13 Fundamentals of Heat Dissipation in 3D IC Packaging …
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13.3 Air Cooling of 3D ICs
Stacking two or more IC chips under conduction cooled configuration allows using
conventional heat sinks on the face of the 3D IC chip stack. Since the heat flux levels
are higher than a single chip, advanced air cooling options may be needed. The
increased thermal resistance in the heat flow pathway also needs to be considered
and can be compensated by a lower thermal resistance in the heat sink.
Advanced air cooling options utilize forced air cooling with air delivered over
the finned heat sink using a fan. As the heat flux (heat dissipation per unit heat sink
footprint area) increases, higher velocities may be needed. Utilizing ducted air inlets
and outlets reduces the noise level as well as the pressure losses at the entrance
and exit from the heat sink. Employing compact heat exchanger surfaces (originally
developed for automotive radiators) in the heat sink matrix results in significantly
higher heat dissipation rates as the heat transfer surface area is significantly increased
due to the secondary fins. Although the pressure drop and fan power are increased,
significantly higher heat dissipation rates are achieved as compared to the natural
convection cooled heat sinks. A comprehensive coverage of compact heat exchanger
design and performance is given by Kays and London [10].
13.4 Jet Impingement and Spray Cooling
High speed jets from a nozzle impinging on a target surface provide an effective
technique to dissipate high heat fluxes. A single jet provides cooling in the immediate
vicinity of the jet impingement area, while an array of jets could be used to cover a
larger area. Air or liquid can be used as the cooling medium, while liquids are most
suitable for higher heat flux dissipation levels.
Although the jets provide a high heat transfer performance, in general, they require
high pressure drops in the range of 50–500 kPa. Apart from the fabrication and
leakage issues, the high velocity jets may lead to vibrations and erosion of the heat
transfer surfaces.
The jet impingement technique cannot be readily applied to conduction cooled
3D ICs. Placing jets in the interlayers can also lead to very complex fluid distribution
system design from the fabrication perspective. Leakage remains a critical concern
due to high pressures employed in delivering the high velocity jets.
Spray cooling involves spraying liquid droplets from a nozzle on a target surface
where they evaporate. This technique employs evaporation as against jet impingement cooling which is based on either on single-phase heat transfer, or in some cases
forced convection boiling. The target surface may be coated with porous coatings
or microstructures to enhance the evaporation rate. The spray distribution pattern,
the effect of returning vapor on the droplet velocity, and controlling the flow rate
to avoid flooding are some of the issues that require careful design considerations.
Although the pressures required for generating sprays are considerably lower than
373
13.3 Air Cooling of 3D ICs
Stacking two or more IC chips under conduction cooled configuration allows using
conventional heat sinks on the face of the 3D IC chip stack. Since the heat flux levels
are higher than a single chip, advanced air cooling options may be needed. The
increased thermal resistance in the heat flow pathway also needs to be considered
and can be compensated by a lower thermal resistance in the heat sink.
Advanced air cooling options utilize forced air cooling with air delivered over
the finned heat sink using a fan. As the heat flux (heat dissipation per unit heat sink
footprint area) increases, higher velocities may be needed. Utilizing ducted air inlets
and outlets reduces the noise level as well as the pressure losses at the entrance
and exit from the heat sink. Employing compact heat exchanger surfaces (originally
developed for automotive radiators) in the heat sink matrix results in significantly
higher heat dissipation rates as the heat transfer surface area is significantly increased
due to the secondary fins. Although the pressure drop and fan power are increased,
significantly higher heat dissipation rates are achieved as compared to the natural
convection cooled heat sinks. A comprehensive coverage of compact heat exchanger
design and performance is given by Kays and London [10].
13.4 Jet Impingement and Spray Cooling
High speed jets from a nozzle impinging on a target surface provide an effective
technique to dissipate high heat fluxes. A single jet provides cooling in the immediate
vicinity of the jet impingement area, while an array of jets could be used to cover a
larger area. Air or liquid can be used as the cooling medium, while liquids are most
suitable for higher heat flux dissipation levels.
Although the jets provide a high heat transfer performance, in general, they require
high pressure drops in the range of 50–500 kPa. Apart from the fabrication and
leakage issues, the high velocity jets may lead to vibrations and erosion of the heat
transfer surfaces.
The jet impingement technique cannot be readily applied to conduction cooled
3D ICs. Placing jets in the interlayers can also lead to very complex fluid distribution
system design from the fabrication perspective. Leakage remains a critical concern
due to high pressures employed in delivering the high velocity jets.
Spray cooling involves spraying liquid droplets from a nozzle on a target surface
where they evaporate. This technique employs evaporation as against jet impingement cooling which is based on either on single-phase heat transfer, or in some cases
forced convection boiling. The target surface may be coated with porous coatings
or microstructures to enhance the evaporation rate. The spray distribution pattern,
the effect of returning vapor on the droplet velocity, and controlling the flow rate
to avoid flooding are some of the issues that require careful design considerations.
Although the pressures required for generating sprays are considerably lower than