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Chip Scale Review November • December • 2018

[ChipScaleReview.com]

Air and liquid cooling solutions advance to meet

thermal needs of ICs

By Josh Perry

[Advanced Thermal Solutions, Inc.]

copper or aluminum and consisting of

a base and fins that extend outward,

which have been essential to the thermal

management of integ rated circuits.

Conduction, convection, and radiation

heat transfer are all at work when using

a heat sink. The heat spreads into the

fins and the flow of air causes a change

of enthalpy across the heat sink surface

to remove the heat.

Temperat u re dist r ibut ion and ai r

distribution in any electronic system

are temporal and spatial. Even between

cards in a telecom chassis you will see

a significant temperature gradient from

the top to the bottom. It is important to

remember that when heat is generated

it goes in every direction, including

into the board upon which the device is

sitting. Because heat is going in every

direction, air management must also

be considered. Heat will be carried by

the airflow to other devices, which will

impact their junction temperatures.

D i f f e r e n t h e a t s i n k ge ome t r i e s

produce different heat transfer rates.

Standard geometries include pin f in

or st raight-f in designs, while more

intricate spread fin or wavy fin arrays

increase the surface area of the heat sink

and improve its thermal performance

(

Figure 2

). Engineers will also need to

optimize spreading resistance from the

component to the heat sink, and in many

cases this can be solved by increasing

the thickness of the base.

nteg rated ci rcuits (ICs) have

revolutionized electronics since

the first one was designed by Texas

Instruments in 1958 and they are now used

in everyday electronics such as computers,

mobile phones, and home appliances. Over

the decades, as semiconductor technology

has advanced, ICs have become smaller,

faster, and more powerful with billions

of transistors in a space no larger than

a fingernail. Despite enhancements in

fabrication techniques and advanced

materials, one thing has remained constant

throughout the years: thermal management

is critical to ensure maximum performance

over the full lifetime of the chip, to avoid

errors, and to optimize mean time between

failures (MTBF).

Heat is a menace for elect ronics,

ir respective of the market sector. It

doesn’t matter what the final product is,

heat must be managed to ensure proper

performance. To maintain operation, the

heat must flow out of a semiconductor

at such a rate as to ensure acceptable

junction temperatures. This heat f low

encounters resistance as it moves from the

junction throughout the device package,

much like electrons face resistance when

flowing through a wire.

In thermodynamic terms, the resistance

described above is known as conduction

resistance and consists of several parts.

From the junction, heat can flow toward

the case of the component, where a heat

sink may be located. This is referred

to as Î

JC

, or junction to case thermal

resistance. Heat can also flow away from

the top surface of the component and

into the board. This is known as junction

to board resistance, or Θ

JB

. Because of

the multiple heat transfer paths within a

component, a single resistance cannot be

used to accurately calculate the junction

temperature. The thermal resistance from

junction to ambient must be broken down

further into a network of resistances

to improve the accuracy of junction

temperature prediction.

As board layouts become denser,

there is a need to design optimized

thermal solutions that use the least

amount of space possible. Simply put,

there is no margin to allow for over-

designed heat sinks with tight component

spacing. When there is more than one

component , t he sit uat ion become s

much more complex than with just a

single component on the board. There is

conduction coupling between components

through the printed circuit board (PCB),

and radiation and convective coupling

between the components and adjacent

cards (

Figure 1

).

Nume r ou s s o l u t i o n s h ave b e e n

designed th rough the years to deal

with excessive heat from increasingly

powerful chips, but air cooling remains

the standard because of cost and ease

of implement at ion. Liqu id cooli ng

solutions are gaining in popularity as

heat dissipation requirements exceed the

capability of air to manage and there are

new techniques, such as direct-to-chip

cooling, which are being researched, but

only infrequently implemented.

Air cooling

Although liquid cooling has been

gai n i ng moment um, ai r cool i ng is

still the first solution for cooling high-

powered chips. A heat sink is a passive

heat exchanger, typically composed of

I

Figure 1:

Heat sinks dissipate thermal energy from

ICs through convection, conduction, and radiation

heat transfer.

Figure 2:

Pin fin heat sinks are a common air cooling

solution in systems with high or ambiguous airflow.