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.