AlSiC For Microprocessors

Figure 1: CPS AlSiC Flip Chip Microprocessor Lids

AlSiC materials properties are ideal for microprocessor applications for more than just the thermal management issues associated with high thermal conductivity and compatible coefficient of thermal expansion (CTE).  Figure 1 shows a  number of AlSiC lid designs (notice the shape capability).

AlSiC also has the advantage of being lightweight with a high strength and stiffness making it great for flip chip microprocessor applications that see shock and vibration.  Shock and vibration considerations should also be considered in automated assembly lines too.

Figure 2: Flip Chip Lid Assembly.
The heat spreader is the AlSiC lid.

Figure 2 shows the typical flip chip lid assembly.  The heat spreader shown in this illustration is the lid.  The lid protects the die and needs to provide a good interface between the die and the lid for thermal dissipation.  The Thermal Interface Material (TIM) 1 is usually a thermal grease material.  This needs to be as thin as possible to minimize the thermal path of heat from the die through the lid.  It is therefor critical that the lid cavity height and tolerance be carefully controlled as well as the flatness of the lid.  This is also influenced by the stiffness of the lid. 

There are two main assembly types for flip chip microprocessor systems: Ceramic and FR4 systems which have two different thermal expansion considerations.  This material is represented in green in Figure 2.  Ceramic systems require lower CTE AlSiC-9 lids and for the FR4 the designer may choose to use AlSiC-12 to match that of the printed circuit board or the designer may also consider using AlSiC-9 to match the CTE of the die.

Being lightweight and stiff makes AlSiC good for large lidded applications, and can enable some new integrated systems packaging concepts (System in Package).  In these applications the designer needs to have a material that can span long distances with out deformation for die protection.  It also needs to be lightweight so that there is minimal orientation dependency.  AlSiC has a strength and stiffness that is slightly greater than Cu yet is is 1/3 the weight of Cu (materials properties).

Figure 3: CPS AlSiC Large SIP lid 55 mm x 70 mm @ 17 g.
The Cu equivalent is 55 g.

Figure 3 shows a large AlSiC SIP lid that is 55 x 70 mm.  Thermal management was not the main consideration in this application and weight was.  A large copper lid is three times as heavy as the AlSiC equivalent, and copper had an orientation dependency in service.  A lid with high mass can also affect the assembly.  The weight of the lid can cause the solder balls in the BGA or under the die to creep during reflow, which may result in solder balls shorting.  The other issue during automated assembly is the shock of acceleration and deceleration that can cause solder balls to debond.  This AlSiC solution enables the designer to consider incorporating more functionality under the lid for shorter electrical distances (faster speed).  All of the electronics are then protected by one large die in this application.

visit the CPS website www.alsic.com for more information.

Properties of Material Used In Electronics

Many materials are required for today's electronics and electronics systems.  In thermal management we need to consider how all these properties affect the system not only during power cycling during the service life, but also in the assembly and testing.

Thermal Conductivity and Coefficient of Thermal Expansion are the most common considered in power electronics.  But the careful design engineer must also consider the weight (density), strength and stiffness too as these have an influence in system reliability.  

Figure 1: CPS AlSiC Products Illustrating Design Functionality

Other considerations, that are not material properties, are what is the materials capability to be fabricated into something that is useful and design capabilities to hold the necessary dimensional tolerances.  These issues will be discussed in the applications in later posts.  Figure 1 shows a wide range of AlSiC products illustrating design functionality.

Figure 2 is a table of physical properties for common materials used in electronic assemblies and their material properties.


I have color coded this chart with the light blue for the packaging materials commonly used.  AlSiC-9 and AlSiC-12 are on top.  AlSiC-9 is applicable for direct mounting of die and ceramic substrates.  AlSiC-12 is good for microprocessor lid assemblies that are mounted on FR-4 boards.


Also included are materials like Copper and Aluminum.  Copper is used in Low power IGBTs.  I can be lower cost since the material is stamped.  Aluminum is used as a low cost heat sink. It also can be stamped but most commonly extruded to have fins for extended surface area for convective or forced air cooling.  The drawbacks of these materials is that both have very high CTE values which will not match that of the electronic die materials (Green) or the substrate materials that are used for electrical isolation.  

Figure 2:  Physical Properties of Materials used in Electronic Systems

Copper Molybdenum (CuMo) and Copper Tungsten (CuW) are considered thermal management materials because they have CTE values that are compatible with the electronic materials and substrate materials.  Both these materials have been used extensively in the military and aerospace industry in hermetic packaging because of the CTE thermal management reliability required in these systems.  The draw back of these materials is that the material is more expensive, it is expensive to machine (most parts are consequently flat) and they are heavy (high density).  They high density is not desirable in weight sensitive aerospace applications or where shock and vibration are a consideration.


In contrast AlSiC materials are thermal management materials having a good thermal dissipation and CTE that is compatible with the electronics and dielectric substrates.  Additionally these materials are light weight, and the fabrication technology allows for functional shapes to be considered. 

Electronic materials are in green contrast in this table.  The notable properties of these materials is that they have very low CTE values and that they have very low fracture strength.  The low strength means that the design engineer really needs to be cognizant of thermal stresses that may be induced during power cycling, and at operation temperatures.


In yellow are some common dielectric substrate materials.  These materials are used for electrical isolation the electronics to the rest of the world.  Their strength and CTE values are relatively low.  So choosing choosing compatible CTE values is necessary when considering heat sink attachment.  

Usually these dielectric materials are metallized with a very thin (~ 300 µm) Copper layer that provides electrical connection between devices and also a solder or braze Cu attachment surface on the side opposite to the electronics to integrate these substrates with a heat sink.  These copper layers are attached by an activated attachment process called Direct Bond Copper or DBC. DBC substrates are commonly used in IGBTs.   In IGBTs the CTE of the assembled materials is very important.


for more information please visit CPS at www.alsic.com.