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# Heat Sink

## Introduction to heatsink?

A heat sink is an object or device that absorbs and dissipates heat from another object mainly electronic devices using thermal contact (either direct or radiant). Heat sinks are used in a wide range of applications wherever efficient heat dissipation is required; major examples include Refrigeration, Heat engines, Cooling electronic devices and lasers.

## Working Principle

• Heat sinks function by efficiently transferring thermal energy "heat" from an object at a relatively high temperature to a second object at a lower temperature with a much greater heat capacity. This rapid transfer of thermal energy quickly brings the first object into thermal equilibrium with the second, lowering the temperature of the first object, fulfilling the heat sink's role as a cooling device.

• Efficient function of a heat sink relies on rapid transfer of thermal energy from the first object to the heat sink, and the heat sink to the Second object.

• The most common design of a heat sink is a metal device with many fins. The high thermal conductivity of the metal combined with its large surface area due to the fins result in the rapid transfer of thermal energy to the surrounding making device cooler which is in thermal contact with it.

• Use of fluids (for example coolants in refrigeration) and thermal interface material (in cooling electronic devices) ensures good transfer of thermal energy to the heat sink. Similarly a fan may improve the transfer of thermal energy from the heat sink to the air by moving cooler air between the fins.

• ## Construction and Material used

• A heat sink usually consists of a base with one or more flat surfaces and an array of comb or fin-like projections to increase the heat sink's surface area contacting the air, and thus increasing the heat dissipation rate.

• While a heat sink is a static object, a fan often aids a heat sink operation by providing airflow over the heat sink. Thus maintaining a larger temperature gradient by replacing the warmed air more quickly than passive convection achieves alone - this is known as a forced air system.

• Heat sinks are made from a good thermal conductor such as copper or aluminum alloy.

• Copper (401 W/(m*K) at 300 K) is significantly heavier and more expensive than aluminum (237 W/(m*K) at 300 K) but is also roughly twice as efficient as a thermal conductor.

• Aluminum has the significant advantage that it can be easily formed by extrusion, thus making complex cross-sections possible. The heat sink's contact surface (the base) must be flat and smooth to ensure the best thermal contact with the object that need to be cooled.

• Frequently, a thermally conductive grease is used to ensure optimal thermal contact; such grease usually contains ceramic materials such as beryllium oxide and aluminum nitride, but may alternatively contain finely divided metal particles, e.g. zinc oxide and colloidal silver.

• Further, a clamping mechanism, screws, or thermal adhesive hold the heat sink tightly onto the component to maximize thermal conductivity, but specifically without pressure that would crush the component.

• ## Classification of Heat Sink

• Active heat sinks:
• These are the type of heat sink in which power supply is required to achieve cooling action.

• Passive heat sinks:
• Passive heat sinks are exactly opposite of active heat sinks as they do not require power supply. They are a simply mechanical device which dissipates heat by convection.

• Stampings:
• It is an old technique of cooling electronic device in which copper or aluminium sheets are stamped into a desired shape

• Extrusions:
• These are the notch type heat sink forming a two dimensional structure allowing to dissipate large heat.

• Bonded/Fabricated Fin:
• Bonded type heat sink have thermal heat transfer supported with air cooling which results in high performance and are used in CPU and other electronic devices/system.

• Castings:
• These are used for strong cooling having high density of pin fin formed by sand, lost core and die casting process in aluminium/copper material.

• Folding Fins:
• In this type the fins are folded to increase the surface area and volume attached to a device via epoxy or brazing allowing high performance with better efficiency.

## Mathematical Calculations

• The governing equation which correlates the total power, temperature difference and the thermal resistance which can be expressed as:

• The thermal resistance is analogous to the electrical resistance used in Ohm's law.

• Thermal Resistance:

• Rj-c is the Junction to case thermal resistance. Usually a parameter that is published by the component manufacturer.

• Rc-s is the thermal resistance across the thermal interface material between the heat sink and the component.

• R s-a is the thermal resistance of the heat sink.

• Junction to Ambient is the sum of the resistances

• ## Heat Sink Selection

• Tj, Rjc and Q will be provided by the component manufacturer.

• Rcs - Thermal resistance of the interface material.

• Ta - Ambient temperature.

• Ta and Rcs are parameters that we can control to a certain extent.

• Rsa is the number that will help us identify a heat sink that will meet our criteria.

• ## Applications of heat sink

 Heat Sink in Voltage regulator Heat Sink in CPU

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