Preliminary
PowerPC 750FL RISC Microprocessor
5.8.1 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
θ
INT 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) must be maintained less than the value specified in Table 3-3 Package
Thermal Characteristics on page 17. 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) can range 30 − 40°C. The air temperature rise within a cabinet (TR) can be in
the range of 5 − 10°C. The thermal resistance of the interface material (θINT) is typically approximately 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) × 5 W
For a Thermalloy heat sink #2328B, the heat sink-to-ambient thermal resistance (θSA) versus air flow velocity
is shown in Figure 5-8 on page 58.
750flds60.fm.6.0
April 27, 2007
System Design Information
Page 57 of 65
IBM [ IBM ]
