Handbook for Sound Engineers

Heatsinks and Relays 369

Table 13-2 summarizes the thermal solution benefits
possible with the proper application of new technolo-
gies, materials and fabrication processes.

Extruded heatsinks have fin thicknesses that are
much greater, thicker than required thermally. They are
thicker to accommodate the strength requirements of the
die, which is close to the melting point of aluminum
during the extrusion process.

Bonded fin and folded fin heatsink designs use sheet
stock for the fins so that they may be optimally sized as
required to carry the thermal load without regard to the
mechanical requirements of the extrusion process.
These heatsinks can, therefore, without compromising
the required open fin spacing, have a greater number of
fins and be convectively much more volumetrically
effective. These sheet metal fins are attached to the
heatsink bases with either thermal epoxy adhesives or
solders. Since this joint only represents ~3 % of the total
thermal resistance of the heatsink, the adhesive choice
is never critical.

Air flow management is the most critical parameter
to control in optimizing the convective heat transfer for
any thermal solution. Baffles, shrouds, and fan sizing
are all very critical in making the most of the convective
portion of the heat transfer thermal solution. Some
months ago we were confronted with an audio amplifier
that had two rows of very hot components. With two
facing extrusions that formed a box shape, we mounted
a fan at the end and blew the air down the chute with

great success. The air flow was fully contained and no

leakage occurred. And so the audio cooling tube was

born.

There should always be a space—0.5–0.8 inches

along the axis of the fan—between the fan outlet and

the fin set that is fully shrouded to force the air to pass

over the convective fin surfaces. This is called the

plenum. Its function is to allow the upstream pressure

generated by the fan to reach an equilibrium and thereby

equalize the air flow through each fin opening.

Audio systems that require fans need to be carefully

designed to have an air flow path that is well defined so

that the fan may be operated at a minimal speed. This

results in the fan generating a minimum of noise.

High-velocity fans are noisy. Noise abatement is very

expensive and seldom truly satisfactory, therefore, the

best solution is to minimize the fan generated noise.

13.1.3 How Heatsinks Work

by Glen Ballou

Heatsinks are used to remove heat from a device, often

the semiconductor junction. To remove heat, there must

be a temperature differential ('T) between the junction

and the air. For this reason, heat removal is always after

the fact. Unfortunately, there is also resistance to heat

transfer between the junction and its case, any insu-

lating material, the heatsink, and the air, Fig. 13-1.

Table 13-2. Thermal Solutions with New Technology, Materials, and Fabrication Processes

Technology/

Material/

Fabrication

Title Volumetric Ratio

Range

Cost Range Comments

M Copper C110 0.8 3.5 × Volumetric ratios are even lower for conduction=limited

applications Weight almost triples (3 ×)

MF Molded Plastic Conductive

Dielectric Elastomeric

0.97 (<200 LFM)

1.03 (>200 LFM)

1.07 (>500 LFM)

0.5–0.7 After

tooling if not

a standard

Saves weight and finishing^3

Hybrids—molded base metal fins

TM Base-Mounted Heat Pipes^3 1 Heat Pipe 0.79

2 Heat Pipe 0.73

1.5–2.0 Aluminum base

TM Base-Mounted Heat Pipes^3 1 Heat Pipe 0.71

2 Heat Pipe 0.66

4.0–4.5 Copper base

TM Base-Mounted Vapor

Chamber^3

0.69 Al

0.58 Cu

2.5 Al

4.8 Cu

Achieves optimum spreading

M Graphite 0.72 6–8 × Relatively fragile

35% reduction in weight

TM Solid-State Heat Pipes

(TPG)^4

0.75 3.6 Al

5.4 Cu

Eliminates burnout as a failure mode

FM Bonded Fin and Folded Fin 0.90 Al

0.76 Cu

2×

3.6 ×

More convective fin surface per unit volume fin shapes

break up boundary film layers for performance gains