PowerPC 740 and PowerPC 750 Microprocessor Datasheet - IBM

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PowerPC 740 and PowerPC 750 MicroprocessorHeat Sink Selection ExampleFor preliminary heat sink sizing, the die-junction temperature can be expressed as follows.T J = T A + T R + (θ JC + θ INT + θ SA ) × P DWhere:T J is the die-junction temperatureT A is the inlet cabinet ambient temperatureT R is the air temperature rise within the system cabinetθ JC is the junction-to-case thermal resistanceθ INT is the thermal resistance of the thermal interface materialθ SA is the heat sink-to-ambient thermal resistanceP D is the power dissipated by the deviceTypical die-junction temperatures (T J ) should be maintained less than the value specified in Table “PackageThermal Characteristics1,” on page 11. The temperature of the air cooling the component greatly dependsupon the ambient inlet air temperature and the air temperature rise within the computer cabinet. An electroniccabinet inlet-air temperature (T A ) may range from 30 to 40°C. The air temperature rise within a cabinet (T R )may be in the range of 5 to 10°C. The thermal resistance of the interface material (θ INT ) is typically about 1°C/W. Assuming a T A of 30°C, a T R of 5°C, a CBGA package θ JC = 0.03, and a power dissipation (P D ) of 5.0 watts,the following expression for T J is obtained.Die-junction temperature: T J = 30°C + 5°C + (0.03°C/W +1.0°C/W + θ SA ) × 5WFor a Thermalloy heat sink #2328B, the heat sink-to-ambient thermal resistance (θ SA ) versus air flow velocityis shown in Figure 24.9/6/2002 Version 2.0 Page 48
PowerPC 740 and PowerPC 750 MicroprocessorFigure 24. Thermalloy #2328B Pin-Fin Heat Sink-to-Ambient Thermal Resistance vs. Air flow VelocityThermalloy #2328B Pin-fin Heat Sink(25 x 28 x 15 mm)8Heat Sink Thermal Resistance (×°C/W)76543210 0.5 1 1.5 2 2.5 3 3.5Approach Air Velocity (m/s)Assuming an air velocity of 0.5m/s, we have an effective θ SA of 7°C/W, thusT J = 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 temperatureof 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-ofmeritused for comparing the thermal performance of various microelectronic packaging technologies, oneshould exercise caution when only using this metric in determining thermal management because no singleparameter can adequately describe three-dimensional heat flow. The final chip-junction operating temperatureis not only a function of the component-level thermal resistance, but the system-level design and its operatingconditions. In addition to the component's power dissipation, a number of factors affect the finaloperating die-junction temperature. These factors might include air flow, board population (local heat flux ofadjacent components), heat sink efficiency, heat sink attach, next-level interconnect technology, system airtemperature rise, etc.Page 49 Version 2.0 9/6/2002
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PowerPC 740 and PowerPC 750 Microprocessor Datasheet - IBM

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