QUICK Definitions

AlSiC                                          

The combination of aluminum metal and silicon carbide ceramic in a composite structure. The name "AlSiC" combines the elemental chemical symbols of these materials, Al for aluminum, and SiC for silicon carbide. These materials are combined and retain their individual characteristics with the aluminum forming a continuous metal matrix and the silicon carbide remaining as a particulate.  The properties of the combined materials are a function of the ratio of these materials. 

 

CPS AlSiC-9 is at a ratio of 63% SiC to 37% Al metal. This combination has a good thermal conductivity and a coefficient of thermal expansion that is compatible with power electronics to allow improved heat dissipation, and it minimizes thermally induced stresses which can result in improved thermal dissipation and increased reliability of power electronics.

 

AlSiC has been made in laboratories since 1955;  CPS has developed process technology that allows AlSiC to be produced on an industrial scale.  CPS started our development of the material in 1987 for hermetic packaging of RX/TX modules used in radar systems. In addition to CTE characteristics, AlSiC is very light weight in contrast to traditional thermal management materials like CuMo and CuW and provides better thermal conductivity compared to iron nickel alloys like Kovar.

 

CPS started high volume manufacturing of AlSiC in 1993, producing baseplates for Motorola for use in power amplifiers in cellular base stations.  In 1995, CPS began shipping AlSiC baseplates for the earliest IGBT power modules. 

Coefficient of Thermal Expansion (CTE)

The measurement of a materials expansion over a given temperature range. 

 

Be aware of the temperature range when making your comparisons of the thermal expansion values of materials.  Often times, an average CTE is reported for room temperature. This is a value of expansion behavior a few degrees on either side of room temperature. You need to be aware of the thermal expansion over your processing conditions (soldering and brazing temperature) and the use temperature range.  Often times this is ignored with tragic consequences.

 

CPS can provide the CTE behavior over a wide range of temperature values. Please consult Dr. Mark Occhionero with your materials CTE compatibility questions.

 

CTE is one component of the thermal management equation.  When two materials are combined with widely different CTE values there will be stresses at the attachment point or within the materials themselves. If the attachment point fails, you lose the thermal dissipation path.  If one of the materials fails then the product, ceases to function. 

 

Often times, the thickness of a compliant joint material is used between two dissimilar CTE value materials to compensate for these thermally induced stresses. This can negatively affect thermal dissipation by increasing the thermal path length also increasing the thermal resistance.

 

With matched CTE values, it is possible to reduce the overall stress on components and minimize the thermal interface layers.  This reduces the thermal resistance and improves the thermal dissipation.

Coolers

A cooler is a base joined with a liquid cooling medium. These bases are usually characterized by a flat "substrate side or electronics side" and a side with pins that extend into a liquid cooling stream to give up the heat of the system.

 

Coolers are primarily used in automotive power module applications where there are power modules for driving the electronic motors, as well as power recovery modules to recover energy in braking.

Composite

A combination of materials wherein the materials remain separate and retain most of their individual characteristics and properties. The combination of the materials provides a a unique set of attributes that is a function of the percentages of the materials that are between the properties of the individual constituents. 

Flip Chip Lid

Lid used to cover and protect the electronics die and components. A lid needs to have good thermal management properties. It should also be light weight and have high strength and stiffness to improve reliability of the attachment and maintain the thinnest thermal interface. Being light weight is important for larger lid solutions to reduce solder ball failure during assembly and use.

IGBT Modules

Insulated Gate Bipolar Transistor module, or IGBT module, is a power electronic device that changes DC voltage and current to a digital approximation of AC current or voltage, or the inverse; or from AC of one voltage and phase to AC of another. Even though IGBTs are the most efficient power conversion device, there are losses which result in heat. If the heat is not managed the electronics can overheat and fail. Heat generated causes the IC devices to expand during heating and contract during cooling when the power is off. In order to dissipate the heat, devices are mounted to a baseplate that rapidly dissipate the heat to the atmosphere or to a cooler. 

 

The AlSiC material property combination of a CTE value that is compatible with the IC device (and ceramic substrate) and a high thermal conductivity makes it an ideal baseplate material for high power density IGBT modules. These modules are used in power conversion applications such as trains and subways (traction), green power grid applications including solar, wind and hydroelectric power (and for traditional powers generation applications too).

 

AlSiC is a good material choice since it is also 1/3 lighter in weight in contrast to copper, the material traditionally used.

Metal Matrix Composite

A composite of two or more materials wherein the metal provides a continuous structure that contains the other constituents of the composite. For AlSiC, aluminum is the metal matrix in which the SiC particles are suspended. 

Thermal Conductivity (CTC)

Material property that is a measurement of a material's capability to conduct heat. Often times the values reported are the Coefficient of Thermal Conduction (CTC) at room temperature. This is not the complete story and one needs to be aware of the thermal conduction at various process temperatures and operation temperatures. This value is usually lower than what is reported at room temperature.

 

CPS can provide the CTC Value over a wide range of temperatures. Please consult with Dr. Mark Occhionero on thermal conductivity measurements and values, especially when you need information for your finite elemental analysis thermal modeling of your assembly,

 

Thermal conductivity is one part of the equation in thermal management assemblies. In order to take advantage of the thermal conductivity for the best thermal dissipation, one needs to minimize the interface between materials.  This requires similar CTE values between the joined materials to minimize failure-causing stress on interface and between the materials. With similar CTE value materials, one can reduce the thermal interface joint which reduces the thermal resistance and improves thermal dissipation performance.

Thermal Dissipation

Thermal dissipation is the ability of a system to remove generated heat from the system and put it into the atmosphere or into a heat sinking system or fluid. Thermal conductivity is one aspect of this equation for materials, but in assemblies we need to consider the influence of thermal path and the thermal resistance of the assembly to remove heat.

Thermal Path

The path of heat conduction from the heat generating device to the atmosphere or to a heat sink. For effective thermal management, this needs to be as short as possible with minimal thermal resistance barriers.

Thermal Management

A term to describe how the heat generated by devices is prevented from damaging systems for devices or causing failure of the device or system. This includes the effective dissipation of heat from the devices, and also preventing or reducing stresses associated with thermal expansion. Effective heat dissipation includes thermal conduction capability and shortest possible path for heat conduction with minimal resistance.

Thermal Resistance

The barrier to the conduction of heat energy between the heat generating device and the atmosphere or heatsink. Poor joints between materials, including joints with voids, joining materials with low thermal conduction, and making good conduction materials too thick affect the thermal resistance of the system.

 

 

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