Nano and Micro Technology-Based Next-Generation Package-Level Cooling Solutions
The design requirement for electronics cooling is to maintain the hottest location (hotspot) on the die (chip) below the specified temperature. Due to the presence of multiple hotspots, the thermal resistance near the die is very high. Total thermal resistance ( Ψ tot ) can be written as: Ψ = DF × (R si +R TIM1 +R spreader )+ Ψ TIM2 + sin k =DF × (R package )+ Ψ TIM2 + Ψ sin k where R package , R si , R TIM1 , and R spreader are the thermal impedances of package, silicon, first-level Thermal Interface Material (TIM), and heat spreader, respectively. Ψ TIM2 and Ψ sink are the thermal resistances of second-level TIM and the heat sink, respectively, and Density Factor (DF) is a factor that accounts for the non-uniformity of heat generation. DF can possibly be greater than 1 if highly non-uniform power distribution exists of it the die size is very small. Since the thermal impedance near the die (R package ) gets multiplied by the DF, any reduction in the package impedance result in a larger reduction in Ψ tot . Because of this reason, the focus of next-generation electronics cooling is on developing efficient cooling solutions near the package. Futuristic cooling solutions may be based on micro and nano technologies. These solutions might include a TIM made from micro and nanoparticles, a nanoparticles, a microchannel heat exchanger, and a Thin Film Thermoelectric Cooler (TFTEC) that is made of thin film superlattices, or nanocomposites, placed directly above the hotspots to provide localized cooling. In this paper, we focus on the technical merits of these technologies and discuss the challenges that must be met to make these technologies a reality for electronics cooling. The main challenges are: a ) to reduce the boundary resistance between the nanoparticles and the host medium for nanoparticles-based TIMs and to increase the reliability performance of TIMs; b) reduce the assembly-related parasitic effects seen in TFTEC (for example, due to the very thin dimension of TFTEC, electrical contact resistance reduces the effective ZT in a package making it much smaller than the intrinsic ZT); and c) pumping requirements and pump reliability for microchannels. Water cannot be used as a coolant because the freezing requirements for electronics cooling is dictated by shipping and handling requirements and is much lower than 0 ° C. Traditional antifreeze liquids have much lower thermal conductivity and higher viscosity than water, forcing very sever pumping requirements in order to get the same thermal performance as water.