Handbook for Sound Engineers

Heatsinks and Relays 371

• Be made of a high thermal conductivity material.


• Have material of high emissivity (painted aluminum
  or copper).


• Have proper ventilation and location (should be
  below, not above, other heat radiators).


• Be placed so that the lowest power device is below
  the higher power devices, and all devices should be
  as low as possible on the heatsink.

The overall effectiveness of a heatsink is dependent
to a great extent on the intimacy of the contact between
the device to be cooled and the surface of the heatsink.
Intimacy between these two is a function of the degree
of conformity between the two surfaces and the amount
of pressure that holds them together. The application of
a silicone oil to the two surfaces will help to minimize
air gaps between the surfaces, improving conduction.
The use of a mica washer between the base of the
device to be cooled and the heatsink will add as much as
0.5°C/W to the thermal resistance of the combination.
Therefore, it is recommended that (whenever possible)
an insulating washer be used to insulate the entire heat-
sink from the chassis to which it is to be mounted. This
permits the solid-state device to be mounted directly to
the surface of the hea tsink (without the mica washer).
In this way, the thermal resistance of the mica washer is
avoided.

Today high thermal conductive/high electrical insu-
lation materials are available to electrically insulate the
transistor case from the heatsink. They come in the form
of silicon rubber insulators, hard-coat-anodized finish
aluminum wafers, and wafers with a high beryllium
content.

A typical heatsink is shown in Fig. 13-3. This sink
has 165 in^2 of radiating surface. The graph in Fig. 13-4
shows the thermal characteristics of a heatsink with a
transistor mounted directly on its surface. A silicone oil
is used to increase the heat transfer. This graph was
made with the heatsink fins in a vertical plane, with air
flowing from convection only. Fig. 13-5 shows the
effect of thermal resistance with forced air blown along
the length of the fin.

A transistor mounting kit is shown in Fig. 13-6.
Several different types of silicon fluids are available to
improve heat transfer from the device to the heatsink.
The fluid is applied between the base of the transistor
and the surface of the heatsink or, if the transistor is
insulated from the heatsink, between the base and the
mica washer and the mica washer and the heatsink. For
diodes pressed into a heatsink, the silicone fluid is
applied to the surface of the diode case before pressing

it into the heatsink. The purpose of the silicone fluid is

to provide good heat transfer by eliminating air gaps.

Thermally conductive adhesives can also be used.

These adhesives offer a high heat transfer, low

shrinkage, and a coefficient of thermal expansion

comparable to copper and aluminum.

The thermal capacity of a cooling fin or heatsink

must be large compared to the thermal capacity of the

Figure 13-3. Typical heatsink for mounting two transistors.

Courtesy Delco Electronics Corp.

Figure 13-4. Thermal characteristics for the heatsink shown

in Fig. 13-3, with forced air cooling. Courtesy of Delco Elec-

tronics Corp.

Figure 13-5. Thermal characteristics for the heatsink shown

in Fig. 13-3, with forced air cooling. Courtesy of Delco Elec-

tronics Corp.

Cooling fins

4.625

4.690

1.335

3.000

3.125

1.370

Chassis insulating

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Temperature differential—°C(mounting stud to ambient air)

Power dissipation—W

40

20

0

0 5 10 15 20 25

60

2.0

1.5

1.0

50 100 150 200

20 W constant dissipation

Thermal resistance—°C/W

Velocity of air flow—ft/min