High performance electronic systems are coping with increasing cooling demands. Conventional solutions realize cooling through combining heat exchangers that are bonded to heat spreaders that are then attached to the chip backside. These are all interconnected with thermal interface materials (TIM) that create a fixed thermal resistance that can’t be overcome by introducing more efficient cooling solutions. Direct cooling on the chip backside would be more efficient, but current direct cooling microchannel solutions create a temperature gradient across the chip surface.
Another option for chip cooling is an impingement-based cooler with distributed coolant outlets. These put the cooling liquid in direct contact with the chip and sprays the liquid perpendicular to the chip surface. This ensures that all the liquid on the chip surface has the same temperature and reduces the contact time between coolant and chip. However, current impingement coolers have the drawback that they are silicon-based and thus expensive, or that their nozzle diameters and use processes are not compatible with the chip packaging process flow.
That is why Imec has been working on a low-cost impingement-based solution for cooling chips at the package level. The chip cooler used polymers instead of silicon, to achieve a cost-effective fabrication. Moreover, Imec’s solution features nozzles of 300 µm, made by high-resolution stereolithography 3D printing. The use of 3D printing allows for customization of the nozzle pattern design to match the heat map and the fabrication of complex internal structures. Moreover, 3D printing allows to efficiently print the whole structure in one part, reducing production cost and time.
“Our impingement chip cooler is actually a 3D printed ‘showerhead’ that sprays the cooling liquid directly onto the bare chip,” said Herman Oprins, senior engineer, Imec. “3D prototyping has improved in resolution, making it available for realizing microfluidic systems such as our chip cooler. 3D printing enables an application-specific design, instead of using a standard design.”
The spouts shoot liquid into a small space in between the cooling system and the surface of the semiconductor before heated coolant is expelled through separate plumbing. Traditional chip cooling systems use heat spreaders attached to the bottom of the substrate, but the thermal materials bonding the heatsinks in place have predetermined heat resistances, which is a major limitation.
Imec’s impingement cooler achieves a high cooling efficiency, with a chip temperature increase of less than 15°C per 100 W/cm2 for a coolant flow rate of 1 l/min. Moreover, it features a pressure drop as low as 0.3 bar, thanks to the smart internal cooler design. It outperforms benchmark conventional cooling solutions in which the thermal interface materials alone already cause a 20°C to 50°C temperature increase. Next to its high efficiency and its cost-effective fabrication, Imec’s cooling solution is much smaller compared to existing solutions, matching the footprint of the chip package enabling chip package reduction and more efficient cooling.