PowerPC 740 and PowerPC 750 Microprocessor
Heat Sink Selection Example
For preliminary heat sink sizing, the die-junction temperature can be expressed as follows.
TJ = TA + TR + (θJC + θINT + θSA) × PD
Where:
TJ is the die-junction temperature
TA is the inlet cabinet ambient temperature
TR is the air temperature rise within the system cabinet
θJC is the junction-to-case thermal resistance
θINT is the thermal resistance of the thermal interface material
θSA is the heat sink-to-ambient thermal resistance
PD is the power dissipated by the device
Typical die-junction temperatures (TJ) should be maintained less than the value specified in Table “Package
Thermal Characteristics1,” on page 11. The temperature of the air cooling the component greatly depends
upon the ambient inlet air temperature and the air temperature rise within the computer cabinet. An electronic
cabinet inlet-air temperature (TA) may range from 30 to 40°C. The air temperature rise within a cabinet (TR)
may be in the range of 5 to 10°C. The thermal resistance of the interface material (θINT) is typically about 1°C/
W. Assuming a TA of 30°C, a TR of 5°C, a CBGA package θJC = 0.03, and a power dissipation (PD) of 5.0 watts,
the following expression for TJ is obtained.
Die-junction temperature: TJ = 30°C + 5°C + (0.03°C/W +1.0°C/W + θSA) × 5W
For a Thermalloy heat sink #2328B, the heat sink-to-ambient thermal resistance (θSA) versus air flow velocity
is shown in Figure 24.
9/6/2002
Version 2.0
Page 48
IBM [ IBM ]
