The Basic Concepts of Thermal Interface

Introduction

What is thermal interface?

Thermal interface is the boundary between two regions of space occupied by materials at different temperatures, where heat transfers from the hotter material to the cooler material. In electronics, thermal interfaces exist between components such as microprocessors, heat sinks, and thermal interface materials (TIMs). The selection and design of thermal interface material can have a significant impact on the thermal performance of an electronic device.

The Importance of Thermal Interfaces in Electronic Devices

Efficient management of the heat generated by electronic devices is essential to preventing damage, maintaining performance, and extending the life of devices. Poor thermal management can lead to component failure, poor device performance, and increased power consumption. Thermal interface materials are critical to increasing the effectiveness of thermal management in electronics by “improving heat transfer” between components.

Background Information

Thermal management overview

Thermal management refers to the process of managing the temperature of electronic devices to ensure optimal performance, reliability, and safety. The goal of thermal management is to remove heat generated by device components and dissipate it efficiently into the surrounding environment. Failure to effectively manage heat can shorten device life, degrade performance, and even lead to catastrophic failure.

Thermal management technologies can be broadly classified into passive and active. Passive methods use the material's natural thermal conductivity to transfer heat from the device to the surrounding environment. For example, there are “heat sinks” that use metal fins to increase the surface area and promote heat dissipation, and “phase change materials” that absorb and release heat while changing from a solid state to a liquid state. The active method uses an external energy source to enhance heat transfer. Examples include “fans” that blow air over a heatsink or through a device to increase convective heat transfer, and “liquid cooling systems” that remove heat from a device by circulating coolant through a series of tubes.

Thermal management is an important aspect of electronics design, especially for power-dense devices such as CPUs, GPUs, and power electronics. Effective thermal management requires careful consideration of device layout, material selection, and heat transfer mechanisms, and may require trade-offs between thermal performance, device size, and cost.

Heat transfer mechanism

Heat transfer mechanisms are fundamental to understanding the principles of thermal management. There are three main modes of heat transfer: conduction, convection, and radiation.

• Conduction:
  Conduction is the transfer of heat through a material without the movement of the material itself. Heat flows from a hotter region to a cooler region, and the heat transfer rate depends on the temperature difference within the region, the thermal conductivity of the material, and the distance the heat travels. Metals have high thermal conductivity and are effective in conducting heat, while insulators have low thermal conductivity and are weak in heat transfer.


• Convection:
  Convection is the transfer of heat by the motion of the fluid itself through a fluid (liquid or gas). When a fluid is heated, it expands and becomes less dense, causing the fluid to rise and create a flow. The heated fluid then transfers heat to the cooler surface and descends, creating a heat transfer cycle. Convection can be natural or forced, depending on whether the fluid flow is driven by natural buoyancy or an external source such as a fan or pump.


• Radiation:
  Radiation refers to the transfer of heat through electromagnetic waves such as infrared rays. All objects emit radiant heat, and the amount of radiation depends on the temperature and the emissivity of the material. Radiant heat transfer is important for thermal management in electronics because it can occur in a vacuum or even between contactless surfaces.

The effectiveness of each heat transfer mode depends on the specific device and environment. All three modes of heat transfer exist to varying degrees in most electronic devices.

Types of Thermal Interface Materials

There are several types of thermal interface materials (TIMs) commonly used in electronic devices to improve heat transfer between components and heat sinks. The choice of TIM depends on a number of factors, including the application, device design, and operating conditions. Commonly used TIM types include:

• Thermal grease:
  Thermal grease is a soft, viscous paste that is applied between components and heatsinks to fill gaps and surface irregularities. It is usually made of a silicone or hydrocarbon base and contains a thermally conductive filler such as a metal oxide. It is relatively inexpensive and easy to apply but may dry out or flow over time. If removing the heatsink after it has hardened, the existing thermal grease must be removed and reapplied. Since it is applied thinly, the heat transfer distance is very short, so a certain level of heat conduction effect can be expected even with materials with low thermal conductivity.


• Thermal tape:
  Thermal tape is a thin adhesive tape that attaches a heatsink to a component. It is usually made of a flexible polymeric material with a thermally conductive adhesive layer. Thermal tape is easy to apply and is useful when space is limited, or the heatsink must be removed.


• Thermal pad:
  A thermal pad is a soft pad placed between a heat source and a heatsink. They are usually made of silicone or polymer materials with embedded thermally conductive fillers. Thermal pads are easy to install, can provide a more reliable interface than thermal grease or tape, and may require more force to compress.


• Phase change material (PCM):
  A phase change material is a material that can absorb and release large amounts of heat by changing from a solid state to a liquid state. They are typically used in applications requiring a fast thermal response, such as portable electronics. PCMs have high thermal conductivity, but have a limited lifetime and are sometimes difficult to apply.


• Metal-based interface materials:
  Metal-based interface materials are usually made of metal alloys that can adapt to surface irregularities and provide high thermal conductivity. They are typically used in high-performance applications such as CPUs and power electronics. However, they can be expensive and difficult to apply.

The choice of TIM depends on a number of factors, including thermal performance, ease of application, and cost. It is important to select a TIM that is compatible with the device material, can withstand operating conditions, and can provide reliable performance over the long term.

Selection Criteria for Thermal Interface Materials

When selecting a thermal interface material (TIM), it is important to consider several criteria to ensure optimal thermal performance and device reliability. The main selection criteria for TIM are:

• Thermal conductivity:
  The thermal conductivity of TIMs is an important factor in determining the heat transfer effect. TIMs with higher thermal conductivity provide better heat transfer, resulting in lower operating temperatures and better device performance. The thermal conductivity of TIMs varies widely, from about 0.5 W/mK for thermal grease to over 10 W/mK for metal-based TIMs. Thermal conductivity is an intrinsic property of a material and is independent of size. Even for materials with the same thermal conductivity, the greater the distance through which heat is conducted, the greater the thermal resistance.


• Thermal resistance:
  The thermal resistance of TIMs is a measure of its resistance to heat flow through the interface, which is proportional to the thickness of the TIM layer and inversely proportional to its area and thermal conductivity. TIMs with lower thermal resistance provide better heat transfer, resulting in lower operating temperatures and better device performance.


• Compressibility:
  TIM's compressibility is a measure of its ability to bridge the gap between a heat source and a heat sink that has microscopic irregularities on its surface. TIMs with high compressibility can provide better contact (reducing air gap due to asperities) and improve heat transfer performance. However, if the compressibility is too high, over time TIM itself (in the case of thermal grease) or silicone oil (in the case of the thermal pad) may bleed out and reduce efficiency.


• Lifespan:
  The lifetime of TIMs is a measure of their ability to maintain thermal performance over time. Some TIMs can dry out, run or break over time, reducing heat transfer efficiency.


• Chemical compatibility:
  TIMs must be chemically compatible with the device materials and environment. Some TIMs can react with certain metals or plastics and cause corrosion or degradation of the device. (In case of thermal pad, contact corrosion problem of low molecular weight siloxane, etc.)


• Ease of application:
  The ease of application of TIMs is an important factor, especially for mass production. Some TIMs may require special equipment or skills (form-in-place thermal compound), but others are simple to apply.


• Cost:
  The cost of TIM is also an important factor to consider. For cost-effective thermal management, the cost of TIM must be balanced with thermal performance, lifetime, and ease of application.

To ensure optimal thermal performance and device reliability, the suitable TIM must be selected by balancing several criteria, including thermal conductivity, compressibility, lifetime, chemical compatibility, ease of application, and cost.

Glossary

When selecting a thermal interface material, it can be helpful to understand terms such as thermal conductivity, thermal resistance, thermal impedance, and thermal transmittance.

• Thermal conductivity:
  
  Its unit is W/mK, and it is a physical property that describes the ability of a substance to transfer heat. Thermal conductivity is derived from the formula shown below.
  
  
  ∆Q

  ∆t

   = kA

  ∆T

  ∆L
  
  
  

  Q: the amount of heat conducted (watt), ∆t: unit time (sec), k: proportional constant (thermal conductivity), A: cross-sectional area where heat is conducted(m²), ∆L: distance where heat is conducted (m), ∆T: the temperature difference between the two sides (kelvin)

  
  

  Assuming that the heat moves in unit time, k is

  
  
  k =  

  Q∙∆L

  A∙∆T
  
  
  
  

  The unit of thermal conductivity becomes W/mK (Watt per meter-Kelvin). Materials with high thermal conductivity conduct heat well, and materials with low thermal conductivity do not conduct heat well. Therefore, materials used for insulation generally have low thermal conductivity.




• Thermal resistance:
  
  Its unit is K/W (Kelvin per Watt). Since thermal resistance is proportional to the distance through which heat is conducted and inversely proportional to thermal conductivity and cross-sectional area, the following formula is derived.
  
  
  R = 

  ∆L

  Ak
  
  
  

  R: thermal resistance, ∆L: distance through which heat is conducted (m), A: cross-sectional area through which heat is conducted (m²), k: thermal conductivity (W/mK)

  
  

  For reference, thermal resistivity is a material constant and is the reciprocal of thermal conductivity. (Unit: mK/W)

  
  
  R_λ = 

  1

  k
  
  
  

  R_λ: thermal resistivity, k: thermal conductivity (W/mK)




• Thermal impedance:
  
  Its unit is square meters-Kelvin per Watt (m²K/W). Thermal impedance is the value obtained by dividing the thickness of a material (the distance through which heat is conducted) by the thermal conductivity and is expressed as: Thermal impedance is a concept that encompasses both thermal resistance and contact resistance and can be useful when calculating the total thermal resistance of a composite material.
  
  
  Z = 

  ∆L

  k
  
  
  

  Z: thermal impedance, ∆L: distance through which heat is conducted (m), k: thermal conductivity (W/mK)

  
  

  For example, the total thermal impedance Z of material A (thermal conductivity k_A, thickness L_A) and material B (thermal conductivity k_B, thickness L_B) placed side by side (series connection) can be calculated as:

  
  
  Z = Z_A + Z_B = 

  L_A

  k_A

    +  

  L_B

  k_B
  
  



• Thermal transmittance:
  
  Its unit is W/m²K(Watt per square meters-Kelvin). The thermal transmittance is the reciprocal of the thermal impedance and is expressed as follows. Just as thermal impedance represents the total thermal resistance of a composite material, thermal transmittance represents the total thermal conductivity of a composite material.
  
  
  U = 

  1

  Z
  
  
  

  U: thermal transmittance, Z: thermal impedance (m²K/W)

Characterization of Thermal Interface Materials

Thermal conductivity measurement technology

Thermal conductivity is an important parameter for thermal interface materials (TIMs) and is typically measured using one of several techniques. These techniques can be largely classified into steady-state techniques and transient techniques, depending on the time and temperature gradient used.

• Steady-State Technique:
  The steady-state technique measures thermal conductivity by measuring the temperature gradient over a sample of known thickness and area under steady-state conditions. The most commonly used steady-state technique is the guarded hot plate method, which measures the heat flow and temperature gradient by placing a sample between two plates at different temperatures. Another technique is to compare a prepared sample to a reference material whose thermal conductivity is already known.


• Transient Technique:
  Transient techniques measure thermal conductivity by measuring the temperature response of a sample to a known heat input over a short period of time. The most commonly used method is the hot wire method, in which a thin wire or strip is heated for a short period of time, and the temperature rise and cooldown curves of the sample are measured. Another technique is laser flash analysis (LFA), which heats the sample with short pulses of laser light and measures the temperature rise and fall.


• Differential scanning calorimetry (DSC):
  Differential scanning calorimetry is a technique for measuring the thermal properties of materials by measuring the heat flow during thermal transitions such as melting or crystallization. DSC can be used to measure the thermal conductivity of TIMs by analyzing the heat flow during thermal transitions.

The choice of measurement technology depends on the specific application and requirements, such as required material properties, temperature range and accuracy. For accurate and reliable thermal conductivity measurements, it is important to select an appropriate technique and carefully consider measurement uncertainties and potential sources of error.

Thermal Resistance Measurement Technology

Thermal resistance is an important parameter for characterizing thermal interface materials (TIMs). In general, it is the same as the thermal conductivity measurement method (steady-state technique, transient technique), and also uses electrical resistance.

• Electrical Resistance: Electrical resistance can be used as a substitute for thermal resistance in some cases, especially for electrically conductive materials. The electrical resistance of the sample is measured using standard electrical measurement techniques, and the thermal resistance is calculated using a conversion factor that depends on the material properties.

Applications of thermal interface materials

Overview of Electronics and Thermal Management Challenges

Electronic devices generate heat as a by-product of operation, and this heat must be dissipated to prevent damage to the device and to maintain its performance. Effective thermal management is essential to ensuring the reliability and lifespan of electronics and is a major challenge in many applications.

Thermal management issues for electronics can vary depending on the type of device, operating environment, and power requirements. However, some common challenges include:

• Miniaturization:
  As electronic devices continue to become smaller, dissipating the heat generated by the components within them becomes more difficult. As devices get smaller, there is less surface area to dissipate heat, and the component packing becomes tighter, leading to higher component temperatures. Parts such as semiconductors are specified to be used within a certain temperature according to specifications, and if this temperature is exceeded, it may stop working or cause malfunction.


• Power Density:
  Electronic devices are becoming more powerful, which can result in higher power densities (the amount of power that can be generated or transmitted per unit volume or area; W/m³, W/m²) and higher calorific value. Higher power densities can create hotspots within the device, reducing performance and increasing the risk of failure.


• Environmental conditions:
  The operating environment can have a significant impact on the thermal management requirements of electronic devices. Ambient temperature, high humidity, and exposure to dust or other contaminants can increase the thermal load on the device and reduce its thermal performance.


• Material selection:
  Materials used in electronic devices can affect thermal performance. For example, the choice of thermal interface material can greatly affect the heat transfer between the component and the heat sink. The choice of materials for the case and other components can also affect the thermal performance of the device.


• Thermal Resistance:
  The thermal resistance of the thermal interface can affect the overall thermal performance of the device. High thermal resistance can result in inefficient heat transfer and high component temperatures.

Effective thermal management is critical to ensuring reliable and long-term operation of electronics. Addressing thermal management challenges requires careful consideration of the device's operating environment, power requirements, and thermal properties of the materials used in the device.

New Applications of Thermal Interface Materials

Thermal interface materials (TIMs) have been widely used in electronics and power electronics to improve thermal management and increase performance. However, there are new TIM applications currently being explored by researchers and industry experts. Here are some of the emerging applications for TIMs:

• LED lights:
  Light-emitting diodes (LEDs) are becoming increasingly popular due to their energy efficiency and long lifetime. However, LEDs generate heat which can affect performance and lifespan. TIMs allow better thermal management of LEDs to better dissipate heat and increase device lifetime.


• Electric motor:
  Electric motors generate heat during operation, which can reduce efficiency and lifetime. TIMs can be used to improve the thermal management of electric motors to improve heat dissipation and increase efficiency and lifetime.


• Solar panels:
  Solar panels convert sunlight into electrical energy, but they can also generate heat during operation. Higher temperatures may affect the performance and lifetime of the panel. TIMs can be used to improve the thermal management of panels, improving heat dissipation and increasing efficiency and lifespan.


• Medical equipment:
  Medical devices generate heat during operation, which can affect performance and safety. TIMs can be used to improve thermal management in medical devices to improve heat dissipation and prevent overheating which can be dangerous to patients.


• Aerospace and defense industries::
  Aerospace and defense industries require high-performance electronics that can withstand extreme conditions. TIMs can be used to improve the thermal management of electronics used in this field to improve heat dissipation and ensure reliable performance in harsh environments.

In conclusion, the new applications of TIMs are diverse and span multiple industries. As researchers continue to explore the potential of TIMs, they will become increasingly essential to improving the performance, efficiency, and reliability of a variety of electronic devices and systems.

Challenges and Future Directions

Current Challenges in Thermal Interface Technology

Thermal Interface Technology (TIT) has made great strides in improving the thermal management of electronic devices and systems, but several challenges remain. Some of the current TIT challenges include:

• Surface roughness::
  One of the major challenges of TIT is surface roughness. The surface roughness of the heat sink and components can affect the contact area and thermal conductivity of the TIM. As a result, the efficiency of the TIM may be reduced, resulting in poor thermal performance.


• Interface thickness:
  The thickness of TIMs is another important parameter affecting thermal conductivity. If the TIM is too thick, it can create a thermal barrier that reduces heat transfer. On the other hand, if the TIM is too thin, it may not fill the gap between the component and the heatsink, resulting in poor thermal contact.


• Thermal stability:
  Another challenge for TITs is the thermal stability of TIMs. Some TIMs may degrade over time due to heat cycling or exposure to high temperatures. This degradation can result in poor thermal performance or even failure of the TIM.


• Compatibility:
  The compatibility of TIMs with components and heat sinks is also an important consideration. Some TIMs can react with components or heat sinks, causing corrosion, performance degradation, or other problems.


• Manufacture and cost::
  Finally, the manufacturing process and cost of TIMs can also be an issue. Some TIMs are difficult to manufacture, resulting in high production costs. Additionally, the cost of a TIM can significantly impact the overall cost of an electronic device or system.

In conclusion, TIT has made significant progress in improving the thermal management of electronics, but there are still some challenges to be addressed. Researchers and industry experts are working to develop new TIMs that address these issues and improve the thermal performance of electronic devices and systems.

New Trends in Thermal Interface Materials

Thermal interface materials (TIMs) constantly evolve to meet the growing demands of electronic devices and systems. Here are some new trends in TIM:

• Graphene-based TIMs:
  Graphene, a two-dimensional material made of carbon atoms, has excellent thermal conductivity and can be used as a thermal interface material. Researchers are exploring ways to use graphene-based TIMs to improve thermal management in electronic devices. Graphene-based TIMs can provide better heat dissipation and improve the overall performance and efficiency of electronic devices.


• Phase change material:
  A phase change material (PCM) is a material that can absorb or release thermal energy during a phase change. Researchers are exploring the use of PCMs as TIMs to improve thermal management in electronics. PCMs can absorb heat during operation and dissipate heat during idle time, improving the overall efficiency and lifespan of the device.


• 3D printing:
  3D printing technology is being used to create custom TIMs to fit the specific geometry of an electronic device. This approach can improve the thermal performance of electronics by maximizing the contact area between the component and the heat sink. 3D printing can also be used to create complex structures that enhance the TIM's thermal conductivity.


• Nanoparticle-based TIM:
  Nanoparticles can be added to existing TIMs to improve thermal conductivity. Researchers are exploring ways to improve the thermal conductivity of TIMs using different types of nanoparticles, including silver, copper, and aluminum oxides. These nanoparticle-based TIMs can provide better heat dissipation and improve the overall thermal performance of electronic devices.


• Hybrid TIM:
  Hybrid TIMs combine different materials to achieve optimal thermal conductivity and stability. For example, researchers are exploring the use of hybrid TIMs that combine liquid metal with conventional thermal paste. Liquid metal provides excellent thermal conductivity, while thermal paste provides stability and prevents corrosion.

In conclusion, the new trend in TIMs is diverse across different materials and fabrication methods. As researchers continue to explore the potential of TIMs, they are likely to become more efficient and effective in improving thermal management in electronic devices and systems.