PowerPC 740 and PowerPC 750 Microprocessor
Figure 24. Thermalloy #2328B Pin-Fin Heat Sink-to-Ambient Thermal Resistance vs. Air flow Velocity
Thermalloy #2328B Pin-fin Heat Sink
(25 x 28 x 15 mm)
8
7
6
5
4
3
2
1
0
0.5
1
1.5
2
2.5
3
3.5
Approach Air Velocity (m/s)
Assuming an air velocity of 0.5m/s, we have an effective θSA of 7°C/W, thus
TJ = 30°C + 5°C + (.03°C/W +1.0°C/W + 7°C/W) × 4.5W,
resulting in a junction temperature of approximately 71°C which is well within the maximum operating temper-
ature of the component.
Other heat sinks are offered by other manufacturers.
Though the junction-to-ambient and the heat sink-to-ambient thermal resistances are a common figure-of-
merit used for comparing the thermal performance of various microelectronic packaging technologies, one
should exercise caution when only using this metric in determining thermal management because no single
parameter can adequately describe three-dimensional heat flow. The final chip-junction operating tempera-
ture is not only a function of the component-level thermal resistance, but the system-level design and its oper-
ating conditions. In addition to the component's power dissipation, a number of factors affect the final
operating die-junction temperature. These factors might include air flow, board population (local heat flux of
adjacent components), heat sink efficiency, heat sink attach, next-level interconnect technology, system air
temperature rise, etc.
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