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27

Chip Scale Review November • December • 2018

[ChipScaleReview.com]

package level, many bare die cooling

solutions have been proposed, mainly

ba s ed on expen sive Si p r oce s si ng

techniques. Imec presents a low-cost

but highly efficient alternative based

on 3D-printing techniques. While this

technique cannot attain the resolution of

Si nozzles, this might not be necessary.

It is true that thermal performance of

smaller apertures is higher, the need

for a higher pump pressure to guide the

cooling fluid through the nozzles, makes

the overall yield limited. Moreover,

imec’s simulations and measurements

demonstrate that a similar performance

can be achieved with mm-size nozzles at

reduced costs.

How many showerheads to cool

a chip?

W i t h a u n i t c e l l m o d e l i m e c

researchers assessed the optimal number

of nozzles, nozzle pitch and diameter

for an 8x8mm

2

test chip. A part of the

nozzle a r ray wit h one i n let sha red

by 4 outlets is considered a unit cell.

The simulations show that the more

nozzles are introduced, the more the

temperature decreases and the more

uniform the temperature profile. And

the more nozzles you need, the smaller

they become for a given chip area. A

higher number of small diameter nozzles

can indeed achieve a better thermal

performance – or decrease in thermal

resistance – but comes at the expense

of the required pump power. Similarly,

increasing the flow rate can result in a

further increase in thermal performance,

but again requires a higher pressure.

Ta k i ng i nt o a c cou nt t h i s t r a deof f

between thermal performance in terms

of thermal resistance and pump power,

the simulations show that it becomes less

energy efficient to increase the number

of nozzles beyond an 8 by 8 array of inlet

nozzles on the chip surface (for a fixed

200µm cavity height) or in other words,

1 nozzle per mm

2

(

Figure 4a

). Moreover,

for a 1mm

2

unit cell the nozzle diameter

at the saturated performance is then on

the order of several hundreds of µms.

A cooler material

The choice of the number of nozzles

and the nozzle diameter impacts the

required fabrication technology, with

finer nozzle diameters requiring more

expensive processing options. For the

8x8mm

2

test chip and accompanying

nozzle d iameters, 3D-pr i nt i ng is a

v iable opt ion (

F i gure 4b

). Recent

advancements in the technique made

it possible to pr i nt st r uct u res with

d iamet e r s r a ng i ng f rom 100µm t o

1mm. S i l i c on p r o c e s s i ng h a s t h e

a d v a n t a g e t h a t i t c a n b e u s e d t o

fabricate small diameter holes below

10μm with deep reactive ion etching

(DRIE) technology. However, the cost

of silicon processing is higher than the

other fabrication methods. Besides, the

simulations demonstrated that further

downscaling of the nozzle diameter is

not necessary due to the saturation in

thermal performance.

Polymer fabrication may be cheaper

t han Si processi ng techn iques, but

does it meet the same performance

standards? After all, polymers have

a much lower thermal conductivit y

compared to Si, resulting in a higher

t h e r m a l r e s i s t a n c e f o r t h e r m a l

conduction. Therefore, good thermal

c ond u c t o r s s u ch a s Cu o r A l a r e

typically used for heat sinks as opposed

t o i n s u l a t o r s , s u c h a s p o l yme r s .

To st udy t he impa c t of t he coole r

mat e r ia l on t he ch ip t empe r at u r e,

imec researchers used a model of the

full cooler that shows the thermal and

hydraulic interaction of the different

nozzles and the impact of the thermal

conductivit y of the cooler mater ial

(

Figure 5

). The results demonstrate

that a polymer cooler has the same

pe r formance a s a Si or Cu coole r,

because the heat removal is in this case

dominated by forced convection in

the coolant. This opens opportunities

for the use of polymer-based low-cost

fabrication options.

The power of printing

A s i d e f r o m t h e p r i c e t a g ,

3D-printing offers several advantages

f o r t h e f a b r i c a t i o n o f a p o l yme r

l i q u i d j e t i m p i n g e m e n t c o o l e r .

Firstly, the cooler can be printed as

a si ngle piece, while, for example,

mechanical machining still requires

gluing separate parts. Secondly, the

technique allows for the fabrication

o f c omp l e x i n t e r n a l s t r u c t u r e s .

This is especially interesting when

creating the intricate combination of

inlets and outlets to guide the f low

in the cavity. Finally, because of the

Figure 5:

a) (top) Model of the full cooler showing b)

(bottom) the flow field of the coolant.

Figure 4:

a) For a constant pumping power (Wp) thermal performance saturates beyond an 8x8 nozzle array; b)

The resulting nozzle diameters for this size of nozzle array are in the fabrication possibilities of 3D-printing.