AN3232

Heatsink selection

Doc ID 17594 Rev 3

9/26

3

Heatsink selection

The critical choice of a heatsinking system depends on the specific device and application

requirements. The decision results in the total power dissipation capability for the amplifier

assembly and greatly influences system performance and cost.

When the operating conditions of the device are such that the dissipated power is low, a

basic thermal core layer without fins may be all that is required for maintaining a low case

temperature, a high MTTF, or adherence to other maximum device ratings. Examples of

applications with low average power dissipation can include low duty-cycle pulsed

amplification or backed-off linear power amplifiers.

Most often, RF power devices are operated at the highest power levels possible, to extract

the most value for the cost. Combining the exceptional performance of recent silicon

improvements, having much higher power efficiency than in previous years, with the thermal

performances of STAC packages, allows the possibility for designers to specify relatively

simple systems like low-cost, extruded aluminum heatsinks with fins to provide sufficient

cooling. Such systems have good thermal and electrical properties for a majority of

applications and operating conditions.

One benefit of STAC packages is to enable designers to “push the envelope” in terms of

power density. First, high thermal flux is managed using a heat spreading copper core layer

between the transistor and a high performance heatsink. Second, for the most demanding

applications, system such as water cooling, heat pipes, and others can be coupled to the

first, affording a means of transferring heat very quickly to a more convenient location.

For a high performance package such as STAC, the preferred heatsink material interposed

between the package base and the heatsink is (UNS C10100) copper. Hereafter this

material layer is referred to as the core. Based on requirements for typical applications, the

thickness range for the core layer is listed in Table 1.

Other copper grades and even aluminum may be used for the core layer based on

application needs. In these cases, it is advised to adjust core thickness appropriately to

control spreading of the heat.

Table 1.

Preferred copper core thickness

Minimum

Typical

Maximum

0.12 in (3.0 mm)

0.22 in (5.6 mm)

0.32 in (8.1 mm)