LIQUID PHASE SINTERED COMPOSITE SOLDERS FOR NEXT GENERATION THERMAL INTERFACE APPLICATIONS

JIA LIU

It is undeniable that electronics are becoming increasingly powerful and that there is continual effort towards miniaturization of these devices and thus increasing heat generation requires a new paradigm in thermal interface materials (TIM) design. This work was aimed at optimizing the processing parameters and characterizing the performance of Cu-In composite solders produced by liquid phase sintering (LPS). These composites comprise a high-melting phase (HMP) such as Cu embedded in a matrix of a low-melting phase (LMP) such as In. Copper contributes to high thermal and electrical conductivity of composites, whereas the soft In matrix helps maintain high shear compliance. This combination of high electrical/thermal conductivities and high shear compliance makes these solders suitable for a range of next-generation thermal interface material (TIM) and interconnect (IC) applications. After considering a range of compositions, a solder with 60 volume percent In was found to possess the requisite combination of high compliance and high conductivity. During the study, interfacial engineering was introduced to slow down the reaction between Cu and In, and hence further improve the performance of composite solders. A dual interfacial layer consisting of Al₂O₃ and Au was used to mitigate the reaction between Cu and In. A 1 nm Al₂O₃ layer was used as a diffusion barrier to prohibit the inter-diffusion between Cu and In, while a 20 nm Au layer was coated on top of the ceramic Al₂O₃ for wetting enhancement. The dual layer increased the thermal conductivity of the solder by a factor of ~2 while reducing the yield strength to make the solder more compliant. The effects of particle size, shape and volume fraction was also studied, and a simple model was utilized to explain the trends in the mechanical and the thermal properties. The optimized Cu-In composite solders were further used to study the performance of solder joints. Mechanical properties under shear and joint thermal resistance were measured as a function of joint thickness, thermal excursion history, and different inter-layers between solder and Cu. Minimizing interfacial contact-resistance is desired, especially when the joint thickness becomes sub-millimeter, and hence role of inter-layer on the contact-resistance was studied.

LIQUID PHASE SINTERED COMPOSITE SOLDERS FOR NEXT GENERATION THERMAL INTERFACE APPLICATIONS

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