Online Catalog

Heat sink selection

Introduction
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In order to select an appropriate heat sink, the process must be done in two steps.

What is the thermal resistance target, C/W?

Example

Ref:Selecting a Heat Sink

First, we need to calculate a thermal resistance target for the heat sink. This is a measure of the heat sink's performance under given conditions at a specified airflow.

For general instructions on how to calculate a target thermal resistance, please click on the "1. Heat Source" section and access the "Help" section.

Design Factors

Example

Once the thermal resistance target has been calculated, a heat sink can then be selected, based on the following 6 design factors below:

1. Heat Source
2. Heat Sink
3. Air Duct
4. Air Flow
5. Thermal Interface
6. Attachment

Click on each design factor and input as much information as possible. It is not necessary to input all of the requested information, if you are not sure. For any omitted information, we will use a typical value.

Heat sink performance depends on many factors, some of which may not covered in this section. We will provide the proposed solution based on our test data and experience. However, due to complex nature of the of the various factors, the final performance could not be guaranteed. Customers are responsible to verify performance in their actual operating environment.

Before a heat sink can be selected, the specification of the heat source(IC device) must be known.

The 2 factors of greatest importance are: heat dissipation (wattage) and maximum allowable junction or case temperature.

In addition IC chip size (heat source contact area) is important, as it will affect the thermal performance and the heat sink attachment method. (Reference: Guideline for small size heat source(bare-chip)) (http://www.micforg.co.jp/en/scoree.html)

This information is typically available and can be obtained from the IC/Chip manufacturer in the form of a data sheet. Many chip manufacturers will also publish their own thermal design guide for their products.

Examples of published IC/Chip specifications:

Intel®G31/P31 Express Chipset Thermal and Mechanical Design Guidelines

Intel Chip Design Guide

The power dissipation and the maximum allowed case temperature are provided. The TDP(Thermal Design Power) is 15.5(W), and the Max. case temperature is 106 degC. The maximum allowed temperature increase is calculated by subtracting the Max. local ambient temperature from the Max. case or junction temperature The Max. local ambient temperature is the temperature of the air that will enter the heat sink cooling the the IC/chip. This value cannot be predetermined and will be your responsibility to specify. However, using 45-55 degC as the Max. local ambient is often used for electronics enclosures and telecommunications equipment, in our experience. If we assume 45 degC for the Max. local ambient temperature, then the maximum allowable temperature rise would be 106-45 = 61C, in this example. The target thermal resistance can then be calculated by dividing the allowable temperature rise by the thermal design power of 15.5 Watts: (106-45)/15.5 = 3.94 C/W

"Altera Device Packaging Specifications" Altera Chip Design Guide

Here is an example of package physical dimension. D1,E1 dimension is important to estimate the thermal performance of the heat sink and the thermal interface material. D, E and A dimension is important for designing a mechanical attachment, such as push pins or screws.

A larger heat sink will typically perform better, as the surface area of the heat sink will become greater. Also, by increasing the volume of the heat sink, the amount of bypass area around the heat sink will be reduced, forcing more airflow to go through the heat sink.

Bypass air flow 1 Bypass air flow 2

However, a larger heat sink may require more complicated mechanical attachment instead of simple and less expensive tape or epoxy attachment. (REF:Online Catalog - Heat Sink Attachment)

If mechanical attachment such as push pins or shoulder screws will be used, the attachment holes need to be located outside of the chip's footprint. Therefore, the heat sink base size needs to be larger than chip size, or the base shape needs to have extended attachment tabs.

If there is a lot of space around the heat sink, a significant amount of the air will flow around the heat sink instead of going through it. We call this bypass air flow.

Having a duct that channels the air, forcing it into the heat sink, will significantly improve the thermal performance.

Bypass air flow Duct

The thermal resistance test data in our catalog is based on fully ducted air flow conditions. Therefore, the performance curve in our catalog is measured under ideal conditions. (Ref:Technical information - Measurement & Calculations).

In the actual environment, if no duct has been created, other components like a daughter card, memory module, PCB components, the chassis, etc..., will act like a duct and help to channel the air.

Depending on the duct size, the heat sink performance will vary. Generally, the performance of a heat sink with a denser fin pattern and higher pressure drop will be more sensitive to the duct size and the amount of bypass airflow.

Typically, the adjacent board or chassis wall will act as a duct, minimizing the amount of bypass space over the top of the heat sink. If no information is entered for the duct size height, we will assume that the duct size is equal to the heat sink size plus 2mm.

The air flow condition can be classified as Natural Convection or Forced convection.

Natural convection is a condition where the air flow is only generated by a a temperature differential and NOT by external means like a fan or blower.

Forced Convection is a condition where air flow IS generated by an external source like a fan or blower.

Greater air speed will result in better thermal performance.

In the actual environment, the air speed generated by a fan or blower will depend on many factors, such as where the components are located relative to the heat sinks, and fin pattern density of the heat sinks that are used.

The air speed used for simulation is ithe inlet air speed, just in front of the heat sink.

Thermal interface material(TIM) fills the small gaps and imperfections between the heat sink and heat source, minimizing the interface thermal resistance.

Please visit Online Catalog - Accessory - Thermal Interface Material.

If you choose adhesive tape for heat sink attachment, the adhesive tape itself is also the thermal interface material.

Pleaes visit Online Catalog - Accessory - Thermally conductive Adhesive Tape.

Generally, as the heat source size becomes smaller, the performance characteristics of the TIM becomes far more significant.

The heat sink attachment method is one of the most important factors regarding heat sink design. The method of heat sink attachment will have an impact on the selection of thermal interface material. For more details, please visit Online Catalog - Heat Sink Attachment.

Design Guide
DuctSize(H)
mm
DuctSize(W)
mm
HeatSink(H)
mm
Ambient
degC
HeatSink(W): mm
HeatSink(L): mm
MaxCaseTemperature:
Wattage: W
AirFlow:
AirSpeed: m/s
ThermalResistance: degC/W
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