Why do we need a Heatsink?

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If any heat source in an electronic device cannot be cooled efficiently through conduction or convection, then a heatsink is required to move the heat away from the heat source so that conduction or convection is more evenly managed. Heat generation is one of the most important challenges for electronic devices since it affects the device&#;s reliability. It is mainly generated from the flow of electrical current through various components in a circuit, such as integrated circuits, transistors, and other electronic components. The primary cause of heat generation in electronic circuits is electronic resistance to the current flow. So, efficient thermal management frameworks are essential to avoid overheating, ensuring a long lifecycle and optimal safety of the electronic devices. [1]

Consequences of Overheating

When a device overheats, it can lead to performance issues, unexpected shutdowns, and even permanent damage to the inside components. There are several techniques for cooling, including different styles of heatsinks, thermoelectric coolers, forced air systems, fans, and heat channels. In power electronics, cooling systems must evacuate excess heat to maintain reliability and power density. So, electronic components should always operate within their rated operating temperature range, the safe operating area (SOA).  [2]

Overheating can happen for different reasons, including intemperate utilization, poor ventilation, and defective cooling systems. Extreme heat can slow and even damage devices [3].

Working Principle and Concept

Heatsinks operate on the fundamental principle that heat transfer from a hot to a cool region is proportional to the available surface area, as governed by the second law of thermodynamics. Heatsinks conduct heat away from critical electronic components, maintaining them within their rated operating temperature range. The absorbed heat is then removed through natural or forced air convection.

Heatsinks function either passively through natural convection or actively using mechanisms like fans or liquid cooling to improve heat dissipation. Key performance factors for heatsink design include:

Design Features

&#; Surface area

&#; Material choice

&#; Fin geometry

&#; Base dimensions

&#; Innovative elements like varying sizes, scales, and protrusions

Optimizing these parameters will enhance heat exchange efficiency or thermal conductivity, resulting improvement of heat transfer coefficient.

Active and Passive Heatsink

Heatsinks are classified into passive and active cooling methods. Passive cooling relies on natural convection and radiation for heat dissipation without external assistance. On the other hand, active cooling employs external devices like fans, pumps, or thermoelectric coolers (TECs) to boost heat transfer and cooling efficiency. The choice of a heatsink is determined by the necessary resistance to meet component thermal requirements with active solutions such as forced air or liquid cooling offering tailored thermal management solutions at different levels. (See Fig. 1) [5]

Figure 1: Natural vs. Forced Convection

Table 1: Heatsinks High-Level Summary

Types of Heatsink by Design

Different types of heatsinks, such as pin fin, plate fin, and stacked fin heatsinks, are commonly utilized in a variety of applications to dissipate heat. Pin-fin heatsinks are effective at improving heat transfer rates by increasing their surface area. Plate fin heatsinks are preferred for their heat transfer rates, lower pressure drop, and reduced thermal resistance when compared to other designs. Stacked fin heatsinks have a larger surface area for efficient heat dissipation. Heat pipes are known for their thermal conductivity and exhibit excellent heat transfer capabilities (see Fig. 2). [13]

Figure 2: Types of Heatsink by Design

Types of Heatsink Based on Manufacturing

Table 2: Considerations by Heatsink Type

Table 3: Materials Summary

Figure 3: Type of Heatsink Based on Manufacturing

Design Optimization

To enhance the thermal performance of heatsinks in both natural and forced convection, various design optimization strategies can be used. Like optimizing fin designs such as trapezoidal, curved, and angled fins can improve heat transfer efficiency, adding a cover plate over straight fins can improve the chimney effect, resulting in more heat transfer. In forced convection, adding internal flow channels can be used to design high-performance heatsinks with efficient heat dissipation capabilities.

Fin efficiency depends on length, thickness, thermal conductivity, and heat transfer coefficient.

Common heatsinks have a fin aspect ratio between 3:1 and 5:1. In forced convection applications, efficiency should be within the 40 to 70 % range. Also, expansion of the surface area, precise fin design, material choice, and an integrated thermal management system are the factors that improve the performance of a heatsink.

Figure 4: Design Optimization in Heatsink

Parametric Approach to Optimize Heatsink Design

To optimize the relationship between design intent and design response:

1. Optimal fin spacing and number of fins are crucial for maximum heat transfer. Thinner fins improve performance by allowing more fins per base plate, while fin height and length significantly affect heatsink conductance and temperature regulation. The results indicate that careful consideration of fin dimensions can lead to an efficient heatsink design that meets specific thermal performance requirements [8].
2. Based on the PhD thesis &#;Design Optimization and Analysis of Heatsinks for Electronic Cooling, Amer Jameel Shareef Al-damook, and Results show that perforated pinned heatsinks improve CPU temperature, fan power consumption, heat transfer rate, and positioning of perforations plays a significant role. The findings suggest that perforated heatsinks are lighter and more efficient for cooling electronic devices and systems [10].

General Rules

These general rules apply to heatsinks:

1. Thermal interface materials (TIMs) play a critical role in efficient heat transfer. TIMs bridge microscopic gaps and imperfections between the mating surfaces, ensuring a low-resistance thermal pathway for heat dissipation. Using TIM enhances thermal conductivity and eliminates air pockets, leading to more efficient heat transfer. All interface surfaces should be smooth and flat. Use heatsink thermal grease or TIM between the heatsink and the device. This will increase the thermal transfer between the two parts; however, excessive quantities of heatsink grease will decrease performance.
2. Mount fins in the vertical plane for optimum natural convective cooling. Do not overcrowd or obstruct devices that use heatsinks. Semiconductors should be spaced to obtain a uniform power density. (see Fig. 4)
3. Do not block airflow around heatsinks, particularly directly above and below items that rely on natural convection.
4. If thermal demands are particularly high, consider using forced convection.

Recommended Heatsink Materials

The most frequent materials used in the construction of heatsinks are those with high thermal conductivity.

Table 4: Thermal Conductivity by Material

Selecting the Right Heatsink

Selecting the right heatsink is based on for key considerations:

1.Determine Heat Generation

Calculate the amount of heat, Q, generated by the device in watts (W). This heat generation is due to electrical resistance and power dissipation within the electronic components.

2. Device & Ambient Temperatures

Find out, maximum allowable junction temperature of the device. This is the highest temperature the device can safely operate at, without risking damage or performance degradation. Also, you must know the maximum ambient air temperature of the system.

3. Convection Type

Identify whether the cooling method will be natural convection or forced convection. For forced convection, you must know air flow velocity (LFM).

4. Calculate Thermal Resistance

The final step is to calculate the required thermal resistance from the device junction to the ambient air using expression:

Thermal Resistance = (Tj - Ta) / Q

Where, Tj is the maximum allowable junction temperature (°C), Ta is the maximum ambient air temperature (°C), Q is the heat generation of the device (W).

Once you have calculated the required thermal resistance, you can select a heatsink that can handle this thermal load and maintain the device within its safe operating temperature range. The choice of heatsink will depend on factors like material, fin design, and whether it uses natural or forced convection. Remember to consider the thermal interface materials and their resistivity values as well.

If you are looking for more details, kindly visit heatsink extrusion profiles.

Figure 5: Comparison Chart of Heatsink Configurations

General Recommendations

Our general recommendations are summarized below:

Table 5: Heatsink Recommendation by Application

Conclusion

Heatsinks maintain optimal operating temperatures, prevent overheating, and ensure the reliability of electronic devices. Heatsink design works on the principle of heat transfer, where the surface area available for conduction or convection influences the amount of heat dissipated. The fabrication of heatsinks is different, with different types of designs like pin fin, plate-fin, and stacked fin heatsinks tailored for specific applications and cooling requirements. Active and passive cooling methods are employed, utilizing natural or forced convection to dissipate heat effectively. Design optimization strategies, such as fin geometry and thermal interface materials, are crucial for enhancing heatsink performance.

Materials like aluminum, copper, heat pipes, graphite, and ceramics are commonly used for their high thermal conductivity. Selecting the right heatsink for a specific application involves calculating heat generation, determining device and ambient temperatures, identifying the convection type, and calculating thermal resistance to ensure that the device will operate within safe temperature limits. Heatsinks are indispensable components in electronics, ensuring efficient heat dissipation and optimal device performance.

About Whizz Systems

Whizz Systems is an end-to-end Electronics Manufacturing Services firm. We specialize in Electronic Design and Manufacturing with an emphasis on NPI. We are based in Silicon Valley, and can smoothly transition projects to our new high-volume facility in Malaysia.

With over 30 years of experience, we have extensive experience supporting major companies like Xllinx, Amazon, Google as well as startups you may or may not have heard of to get their products to market.

Learn more: us at to connect with our team of industry experts about your projects today.

Further Reading/References

HEAT SINK

A heat sink is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to fluid medium, often air or a liquid coolant, where it is dissipated away from the device, thereby allowing regulation of the device's temperature. In computers, heat sinks are used to cool CPUs, GPUs, and some chipsets and RAM modules. Heat sinks are used with high-power semiconductor devices such as power transistors and optoelectronics such as lasers and light-emitting diodes (LEDs), where the heat dissipation ability of the component itself is insufficient to moderate its temperature.

For any electronics device, &#;Thermal Management&#; is the important part which decides the performance and efficiency of the device during its entire product life. This thing will highlight into the Selection and Design of Heat Sinks.

 

How heat is Transferred ?

The Heat is transferred from higher temperature to lower temperature by the following 3 Ways- 

Conduction: It happens due to movement of molecules. Molecules at higher temperatures vibrate at a higher amplitude and pass the energy to the lower temperature molecules. 

Convection: Heat is transferred from a hot surface to a cool surface by the bulk movement of fluids (air or liquid). 

Radiation: Heat transfer happens in the form of Electromagnetic radiation.

Most of the electronic device cooling systems use conduction and convection type heat transfer for thermal management.

 

Determine Power Dissipation by the Device:

The first step in determining the Heat Sink design is to figure out how much power the Semiconductor device dissipates. There are different methodologies applied to calculate the Power Loss in the Semiconductor device. The below formula provides information about how the device power dissipation can be calculated.

Power Dissipation = (Vin &#; Vout) * Iout

eg, If the input is 9V and it is providing 2A of current, then Power Dissipation = (9V &#; 5V) * 2A = 8W.

 

Extract Datasheet Values:

The next step after obtaining the device power dissipation is extracting thermal resistance values from the Datasheet of the component.

As per the datasheet of Posstive-Voltage regulators, the maximum operating virtual junction temperature Tj = 125˚C. So under any operating conditions, the Tj should be managed below 125˚C.

After determining the maximum junction temperature rating, the device's thermal resistance must be determined. To evaluate the thermal resistance, we must first finalize the sort of packaging that will be used for our design. The junction-to-ambient thermal resistance R(JA) = 19 °C/W TO-220 package is chosen.

Calculating the maximum junction temperature without a heat sink-

Tj = Ta + R&#;(JA) * Power

Ta = Ambient Temperature (Environment Temperature)

Tj = 35 + (19 * 8)  (assuming Ta = 35)

= 187˚C

As per the calculation, if the device is used without a heat sink, the maximum junction temperature of the device reaching 187˚C. This should be reduced below 125˚C and hence the Heat Sink should be used.

 

Obtaining Junction to Case Thermal Resistance Value:

The below image provides details about the different sections of the semiconductor thermal system. The R&#;(JC) can be obtained from the device datasheet (here LM). For the LM - TO-220 package, the junction to case temperature is 3˚C/W.

 

Choosing a Correct Heat Sink for your Component by Calculating the size of heat sink.

 

Heat Source Power (Q): This is the thermal design power (TDP) of the chip, which is the maximum amount of heat it can create in watts without going outside its thermal envelope. It should be given by the chip manufacturer or an ASIC engineer.

Tc Max: The chip case's maximum temperature. The manufacturer will usually give this for most chip designs. The maximum Tj (junction temperature) for bare die chips will be provided. Use the Tj spec instead of Tc max in this case.

Max Ambient: The maximum ambient temperature at which the device is intended to operate.

Thermal Budget: (Tc Max - Max Ambient). The sum total of all ΔTs in the network, from Tc to Air temperature rise cannot exceed this limit.

Note &#; thermal budgets below 40 degrees Celsius are generally good candidates for two-phase cooling using heat pipes or vapor chambers.

Volumetric Thermal Resistance (Rv): This equation and subsequent guidelines have been shown to closely estimate heat sink volume: 

V=(Q*Rv)/ΔT.

The first step in using the chart below is to know the available airflow across the heat sink. As you know, the higher the airflow the smaller the heat sink. The first challenge you&#;ll probably face is that the fan manufacturer has given you the &#;bulk airflow&#;, usually in cubic feet per minute (CFM), but not the air velocity. It&#;s easy to determine the velocity if you know the size of the heat sink.

 

Here are some rough guidelines:

• Open air natural convection develops about 40 LFM which is about just enough to blow out a match.
• 1 m/s or 200 LFM you can feel the flow but not hear it.
• 2.5 m/s or 500 LFM is a good flow that will blow out a whole bunch of candles and you can begin to hear the noise, especially in a quiet environment.
• 5 m/s or LFM is going to be noisy. Not to be used in any noise-sensitive environment.

 

Firstly you have to select the appropriate airflow, the next step is choosing from a range of Rv. In the case of moderate air (2.5 m/s) the range is 80-150. The published rule is as follows:

• For heat sinks smaller than 300 cm³ you use the lower limit, in this case 80.
• For large heat sinks, greater than 1,000 cm³ use the upper limit.

If you initially use 110 Rv value and the estimated volume of the heat sink is less than 300 cm³, change the Rv to 80, which is the lower end of the Rv value for moderate airflow.

When designing devices that must work at altitude, it&#;s important to de-rate the Rv. A solid rule of thumb is 10% for every mile of altitude. For example, at one mile high we&#;d divide 80 Rv by 0.9 to end up with a de-rated Rv of roughly 89.

After volume of heat sink is determined, the last step is to assign some length, width and height dimensions. It is generally depend on the space available to attach the heat sink.

There are two types of heat sinks: active and passive. Example Peltier Heat Sink TEC1- is a Suitable for a Thermoelectric cooling system.

 

Factors to Improve Thermal Performance of a Heat Sink

1. Heat Sink Design: Shape and Size of Fins and there Arrangement.
2. Airflow Direction: Airflow should be parallel to the fins
3. Air Speed and Temperature
4. Heat Sink Material: Copper or Aluminium
5. Thermal Interface Material: Heat Conductive Paste
6. Thermal Resistance of Heat Sink

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