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26

Chip Scale Review November • December • 2018

[ChipScaleReview.com]

A cold shower for chips

By Herman Oprins, Tiwei Wei, Vladimir Cherman, Eric Beyne

[imec]

ooling the elect ronics in

large data centers comes

wit h a he av y p r ic e t ag.

It is e st imat ed t hat almost hal f of

the electricity bill for such centers is

dedicated to powering the ventilators for

air cooling. Moreover, with increasing

power-intensive applications and server

densities, air cooling capacity is quickly

reaching its limits and server rooms are

running out of space for the large fans.

Liquid coolants are gaining in importance

as a more economically viable alternative

to meet future cooling and footprint

demands. In order to cool down 100W,

a fan and heat sink combination of

several 100cm

3

is needed, compared to

a liquid cooling unit that is a factor of

1000 times smaller (

Figure 1

). Because

the cooler has similar dimensions to the

chip package, it can also be placed in

close proximity to the heat source, which

allows for packing more chips in a rack,

thereby increasing compute density.

Many large dat a centers are now

making the jump f rom air to liquid

cooling. While the transition already

means a signif icant improvement in

cooling efficiency, there is still a lot to

gain in the specific design of the cooler.

The most common method to achieve

liquid cooling is through the use of

“cold plates.” These metal plates with

flow paths guide liquids and are directly

glued onto the chip. Drawbacks of this

technique are the presence of the thermal

interface material (TIM), creating a fixed

thermal resistance, and the formation of

a temperature gradient across the chip

surface. Especially the layer of glue

presents a real thermal bottleneck for the

heat removal that cannot be addressed by

further optimization of the cooler itself.

Liquid jet impingement cooling might be

a solution. By opening up the backside

of the chip and allowing direct contact

between the liquid coolant and chip

(

Figure 2

), the TIM thermal resistance is

avoided. Moreover, vertical impingement

ensures that all liquid that hits the chip

surface has the same inlet temperature.

Liquid economy

Imec’s jet impingement cooler directly

cools the backside of high-performance

chips or chip stacks, but is fabricated

using low-cost polymer fabr ication

techniques. The key part of the system

is an ar ray of nozzles that acts as a

“showerhead” for each chip (

Figure

3

). The nozzle array is made up of the

distributed inlets and outlets on the

nozzle plate. The inlet tubes in the inlet

plenum feed the liquid into the individual

inlet nozzles, while the outlet tubes in

the layer above collect the outflow. The

interaction between inlet and outlet flows

happens in the cavity region where the

fluid is in contact with the chip.

Impingement cooling solutions have

already been successfully implemented

f o r l a r g e (30 -50 cm) mo d u l e s f o r

p owe r e l e c t r o n i c s t h a t g e n e r a t e

kilowatts. For the integration to chip or

C

Figure 1:

a) (left) Air cooling installations have a larger footprint than liquid alternatives; b) (middle) Micro-machined

liquid coolers and especially c) (right) the current 3D-printed liquid coolers can be up to an order of magnitude smaller.

Figure 3:

a) (top) Cross section of the 3D-printed

cooler; b) (middle) Top view of the nozzle plate with

distribution of NxN inlets and outlets; and c) (bottom)

3D-printed model of the internal structures.

Figure 2:

a) (top) The liquid jet impingement cooler is

attached to the chip; b) (bottom) The nozzle array sprays

cooling fluid directly onto the open backside of the chip.