Microprocessor RoHS Requirements: Demands a need for lightweight high stiffness AlSiC Microprocessor Lids

The Problem
      Recent RoHS requirements for epoxies and solders used to attach microprocessor lids to the printed circuit boards means changes to microprocessor assemblies behavior during thermal cycling.  Often times these new “safer” materials lose desirable properties and become less compliant with the formulation change.   As a result, thermally induced stresses are not compensated by the epoxy and the microprocessor can bow and flex due to thermal expansions differences between the assembled materials.

       Flexing of a microprocessor assembly has deleterious consequences.  In the best case the system will have performance issues as the system can flex causing the thermal interface distance to change as position of the lid and printed circuit moves with respect to the die or chip.  An increase in TIM1 thickness will increase the thermal path and degrade the thermal resistance and cause the thermal interface material to “pump out” from between the die and heat sink material.   A decrease in TIM1 thermal interface distance ultimately improves the thermal dissipation performance, but ultimately will “pump out” the TIM1 thermal interface material eventually degrading the thermal dissipation the performance.  In the most severe cases bowing and flexing of the circuit assembly if too great can stress the die, solder ball connections to the point of failure.

The Solutions

       The best way to counteract these thermally induced stresses without the aid of this stress relief mechanism is to choose materials that are compatible in terms of their thermal expansion coefficients.  This is not always possible, because there are many materials components in these assemblies with different CTE values making a “one compatible CTE solution” very difficult to manage.

       An alternate approach is to increase the stiffness of the system.  This can be done by choosing a stiffer material(s) or by increasing the thickness of the materials in the system presently.   Increasing thickness to for increased stiffness may not meet the critical height requirements of the system.  Also the increased thickness will increase weight of the system which can result in an assembly that is less tolerant to shock and vibration.  Also the weight of the material may create orientation dependence to avoid damage created by material creep.  Copper has a density of 8.9 g/cm3; increasing thickness for improved stiffness will add significant weight.

       Increased material stiffness without a increase in material density (decreased material density is more desirable) would be the solution.  AlSiC, which has a density of 3 g/cm3 – 1/3 the density of copper, has a stiffness value that is greater than the copper equivalent.   And since AlSiC has significantly lower density increase thickness to achieve  greater assembly stiffness is possible without significant weight penalty (it will always be 1/3 lighter weight than the copper equivalent).

       And, by the way, No Restricted Hazardous Substances (RoHS) are used in the fabrication of AlSiC Heatsinks and Microprocessor lids.

      Please visit the CPS website for more information www.alsic.com also contact Bo Sullivan for discussion of your application.

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.