Maximizing Thermal Performance With Thermal Paste

Thermal paste, commonly called thermal compound or grease, is a dense material that accelerates thermal conductivity between heat-generating components of electronic devices and systems. Thermal paste is vital in managing heat in high-power converters. For instance, in transformer coils and cooling systems such as liquid coolants. This helps to transfer heat away from the components, preventing overheating and improving the device's overall performance and lifespan. The components would be more likely to be thermal paste due to heat-related issues. By the end of this guide, power system engineers will better understand thermal paste and how it can help them optimize the thermal performance of their high-power, high-volume macro electronics.

 

Why Is Thermal Paste Necessary?

Thermal paste is necessary for high-volume macro-electronics because they generate a significant amount of heat during operation, leading to damage and failures of the components. Therm paste improves thermal conductivity by filling the microscopic gaps between heat-generating components and cooling systems. A filling helps transfer heat away from the Components, preventing overheating and prolonging the device’s lifespan.

Figure 1 shows the different types of adhesion between the two contact surfaces, such as a power semiconductor and a heat sink, in the high-voltage industry. These include mechanical adhesion, such as clips and screws, and thermal adhesion, such as thermal tape or epoxy. In particular:

1. The first figure shows that achieving ideal adhesion between the two contact surfaces, such as a power semiconductor and a heat sink, is impossible

2. The second figure illustrates the typical contact without thermal paste, where the surfaces are not perfectly smooth, and microscopic imperfections contain air, which can prevent the passage of heat

3. However, the third figure shows that thermal paste can significantly improve adhesion, allowing for better heat transfer and device performance. It's worth noting that different types of thermal paste have varying thermal conductivity coefficients, depending on the materials used, and can be optimized for specific high-voltage applications

Different thermal conductivity coefficients depend on the type of material used.

 

Figure 1. The different types of contact between the electronic component and the heat sink. Image used courtesy of EETech

 

Thermal Paste Conductivity

Thermal conductivity measures a material's ability to conduct heat and is the amount of heat passing through a given unit area of a material per unit time per unit temperature gradient. High-conductivity materials can transfer heat more efficiently than those with low thermal conductivity. Thermal conductivity is denoted by k.

\[K=\frac{W}{m.K}\]

Thermal paste is essential for efficient heat transfer involving high-voltage applications, with thermal conductivity being an important factor. Let's consider an example of a power semiconductor generating 200 W of heat, and a heat sink with S.A of 10 cm^2, assuming a temperature difference of 5 °C between the two.

If we use a thermal paste with a high thermal conductivity of 5 W/Mk, The required thickness to achieve efficient heat transfer would be 0.02mm. This means the heat transfer rate would be approximately 100 W/m²K, resulting in a temperature drop of around 1°C across the paste.

If we use a thermal paste with high thermal conductivity of 5 W/mK, the required thickness to achieve an efficient heat transfer rate would be approximately 100 W/m²K, resulting in a temperature drop of around 1°C across the paste. This is illustrated below.

Power generated: 200 W

The surface area of heat sink:10 cm²

Temperature difference: 5 ^oC

Thermal paste with thermal conductivity of 5 W/Mk

Therefore

\[Thickness=\frac{power\times thermal\,resistance\times area}{temperature\,difference}\]

\[=\frac{200\times0.01\div5}{10^{-4}}\]

Thickness =  0.02 mm

\[Heat\,transfer\,rate=\frac{thermal\,conductivity}{thickness}\]

\[=\frac{5}{0.02\times10^{-3}}\]

Heat transfer rate = 100  W/m²K

\[The\,temperature\,drop\,across\,paste=\frac{heat\,transfer\,rate\times surface\,area\times temperature\,difference}{power\,generated}\]

\[=\frac{100\times10^{-4}\times5}{200}\]

     The temperature drop across paste = 0.00625^oC

Thermal paste with thermal conductivity of 3 W/Mk:

\[Thickness=\frac{200\times0.001\div3}{10^{-4}}\]

Thickness = 0.03 mm

\[Heat\,transfer\,rate=\frac{3}{0.03\times10^{-3}}\]

= 66.7 W/M²K

Therefore

Temperature drop = 66.7 X 10^-4 X 5/200

Temperature drop = 0.000835 °C/W x 5 ^oC

Temperature drop = 0.004175 ^oC

The table below illustrates the thermal conductivity values of thermal paste and other commonly used materials.

 

Table 1. Illustrates the thermal conductivity values of thermal paste and other commonly used materials.

Material	Thermal conductivity at 20oC(W/mK)
Steel with 5% Ni	29
Steel with 30% Ni	105
Water	0.63
Alcohol	0.21
Aluminum	210
Air	0.026
Coal	0.15
Marble	2.8
Gold	300
Porcelain	0.9
Copper	302
Cork	0.051
Glass	0.7
Thermal paste	7.5

 

Copper and aluminum are the best thermal conductors, as shown in the table above. Air is a poor conductor of thermal paste. Thermal paste does not have equal values as Aluminum and Copper but makes excellent contributions to heat transfer.

Consider a high-voltage application with a power consumption of 48 W at an ambient temperature of 20 °C. Figure 2 illustrates three different operating configurations of the system.

The first configuration represents an ideal scenario where the CPU and heatsink are at the perfect molecular level. In this case, the maximum temperature reached by the system on the CPU is 40°C, which is a favorable temperature for the reliable operation of the system.

The second configuration depicts a thermal paste between the CPU and heatsink. In this case, the thermal paste helps fill the microscopic gaps between the surfaces and provides better thermal conductivity. As a result, the maximum temperature reached by the system on the CPU is 44°C, which is only slightly higher than the ideal scenario, but still within acceptable limits for high-power electronics.

The third configuration represents a critical scenario where the CPU and heatsink contact is precarious, without thermal paste, and with many air bubbles between the surfaces.

In this case, air bubbles significantly hinder heat transfer, resulting in a maximum temperature of 78°C on the CPU. This is a very high temperature, and the system's operation under such conditions is unreliable, making it necessary for the designer to take necessary precautions to ensure optimal cooling.

 

Figure 2. illustrates three different operating configurations of the CPU system. Image used courtesy of EETech

 

Dissipation Space

Dissipation space is one of the most important aspects we cannot afford to discuss. Heat sinks are constructed to achieve the largest surface area exposed to air. Protuberances are the ones that create the largest possible exposed surface area.

Figure 3 demonstrates the effect of the number of cooling fins on the temperature of a high-power, high-volume macro electronic device (in this case, a CPU).

The power applied to the device is constant in each illustration.

Removing the cooling fins and heatsink causes the CPU temperature to increase.

The temperature reached by the CPU with different numbers of cooling fins and heatsink configurations are:

• Heatsink complete with 8 cooling fins: 78.08°C
• Heatsink fitted with 7 cooling fins: 78.36°C
• Heatsink fitted with 6 cooling fins: 79.18°C
• Heatsink provided with 5 cooling fins: 80.65°C
• Heatsink fitted with 4 cooling fins: 82.71°C
• Heatsink equipped with 3 cooling fins: 84.84°C
• Heatsink equipped with 2 cooling fins: 86.58°C
• Heatsink fitted with 1 cooling fin: 87.88°C
• Heatsink without cooling fins: 89.12°C
• Without heatsink: 151.4°C

 

Figure 3. Demonstrates the effect of the number of cooling fins on the temperature of a high-power CPU. Image courtesy of EETech

 

How to Enhance Durability With Thermal Paste

Thermal paste helps improve the thermal conductivity between the component and the heatsink, dissipating heat more effectively and protecting the electronic components from overheating.  Here are some ways thermal paste enhances durability in high-voltage applications:

• Reducing thermal resistance: Thermal paste fills microscopic gaps in the surface, allowing better heat transfer and lowering the overall system resistance.
• Prevents overheating: By facilitating better heat transfer, thermal paste prevents the buildup of excess heat in the electronic components, which can cause damage and shorten their lifespan.
• Improving stability: Temperature changes can make electronic components susceptible to thermal expansion and contraction. The thermal paste can help improve electronic devices' stability and longevity by maintaining consistent temperatures and reducing the risk of overheating.
• Ensuring optimal performance: High-voltage electronic devices require efficient cooling to Maintain optimal performance. Using thermal paste helps to improve heat dissipation and prevent thermal throttling, ensuring that the device can operate at peak performance for longer periods. 

 

Benefits of Thermal Paste 

Thermal paste, when applied correctly, can provide several benefits that can optimize the thermal performance of electronics, which include:

• Improved heat transfer: Thermal paste fills in microscopic gaps between the CPU and heatsink, increasing surface contact and thermal conductivity, which helps to dissipate heat more efficiently.
• Enhanced durability: By lowering the operating temperature of electronic components, the thermal paste extends the lifespan of devices, reducing the risk of overheating and failure.
• Reduced noise: In systems with high-powered components, cooling fans can become noisy due to increased RPM. Using thermal paste to improve heat transfer, cooling fans can run at lower RPMs, reducing noise pollution.
• Increased performance: Electronic devices can experience performance degradation due to high temperatures. Devices can maintain their performance levels by using a thermal paste to lower operating temperatures for longer periods.
• Cost-effective: Thermal paste is an inexpensive solution to a common problem in electronic devices, making it a cost-effective option for improving device performance and longevity.

 

How to Apply Thermal Paste

The process of applying thermal paste in high-voltage electronics is similar to the process used in standard electronic devices. However, there are a few additional factors to consider due to higher voltage and power output. Here is a step-by-step guide to applying thermal paste correctly in high-voltage electronics:

1. Turn off and unplug the device: Before applying thermal paste, it is essential to turn off and unplug the high-voltage electronic device to avoid any electrical hazards

2. Remove the heat sink and clean the surfaces: Like in standard electronic devices, the heat sink and CPU surfaces must be cleaned thoroughly using isopropyl alcohol or other recommended cleaning agents. Ensuring cleaning does not cause electrical discharge in high-voltage electronics is advisable

3. Apply a small amount of thermal paste: Apply a small amount of thermal paste (typically pea-sized) on the center of the CPU. In high-voltage electronics, it is essential to use a thermal paste with a high thermal conductivity coefficient to ensure optimal heat transfer

4. Spread the thermal paste evenly: Use a plastic card or other recommended tool to spread the thermal paste evenly over the entire CPU surface. Be keen not to apply too much pressure, which can cause air bubbles to form and reduce heat transfer efficiency

5. Reinstall the heat sink: Once the thermal paste is evenly spread, reattach it to the CPU and secure it. Ensure the heat sink is aligned correctly and tightened evenly to avoid warping or damaging the CPU

6. Power on the device: Once the heat sink is reattached, power on the high-voltage electronic device and monitor the temperature closely. If the temperature is too high, consider reapplying thermal paste or upgrading the cooling system

Following the manufacturer's instructions and recommendations when applying thermal paste in high-voltage electronics is crucial to ensure optimal performance and safety.

 

Takeaways of Thermal Paste

• Thermal paste is critical for ensuring electronic devices' longevity and optimal performance, particularly those operating at high voltages. It helps to improve heat transfer between the device's components and the heat sink, thereby preventing overheating and component failure.
• Thermal paste is used to fill microscopic imperfections between the surfaces of electronic components and heat sinks, improving heat transfer.
• Applying thermal paste correctly is important for the optimal performance and durability of electronic devices.
• Common mistakes during application include using too much or too little paste and applying it unevenly.
• Factors to consider when choosing thermal paste include thermal conductivity, viscosity, and electrical conductivity.
• Best practices for maintaining thermal paste include cleaning and reapplying as needed.
• The future outlook for thermal paste technology is promising, with ongoing research and development to improve its performance and compatibility with emerging technologies.

 

Featured image used courtesy of Adobe Stock