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.