Abstract:

A novel fluxless soldering process is presented, that enables lead-free soldering of semiconductor die-to-heat spreader (and heat spreader-to-heat sink structures) at room temperature. The process is based on the use of reactive multilayer foils to locally melt the solder interface. Silicon-copper samples joined with indium solder are thermally characterized for a range of die sizes and bond line thicknesses. The thermal resistance of the solder joints is found to be an order of magnitude lower than for conventional thermal interface materials (TIMs), with good thermal fatigue resistance. The predicted thermo-mechanical behavior of the solder interface in a central processing unit (CPU) application indicates that such joints would survive application environments without causing die cracking. The soldering technology employed could greatly enhance the thermal performance of power IC packages such as CPUs, by enabling the adoption of a solder-based TIM between the die and integrated heat spreader.

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