Heat Spreader

A heat spreader is most often simply a copper plate, having high thermal conductivity. Functionally, it is a heat exchanger that moves heat between a heat source and a secondary heat exchanger whose surface area and geometry are more favorable. By definition, the heat is "spread out", such that the secondary heat exchanger increases the heat capacity of the assembly. These properties make it a better match for an air heat exchanger, since the low heat conduction for air in convection is matched with higher surface area.

A heat spreader is generally used when the heat source tends to have a high heat-flux density, (high heat flow per unit area), and for whatever reason, heat can not be conducted away effectively by the secondary heat exchanger. For instance, this may be because it is air-cooled, giving it a lower heat transfer coefficient than if it were liquid-cooled. A high enough heat exchanger transfer coefficient is sufficient to avoid the need for a heat spreader.

The use of a heat spreader is an important part of an economically optimal design for transferring heat from high to low heat flux media.
 

From wikipedia: http://en.wikipedia.org/wiki/Heat_spreader

What is a Heat Sink?

A heat sink is a term for a component or assembly that transfers heat generated within a solid material to a fluid medium, such as air or a liquid. Examples of heat sinks are the heat exchangers used in refrigeration and air conditioning systems and the radiator (also a heat exchanger) in a car. Heat sinks also help to cool electronic and optoelectronic devices, such as higher-power lasers and light emitting diodes (LEDs).

A heat sink is physically designed to increase the surface area in contact with the cooling fluid surrounding it, such as the air. Approach air velocity, choice of material, fin (or other protrusion) design and surface treatment are some of the design factors which influence the thermal resistance, i.e. thermal performance, of a heat sink. One engineering application of heat sinks is in the thermal management of electronics, often computer central processing unit (CPU) or graphics processors. For these, heat sink attachment methods and thermal interface materials also influence the eventual junction or die temperature of the processor(s). Thermal adhesive (also known as thermal grease) is added to the base of the heatsink to help its thermal performance. Theoretical, experimental and numerical methods can be used to determine a heat sink's thermal performance.

From wikipedia.

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Heat Sink Materials

Aluminum is the most common material because of lower cost, ease of manufacturing, and the existing 

infrastructure.  A higher conductivity material is preferred, however, copper is beginning to find more use for both heat sinks and chip IC lead frames.  A material that is more conductive than aluminum, but not as costly as copper would be viable, especially if easily shaped. 

Newer materials, both clads and alloys, have become available in recent years, however, copper-tungsten (Cu/W) is a great material and is offered by THT.  The very low CTE values are its main feature. Copper- Molybdenum (Cu/Mo) also has a low CTE and is available at THT. 

Composite heat sinks are emerging and this area is getting increasingly more attention since Nanotechnology can be applied.  We can expect other composites to be developed with nano materials such as carbon Nanotubes (CNT) in the near future. Metal and nano materials may possibly be used together as composites. 

Some of this blog post comes from "Heat Sink Materials" by Ken Gilleo, PhD

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Is There a Market for New Heat Sink Materials with Low CTE?

This is an excerpt from "Heat Sink Materials" by Ken Gilleo, PhD

Heat sinks have been used before the dawn of solid-state electronics and the industry is well established on a global basis including plants in China. Aluminum is the de facto standard material even though it is far from ideal. Copper has better thermal conductivity and is gaining share as smaller, more efficient heat sinks and related products are required. Copper alloys and constructions using copper are also being used today. It should be noted that low CTE alloys are already in use and a new material must be compared to these products. Non-metals are also likely to appear soon as nanotechnology attacks thermal management problems.

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Video about Heat Sinks for RF and Microwave Packaging

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Copper Tungsten uses & Formulation Processes

This is from Intatech: http://intatech.com/CuW.html

Copper tungsten is used for primarily three reasons:


1. Matching coefficient of thermal expansion to a mating material
2. Thermal conductivity for heat sinks
3. Density for RF shielding


There are two formulation processes for CuW

1. Powdered Metal:

A blending of tungsten, copper and various binders are formed under high pressure and then sintered at high temperatures where the material becomes fully dense.

2. Infiltration:

The process of infiltrating copper into a tungsten skeleton. This process is performed under pressure and high temperature.


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Copper Tungsten

This refractory metal composite materials are a combination of tungsten combined with copper or silver. The manufacturing process is to press the refractory, sinter the pressed compact at a high temperature, and infiltrate with copper or silver. All this is done under very closely controlled conditions. The result is a relatively hard materials with superior arc and wear resistance, high physical properties at elevated temperatures, and good electrical and thermal conductivity.


Advantages:
- Higher thermal conductivity
- Low thermal expansion
- High arc resistance combined with good electrical conductivity


Applications:
- Heat Sinks as passive cooling elements of electronic devices
- Electrodes for resistance welding
- Electrodes in electric spark erosion cutting machines
- Arcing contacts and vacuum contacts in high and medium voltage breakers or vacuum interruptions


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