Digital Equipment AXPpci 33, Design Manual

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Environmental Data
Table 3–13 DECchip 21068A (100 MHz) Cooling Using Fan/Heatsink
Fan
Voltage AXPpci 33
CPU Power
T c
Max Theta jc Theta ja Theta ca
Maximum
Ambient
Temperature Notes
12 V 10 W 93°C 0.70°C/W 2.10°C/W 1.30°C/W 80°C 1, 2
Using part number 12–35739–34 for heatsink and specified fan.
Notes:
1 If the recommended thermal pad (thermal interface between the 21068A and heatsink) is not used, the Theta jc
and Theta ja values will increase by 0.45°C/W. The end result will be a decrease in maximum allowable ambient
temperature by 5°C.
2 Operation above 40°C ambient temperature will require further thermal analysis by the system integrator. This
ensures that maximum operating temperatures are not exceeded for any other device including storage devices,
power supplies, PCI or ISA option modules, or other chips on the MLB.
3.1.2.5 Custom Heatsink Design
If you use alternative heatsinks, use this formula to determine the
maximum ambient air temperature for the DECchip 21066:
Tambient = Tc - [Power (Interface Thermal Resi Heatsink Thermal Resistance)]
where the variables have these meanings:
T
c
Maximum device case temperatures based on
processor type:
For DECchip 21068–66, 93°C.
For DECchip 21068A–100, 93°C.
For DECchip 21066–166, 85°C
For DECchip 21066A–233, 84°C
Power
Worst-case values based on processor type:
For DECchip 21068–66, 9 W.
For DECchip 21068A–100, 10 W.
For DECchip 21066–166, 21 W.
For DECchip 21066A–233, 23 W.
Interface Thermal Resistance °C/W between the DECchip 21066 and the
heatsink.
Heatsink Thermal Resistance
°C/W between the heatsink and ambient air
flow at some known velocity.
When using other heatsink designs, consider:
• If the custom heatsink footprint is larger than the DECchip 21066
PGA chip, interference may occur with side 2 of the ISA module in slot
2 (J23).
• If the custom heatsink footprint is larger than the DECchip 21066
PGA chip, the length of the ISA modules in slots 4 and 5 (J21 and J20)
may be reduced.
• Operation of the ZIF socket lever is inhibited with a larger heatsink,
requiring that the heatsink be installed after the DECchip 21066
processor is installed into the ZIF socket. This practice is not
recommended.
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