Intel
®
Pentium
®
III Processor in the FC-PGA2 Package Thermal Design Guidelines
R
24
249660-001
Equation 2. Case to Ambient Thermal Resistance
θ
CA
=
θ
CS
+
θ
SA
Where:
θ
CA
=
thermal resistance from case to local-ambient (°C/W)
T
CASE
=
processor case temperature (°C)
T
LA
=
local-ambient temperature in chassis around processor (°C)
TDP
=
processor Thermal Design Power dissipation (W) (assume all power goes to the case)
θ
CS
=
case-to-sink thermal resistance (°C/W)
θ
SA
=
sink to local-ambient thermal resistance (°C/W)
7.2.
Thermal Solution Performance
All processor thermal solutions interface to the processor at the IHS. The system thermal solution must
adequately control the local-ambient air around the processor (T
LA
). The lower the thermal resistance
between the processor and the local-ambient air, the more efficient the thermal solution is. The required
θ
CA
is dependent upon the maximum allowed processor temperature (T
CASE
), the local-ambient
temperature (T
LA
) and the processor thermal design power (TDP). For actual specifications on FC-PGA2
packaged processors, refer to the processor datasheet.
Use Equation 1 and Equation 2 (
above
) to determine a target
θ
CA
and
θ
SA
using the following
assumptions.
T
CASE
=
67 °C, hypothetical maximum case temperature specification
T
LA
=
Assume 45°C, a typical value for desktop systems
TDP
=
Assume 30 W, hypothetical thermal design power
θ
CS
=
Assume 0.15 °C/W, hypothetical TIM resistance
Solving for the equation 1 from above:
θ
CA
=
(T
CASE
- T
LA
) / TDP
= (67 – 45) / 30
= 0.73 °C/W
Solving for equation 2 from above:
θ
CA
=
θ
CS
+
θ
SA
θ
SA
=
θ
CA
−
θ
CS
= 0.73
−
0.15
= 0.58 °C/W
