Evaporative hotspot cooling of microprocessors using electrowetting on dielectric (EWOD)
Abstract
The increased use of smart devices and great strides in fabrication technologies
have resulted in densely packed electronics. Combined with 3D packaging, high heat
uxes have become the norm. The need for advanced thermal management solutions
has risen. Most devices based on their load pro le and architecture are susceptible
to non-uniformities in heat
uxes. These non-uniformities with very high heat
uxes
are known as hotspots. Mitigation of hotspots is very important for the normal
functioning and long term reliability of microprocessors.
Multiple attempts have been made at developing microscale thermal solutions.
Most prominent among them are microchannels and heat pipes. The shrinking size
of electronics demands compact thermal solutions. One of them is electrowetting on
dielectric (EWOD) digital micro
uidics. They are highly suitable due to their unique
features such as no moving mechanical parts, low power consumption and pump-less
operation. Moreover, unlike microchannels, they are immune to
ow instabilities.
This research focuses on utilizing EWOD devices to cool hotspots evaporatively.
Indium tin oxide (ITO) as well as nickel thin lm resistors are integrated in the devices
to simulate hotspots as well as make temperature measurements. The experiments
were carried out in ambient conditions. Cooling of the hotspot is studied by measuring
the hotspot temperature with di erent heat
uxes and water droplet delivery speeds.
Heat
uxes higher than 40 W/cm2 resulted in enhanced cooling due to phase change
happening at the advancing and receding menisci of droplets. Sustained droplet
motion cools and maintains the hotspot temperature.
vi
Phase change phenomena although present at high heat
uxes, was not the
main mode of heat transfer. To increase the role of evaporation, a superhydrophilic
area was integrated in the electrowetting device. The superhydrophilic region, owing
to its near zero contact angle, increases the length of the transition region in the thin
lm[1]. Visuals of spreading of the liquid thin lm synchronized with the hotspot
temperature data was used to study evaporative hotspot cooling. Di erent spreading
regimes were observed corresponding to various heat transfer modes governing heat
dissipation.Also, a thermal resistance analysis was conducted to understand the heat
transfer mechanism. The e ect of various evaporation parameters was studied.
The work focuses on studying evaporative heat transfer in hotspot cooling.
While doing so, it also elucidates the challenges faced in designing a purely evaporative
hotspot cooling solution.