How to Optimize Thermal Management with Heat Spreaders and Gap Fillers

Good thermal management is important to ensure the performance and reliability of Electronic devices. It’s conceptually simple, beginning with the transfer of unwanted heat away from the source, and spreading it over a larger area for effective dissipation and cooling. But in many cases, the implementation can be challenging.

The surfaces of heat-generating devices are typically not smooth enough to have the low thermal impedance needed to ensure good heat transfer. For some devices the surfaces are not planar, increasing the thermal management challenge. Also, the components that need to be cooled can be deep inside the system, further complicating the extraction of potentially damaging heat.

Thermal pastes and greases can be used to improve thermal conductivity but getting the needed coverage to ensure good thermal transfer and avoiding over-application that can cause contamination of circuit board traces and result in short circuits, can be tricky. Additionally, thermal pastes and greases cannot spread heat laterally away from the source.

Instead, designers can turn to a variety of thermal interface materials (TIMs), including gap fillers and heat spreaders to provide the consistently low thermal impedances needed for effective heat transfer, while eliminating any contamination concerns. To meet specific system needs, TIMs can be structured to transfer heat vertically or spread heat laterally. TIMs are available in a variety of thicknesses to match the requirements of specific applications, are mechanically stable at elevated operating temperatures for good reliability, can provide high electrical isolation, and they are easy to apply.

This article reviews thermal management and provides general TIM selection guidelines. It then presents several TIM options from Würth Elektronik and examines the application and design considerations for each.

What are TIMs?

TIMs are placed between a heat source and a cooling assembly to improve thermal coupling and heat flow. Two factors increase the efficiency of the thermal coupling. First is the ability of the TIM to conform to microscopic surface irregularities, eliminating all pockets of insulating air that reduce the thermal conductivity of the interface (Figure 1). Second, TIMs have the thermal conductivity required to effectively transfer heat from the source to the cooling assembly. Thermal conductivity, K, is quantified as watts per meter per degree Kelvin (W/mK). It’s measured using ASTM D5470, “Standard Test Method for Thermal Transmission Properties of Thermally Conductive Electrical Insulation Materials.”

How to Optimize Thermal Management with Heat Spreaders and Gap FillersFigure 1: A TIM (blue) is used to fill in the microscopic irregularities that exist in the surfaces of components and cooling assemblies to improve thermal coupling. (Image source: Würth Elektronik)

In addition to thermal conductivity, there are several considerations when selecting a TIM:

  • Operating temperature range is important since various TIMs are specified for different temperature ranges.
  • The distance between the mating surfaces and whether the TIM needs to be compressed to deliver optimal thermal transfer.
  • Compression pressure withstand capability of the TIM.
  • Some TIMs are available with adhesives applied to their surfaces that enables mechanical fixing.
  • Electrical insulation property of the TIM as some materials can be used to provide electrical isolation.
  • Some TIMs are available as standard parts with no minimum order quantity and no tooling costs, while others are available in custom shapes that can be optimized for specific application requirements.

Gap filler choices

The WE-TGF silicone gap filler is a general-purpose material designed to be used in low-pressure applications that benefit from electrical isolation, where the TIM is compressed between 10% and 30% of its thickness. Exceeding the recommended compression level may result in silicone oil expulsion, reducing the expected lifetime of the material, and possibly contaminating the printed circuit board (pc board). These TIMs are designed for use between two mechanically secure surfaces as they do not have any additional adhesive beyond their natural tackiness. Thicknesses from 0.5 to 18 millimeters (mm) are available with thermal conductivities between 1 and 3 W/mK. Thicknesses from 0.5 to 3 mm support higher levels of thermal conductivity (Figure 2).

How to Optimize Thermal Management with Heat Spreaders and Gap FillersFigure 2: Thermal gap fillers from Würth are available to meet the needs of a wide variety of applications. (Image source: Würth Elektronik)

For example, part number 40001020 is a 400 x 200 mm pad that is 2 mm thick with a K of 1 W/mK, and a dielectric strength or electrical break down rating (EBR) of 8 kV/mm. The soft and electrically insulative characteristics of WE-TGF gap fillers make them suited for use between one or more electronic components and a cooling assembly (Figure 3).

How to Optimize Thermal Management with Heat Spreaders and Gap FillersFigure 3: A silicone elastomer gap filler pad is designed to fill a gap between one or multiple electronic components and a cooling assembly, such as a heatsink, cooling plate, or metal housing. (Image source: Würth Elektronik)

For thermal management applications that need electrical isolation and a thinner profile, designers can use the WE-TINS thermally conductive silicone insulator pad with K from 1.6 to 3.5 W/mK and a thickness of 0.23 mm. Part number 404035025 has a K of 3.5 W/mK and an EBR of 6 kV/mm. Like all parts in the WE-TINS series, the 404035025 combines thermally conductive silicone rubber and a fiberglass mesh. The mesh adds mechanical strength and is puncture and shear resistant. As a result of the mechanical properties of the structure, these TIMs can be compressed as desired, and they have high tensile strength.

Thermal phase-changing materials and thermal transfer tapes are thinner yet, with profiles of only 0.02 mm. For example, the WE-PCM series of phase-changing TIM changes from a solid to a liquid at a specific temperature, providing a complete wet-out of the interface without any spills or overflow. They are designed for use with high-performance integrated circuits or power components and cooling assemblies. For example, part number 402150101020 measures 100 mm square with adhesive on both sides, a K of 5 W/mK, an EBR of 3 kV/mm, and a phase change temperature of 55 degrees Celsius (°C).

WE-TTT thermal transfer tape is a double-sided tape that enables mechanical fixing of both contact surfaces. It has a K of 1 W/mK and an EBR of 4 kV/mm, and is designed for low-pressure applications. It is available in widths of 8 mm (part number 403012008) and 50 mm (part number 403012050) on 25 meter (m) rolls.

Graphite heat spreading solutions

Synthetic graphite based TIMs offer the highest levels of thermal conductivity (Figure 4). Part number 4051210297017 in the WE-TGS family is a synthetic graphite heat spreader measuring 297 x 210 mm with a K of 1800 W/mK, that provides no electrical isolation. The combination of high thermal conductivity, light weight, and thinness (0.03 mm) makes these graphite sheets useful in a wide range of applications from high power semiconductor modules to handheld devices.

How to Optimize Thermal Management with Heat Spreaders and Gap FillersFigure 4: Graphite heat spreaders offer high thermal conductivities in multiple dimensions and are as thin as 0.03 mm. (Image source: Würth Elektronik)

The WE-TGFG series combines graphite sheets with foam pads to produce unique thermal management solutions with a K of 400 W/mK and an EBR of 1 kV/mm. Long gaskets can be fabricated to serve as heat spreaders, transferring heat laterally from the source to a cooling assembly located in another part of the system (Figure 5). For example, part 407150045015 measures 45 mm long, 15 mm wide and 1.5 mm thick, and can be used in applications that benefit from gap-filling and lateral heat transfer.

How to Optimize Thermal Management with Heat Spreaders and Gap FillersFigure 5: A TIM placed on top of a hot component can act as a heat spreader, transferring the heat laterally away from the component. (Image source: Würth Elektronik)

Achieving higher thermal conductivities with silicon pads like the WE-TGF gap fillers requires that the pad be made thinner. Designers can turn to the WE-TGFG TIMs to fill gaps of up to 25 mm with a much higher thermal conductivity than is possible with silicone pads, and WE-TGFG parts can be made in custom geometries to fit into non-planar spaces (Figure 6).

How to Optimize Thermal Management with Heat Spreaders and Gap FillersFigure 6: A graphite foam gasket (center) can be fabricated with various geometries and be used to interface between a heat source (bottom) and a non-planar heat dissipation element (top). (Image source: Würth Elektronik)

Combining TIMs for improved performance

TIMs can be combined to provide higher levels of performance. For example, a WE-TGS graphite heat spreader can be combined with a WE-TGF silicone gap filler to enable the use of a heatsink with a footprint larger than the heat source, increasing the cooling ability of the overall assembly (Figure 7).

How to Optimize Thermal Management with Heat Spreaders and Gap FillersFigure 7: Combining a WE-TGS graphite heat spreader (TIM 1) with a WE-TGF silicone gap filler (TIM 2) can enable the use of a larger heatsink than the footprint of the hot component, providing enhanced cooling. (Image source: Würth Elektronik)

General application guidelines

Regardless of the TIM or TIMs being used, there are a few general application guidelines that designers need to consider:

  • The surfaces of the component and cooling assembly need to be clean and dry. A lint-free swab or wipe and isopropyl alcohol should be used to remove any surface contamination.
  • When using TIMs that require compression, the material should be compressed with even pressure across the entire surface. The material can be damaged if the applied pressure exceeds the specified rating.
  • All surface air bubbles and/or gaps must be eliminated to realize the best thermal conductivity.
  • The operating temperature of the TIM must be able to accommodate the combination of the ambient temperature and the temperature rise of the component being cooled.

Conclusion

Thermal management is a problem across a wide swath of electronic system designs. As shown, designers can turn to a wide range of TIMs made with a variety of materials including silicones, phase change materials, graphite, and foam pads. The use of TIMs can deliver the consistently low thermal impedances needed for effective heat transfer while eliminating any contamination concerns that can arise when using thermal pastes or greases.

While pastes and greases only transfer heat vertically, designers can choose from gap-filler TIMs that conduct heat vertically or heat spreaders that can conduct heat laterally. Finally, many TIMs are available with no minimum order quantity or tooling costs, making them an economical choice for thermal management designs.

Recommended reading

  1. An Introduction to Thermal Management
  2. How to Stay Cool: The Basics of Heat Sink Selection and Application