Handbook for Sound Engineers

Heatsinks and Relays 367

13.1 Heatsinks

13.1.1 Thermal Management of Today’s Audio
Systems

By Henry Villaume, Villaume Associates, LLC

Today’s audio systems, like all electronic systems are
being powered by smaller devices, packaged in smaller
systems that are generating more heat. We need to
increase our level of understanding on all of the latest
techniques for the management of this added heat in as
effective a means as possible. Let’s first start with the
understanding of the three methods of heat transfer—
specifically, convection, conduction, and radiation as all
three methods of heat transfer contribute to the
complete thermal management provided by the heat-
sinks installed in an audio system.

13.1.1.1 Convection

Convection is the transfer of heat from a solid surface to
the surrounding gas, which is always air in the case of a
typical audio system. This method of heat transfer
drives the amount of required fin surface area that is
accessible to the surrounding air so that it may heat up
the surrounding air, allow it to move away, and make
room for the process to repeat itself. This process can be
greatly accelerated with the use of a fan to provide more
energy to the moving of the air than just the natural
buoyant force of the heated air.

Natural convection is when there is no external fan
and the heat transfer occurs with very low air flow rates,
typically as low as 35 linear feet per minute (lfm) for
obstructed natural convection to 75 lfm for optimum
unobstructed vertical natural convection. Natural
convection is never zero air flow rate because without
air movement there would be no heat transfer. Think of
the closed cell plastic foam insulation. It works as an
insulator because the closed cell prevents the air from
moving away.

Forced convection is when a system fan imparts a
velocity to the air surrounding the heatsink fins. The fan
may be physically attached to the convective fin surface
area of the heatsink to increase the air velocity over the
fin surfaces. There is impingement flow—fan blows
down from on top of the fins—and through flow—fan
blows from the side across the fin set.

Forced convection thermal systems are most gener-
ally significantly smaller (50% or more) than their
natural convection equivalents. The penalties for the

smaller size are the added power to operate the fan, an

added failure mechanism, the added cost, and the noise

from the fan. Fan noise is probably the most important

consideration when applying them in audio systems.

13.1.1.2 Conduction

Conduction is the transfer of heat from one solid to the

next adjacent solid. The amount and thermal gradient of

heat transfer are dependent on the surface finishes

—flatness and roughness—and the interfacial pressure

generated by the attachment system. This mechanically

generated force is accomplished by screws, springs,

snap assemblies, etc. The thermal effectiveness of a

conductive interface is measured by the resultant

temperature gradient in °C. This may be calculated from

the interface thermal resistance at the mounted pressure

times the watts of energy moving across the joint

divided by the cross-sectional area. These temperature

gradients are most significant for high wattage compo-

nents in small packages—divisors less than 1.0 are, in

actual effect, multipliers. Good thermal solutions have

attachment systems that generate pressures of

25–50 psi.

Table 13-1 compares the thermal performance of

most of the common interface material groups with a

dry joint—this makes amply clear why it is never

acceptable to specify or default through design inaction

to a dry joint.

Table 13-1. Thermal Performance of Common

Interface Materials

Interface Material

Group

Thermal

Performance

Comments

Range in

°C in²/W

Dry Mating Sur-

faces

3.0-12.0 Too much uncertainty to use.

Too big a thermal gradient

Gap Fillers 0.4-4.0 Minimize thickness required

Spring mechanical load

Electrically

Insulating

0.2- 1.5 Maximize mechanical loads

High Performance

Pads

0.09-0.35 Minimize thickness

Maximize mechanical loads

Phase Change Pads 0.02-0.14 Must follow application

method

Spring mechanical load

Low Performance

Grease

0.04-0.16 Screen apply

Spring mechanical load

High Performance

Grease

0.009-0.04 Must Screen apply

Spring mechanical load

Best at high loads(> 50 psi)