liquid dispensed, they can provide very effective gap filling that leaves very few air pockets between the component and its heat sink. They are strong enough to attach components where a large mechanical mount is unavailable, or they can be used to bond a device directly to its enclosure. However, the thermal expansion coefficients of the hardened epoxy and bonding surfaces must be similar, and manual assembly may be required to properly apply these materials during production.

THERMAL PADS

A more flexible solution with a range of possible thermal conductivity values is thermal pads, which are generally cut into a specific shape and applied directly to the target component. These pads are flexible when applied and are broadly applicable to large and small components. They can also be incorporated into automated assembly processes. One disadvantage is that,  if not manufactured specifically to fit a component, a thermal pad would have to be manually cut and applied to the component. These material are also not the best option for components with rough surfaces and can leave some air gaps between the mating surfaces.

The table above shows a summary of advantages and disadvantages of various thermal interface materials.

ADVANTAGES OF THERMAL PADS AS THERMAL INTERFACE MATERIALS

Among the thermal interface material options shown above, only one class of material can be manufactured with standard form factors to fit critical components: thermal pads. Thermal pads are superior thermal interface materials from the perspectives of performance, assembly, and applicability. The major advantages of thermal pads are: 

• Thermal conductivity: The materials used in thermal pads have among the highest thermal conductivity values among all thermal interface materials. Some thermal pads can outperform advanced materials like graphite thermal pads in terms of z-axis thermal
• Assembly: Thermal pads can also be used in automated assembly processes, making them competitive with other liquid or paste-based thermal interface If manufactured in die-cut form factors, no manual application or trimming is needed when applied to a standardized heat sink.
• Ease of application: Thermal pad materials can be die-cut to fit standard component packaging. This means that thermal pads can be used on components where thermal greases or pastes can’t be used, such as high power MOSFETs and power regulator ICs. They can also be used to provide a heat dissipation path into an enclosure.

Thanks to their ease of application and broad applicability, thermal pads should be the first choice for heat management in advanced electronics, particularly for attaching heat sinks to standard packages. This is also the case in small form factor electronics, where the enclosure might be used as the heat sink element.

CONCLUSION

Thermal interface materials will continue to remain a standard strategy for attaching heatsinks to standardized component packages. Thermal pads that match these standard package sizes have the greatest advantages in terms of assembly thanks to their ease of application. Today’s advanced thermal interface materials can provide strong adhesion and gap filling without the messiness of thermal greases or thermal paste, yet they meet the demanding reliability requirements in multiple industries.

When you’re looking for thermal interface materials that are easy to apply and remain reliable with long life time, look at Ohmite’s line of thermal pads for heat sinkable devices. Ohmite’s die-cut products are available in high volume to match standard device footprints for through-hole or surface-mount components.  

Contact Ohmite today to learn more about our thermal management materials.