Heat that if left unchecked could reduce the life span or even destroy outright the processor that created it. Thermal management increasingly needed when dealing with electronic devices to satisfy the objectives and of functional integrity and operational integrity. A lower temperature will yield satisfactory function and increase the expected life of the electronics. The application of a heat sink satisfies those two objectives. A heat sink is a heat dissipation device that consists of a base and fins that extend outward. The performance of the heat sink is affected by air velocity, choice of the material, fin design and surface treatment. When a chip heats up from use the heat sink removes and disperses the heat to keep chip at a proper operational temperature. The heat comes in to the fins and then the flow takes it away by change of the enthalpy across the surface. Without the use of the heat sink the chip would over heat which could destroy the entire system. To further heat dissipation ability of a heat sink the meat can be anodized. Anodizing is the electrochemical process that thickens and toughens the naturally occurring protective oxide layer on the surface of metal. This layer enhances the radiation heat transfer and can also be colour dyed to appeal more to the user.
As the name suggests the heat sink absorbs the heat which is generated by the electronic devices. Then it transfers heat to the surrounding. Now this surrounding could be water, air or sometimes even oil. So these heat sinks are available in various size and shape but the purpose of the every heat sink is to remove the heat from the component and is widely use in the electronic devices. In computer peripherals then they are used in graphics cards, CPU and sometime even in the RAM module. In a high performance RAM module they are used as a heat spreader. They are used in the LED modules as well as in the electronic components like high power MOSFETs and the transistors. The purpose of the heat sink is to remove the heat form the electronic devices so that they can be operated in the allowable temperature range.
If see the electronic devices not all the energy is converted in to useful work. For example if we take an example of the LED then only some fraction of the applied power getting converted in to the light. The remaining power is getting dissipated in the form of heat. Every electronic device consists of a finite number of resistors and these resistors dissipate the power in the form of heat.
The processor consists of million and billions of little transistor and in this processor even if all the transistors are not in use at once still they consume some power and this power or energy getting dissipated in the form of heat. As we know that all the electronic devices has some finite resistance and the power which is getting dissipated across the equivalent resistor is converted in to the heat. Due to this heat the device temperature increases which not only reduces the life time of the device but sometimes even it can damage the device and that is why it is quite important to maintain the device temperature within the acceptable range. That is the reason the heat sink is used with the electronic devices and particularly with those devices which generates considerable amount of heat. So the heat sink operation is based on Fourier law of heat that means whenever the temperature gradient exists in the body then the heat transfers from the high temperature section to the low temperature section. This heat can transfer in three different ways:
Whenever we connect the heat sink to a device then how the heat transfer occurs.
As shown in the schematic the heat sink is connected to the device and this device transfer the heat to the heat sink via conduction. Now to maximize this heat transfer the device and the heat sink should be in the proper contact. But even if we connect these two surfaces properly then also at the microscopic level, some air still remains between the two surfaces. Due to this air gap the heat transfer between the two surfaces will get affected. So using the thermal compounds like heat sink compounds the air gap can be minimized. Due to that the heat transfer between the device and heat sink can be maximized. Now from the heat sink the heat is transfer to the surrounding through the convection method and minor amount of heat is also transferred using the radiation method. In this way the heat is transfer from the device to the surrounding.
Heat sinks in the computer:
When we first open the computer or other electronic device one of the first thing we would see is one or more large metal objects called heat sinks. Inside the computer heat sink is found on the CPU, graphic card and mother board, inside the power supply and even in other places as needed. When we see the heat sinks it looks different from each other but they all serve the same basic purpose to remove heat from delicate components and extend their life time.
Heat sink materials:
The most common material used for a heat sink is aluminium, due to its high thermal conductivity and its ability to be machined easily.
Copper is sometime used because of its thermal conductivity which is nearly double that of the aluminium but it is more expensive and less machinable. There are many types of the heat sinks which are:
- Folded fin
- Single fin
|Most applications||Low – high||Cost effective||Limited dimension|
|Low power||low||In expensive||Low performance|
|Large applications||Moderate||Large sizes||Expensive|
|Ducted air||Very high||High heat flux density||Expensive need ducting|
|Many application||Medium||Inexpensive||Limited in design and flow management|
|Swaged||High power applications||Medium||Good for power device||Heavy and bulky, limited ability for flow management|
|Single fin assembly||All application||Low high||Light weight and low profile with high degree of flow management options||Expensive|
|Skived||Many applications||Medium high||High fin density||Thick base, higher weight, directional sensitive|
Design of the heat sink:
The heat sink design in such a way that the surface area which is in the contact with the cooling medium can be maximized so the idea is that the contact area of the heat sink should be at least 10 to 20 times more than the surface area of the device.
That is why the fin type arrangement is used with the heat sink. So the performance of the heat sink which means that how will it is able to transfer the heat depends upon the multiple parameters which are:
- Area of the heat sink
- Fin density
- Fin spacing
- Air flow
Types of the heat sink:
It also depends upon the material which is used for the construction of the heat sink. So usually either aluminium or copper is used for the construction of the heat sink. It also depends on the air flow. The heat sink can be classified in to two categories:
- Active heat sink
- Passive heat sink
This is the most basic heat sink and it consists of a simple flat piece of metal. It moderately improves heat dissipation because while metal will transfer heat to the surrounding air faster than plastic. It would be much more effective if it also increased the size of the area of the surface that is being used to transfer.
Passive heat sink:
In case of the passive heat sink there is no mechanical moving part and the heat is transferred through the natural convection. These are basically heat spreader with structure on top of them that dramatically increase the surface area that can be used to dissipate heat to the surrounding air. They are much more effective than the heat spreader but they are also more expensive to make and they take up more space.
Active heat sink:
In case of the active heat sink the heat is transferred using the forced convection and for that the devices such as fan or blower is used.
So due to the force convection the active sink can remove the heat more efficiently. So for the same heat transfer the active heat sink requires less space compared to the passive heat sink. But as it involves the mechanical moving part so it requires more maintenance compared to the passive heat sink.
Heat pipe or vapour chamber heat sink:
Now apart from this forced cooling use the fans sometimes the vapour chamber is also used in many applications. The vapour chamber heat sink is used for very hot components like CPU’s or graphic cards. The limiting factor of a standard thinned heat sink performance is no longer the speed at which the fins can be used to dissipate the heat to the air but rather the speed at which the heat can be moved away from the very small processor to the fins in the first place. Heat pipes or vapour usually consists of an outer copper wall and a material inside that is constantly changing phases between the liquid and gas. They can be used to carry heat away from a small heat source extremely quickly to a large array of heat sinks fins where it can be dissipated to the air. The vapour chamber heat sink is constructed from the sealed metal plates and then it is filled with small amount of fluid such as deionized water.
This water allows the heat to be rapidly dispersed away from the source. Whenever the heat is applied to the vapour chamber then the fluid which is near the location immediately vaporizes and then it fills the entire volume of the chamber. Now when this vapour chamber comes in to contact with the cooler wall surfaces then it condenses and releases heat to the cooler wall surface. This condensed fluid returns back to the heat source via capillary action.
The heat transfer through the heat sink can be defined in thermal resistance and this thermal resistance is defined in the unit of either degree centigrade per watt or degree kelvin per watt which means that the rise in the temperature of a particular device when we apply the one watt of power and the thermal resistance of any heat sink should be as low as possible.
So for one heat sink the thermal resistance is 5 degree centigrade per watt that if 5 watt power is absorb by the heat sink then the temperature of the heat sink will be rise by 25 degree centigrade. If the ambient temperature is 25° C then the heat sink will be at 50° C.
How to improve the performance of the heat sink:
Lower the ambient temperature:
We can improve the performance of the heat sink by lowering the ambient temperature like if cracking open a window lower the room temperature by 5° C it will decrease the heat sink temperature by about 5° C.
More Air flow:
The faster the air moves over the heat sink the better it will perform.
Better thermal interface material:
A better thermal interface material no two pieces of metal will ever up perfectly and thermal interface material fills in the micro gaps for better heat conduction or better heat transfer between them replacing the sub parts solution that come pre-installed on the components with high performance. Thermal compound can easily lower temperature by several degrees.
A good solid mount improves the contact between a chip and a heat sink and ensures effective thermal transfer often a heat sink that is not performing as expected is being held back by an air bubbled trapped in between a small component nearby that is interfering with the heat sink mounting pressure.