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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High Density Tungsten

High density tungsten alloys are materials with a high tungsten content and low amounts of Ni Fe or Ni Cu.


Advantages:


- High density
- Good machinability
- Good mechanical properties
- High modulus of elasticity
- Harmless to health and to the environment


Applications:


- Electrodes for resistance welding
- Protection shields for nuclear radiation
- Balancing weights 
- Electroheat upsetting anvil block
- High voltage electrical contact
- Substitutional material for uranium
- Balls in bomb


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Pure Tungsten

Tungsten has the highest melting point (3422°C) of all the non-alloyed metals. It can keep its strength and stiffness at high temperatures. Tungsten has good thermal conductivity, excellent corrosion resistance, and lowest thermal expansion of any pure metal.

Tungsten is suitable for aerospace and high temperature uses which include electrical, heating, and welding applications. Hardness and density properties make this metal ideal for making heavy metal alloys such as copper tungsten that are used in armament, heat sinks, and high density applications.

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Molybdenum Properties

Molybdenum can withstand extreme temperatures and keep its strength and stiffness at high temperatures. The melting point of Molybdenum is 2623°C.

The ability of molybdenum to withstand extreme temperatures without significantly expanding or softening makes it useful in applications that involve intense heat, including the manufacture of aircraft parts, electrical contacts, industrial motors, and filaments. Molybdenum is also used in alloys for its high corrosion resistance and weld ability.

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Cu/Mo70Cu/Cu (CPC) Heat Sinks

Cu/Mo70Cu/Cu (CPC) is a sandwiched composite similar to Cu/Mo/Cu comprising a Mo70-Cu alloy core layer and two copper clad layers. The ratio of the thickness in Cu Mo70Cu and Cu layers is 1:4:1. It has different CTEs in the X and the Y direction. Its thermal conductivity is higher than those of W/Cu, Mo/Cu, Cu/Mo/Cu and it is much cheaper.


Advantages of CPC Heat Sinks:


- Can be more easily stamped into components than CMC
- Very strong interface bonding that can resist 850 degrees C heat shock repeatedly 
- No magnesium
- Higher thermal conductivity at lower cost


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Cu/Mo/Cu Heat Sinks (CMC)

Cu/Mo/Cu (CMC) is a sandwiched composite comprising a molybdenum core layer and two copper clad layers. It has adjustable CTE, high thermal conductivity, and high strength. All types of Cu/Mo/Cu sheets can be stamped into components.


Advantages of CMC Heat sinks:


1. Large size sheets available (length up to 400mm, width up to 200mm)
2. Can be stamped into components
3. Very strong interface bonding that can resist 850°C heat shock repeatedly
4. Adjustable CTE matching that of semiconductor and ceramic materials
5. High thermal conductivity
6. No magnetism


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Molybdenum-Copper Heat Sinks

Molybdenum-Copper heat sinks are composites of molybdenum and copper. Similar to W-Cu, CTE of Mo-Cu can also be tailored by adjusting the content of molybdenum. Mo-Cu is much lighter than W-Cu so that it is suitable for aeronautic and astronautic applications.  These heat sinks are widely used in applications such as optoelectronics packages, Microwave Packages, C Packages, Laser Submounts, etc.


Advantages of Molybdenum-Copper heat sinks:


High thermal conductivity since no sintering additives have been used


Excellent hermeticity


Relatively small density


Stampable sheets available (Mo content no more than 75 wt%)


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Copper Tungsten Heat Sinks

They are composites of tungsten and copper. By adjusting the content of tungsten, we can have its coefficient of thermal expansion (CTE) designed to match those of materials such as ceramics (Al2O3, BeO), semiconductors (Si), and metals (Kovar), etc.


Advantages of CuW Heat Sinks:

• High thermal conductivity
• Excellent hermeticity
• Excellent flatness, surface finish, and size control

Copper Tungsten is one of the most popular refractory metal based heat sink materials offered today.  For more information, please visit us at http://www.torreyhillstech.com 

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Introduction to THT Heat Sink Materials

Torrey Hills’ heat sinks manufacturing facility specializes in the R&D and production of high-tech electronic packaging materials, namely copper tungsten, molybdenum copper, and copper molybdenum copper heat sinks. The facility is top in the field of electronic packaging materials with advanced material manufacture technologies, complete materials detective devices and modern management system.

Our products are widely used in applications such as optoelectronics packages, RF and microwave packages, C packages, laser submounts, etc.

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