Table of Contents
Advertisement
Thermal Metrology
For active heatsinks, it is important to avoid taking measurement in the dead flow
zone that usually develops above the fan hub and hub spokes. Measurements should
be taken at four different locations uniformly placed at the center of the annulus
formed by the fan hub and the fan housing to evaluate the uniformity of the air
temperature at the fan inlet. The thermocouples should be placed approximately
3 mm to 8 mm [0.1 to 0.3 in] above the fan hub vertically and halfway between the
fan hub and the fan housing horizontally as shown in the ATX heatsink in Figure 3-2
(avoiding the hub spokes). Using an open bench to characterize an active heatsink can
be useful, and usually ensures more uniform temperatures at the fan inlet. However,
additional tests that include a solid barrier above the test motherboard surface can
help evaluate the potential impact of the chassis. This barrier is typically clear
Plexiglas*, extending at least 100 mm [4 in] in all directions beyond the edge of the
thermal solution. Typical distance from the motherboard to the barrier is 81 mm
[3.2 in]. For even more realistic airflow, the motherboard should be populated with
significant elements like memory cards, graphic card, and chipset heatsink. If a barrier
is used, the thermocouple can be taped directly to the barrier with a clear tape at the
horizontal location as previously described, half way between the fan hub and the fan
housing. If a variable speed fan is used, it may be useful to add a thermocouple taped
to the barrier above the location of the temperature sensor used by the fan to check
its speed setting against air temperature. When measuring T
in a chassis with a live
A
motherboard, add-in cards, and other system components, it is likely that the T
A
measurements will reveal a highly non-uniform temperature distribution across the
inlet fan section.
For passive heatsinks, thermocouples should be placed approximately 13 mm to
25 mm [0.5 to 1.0 in] away from processor and heatsink as shown in Figure 3-3. The
thermocouples should be placed approximately 51 mm [2.0 in] above the baseboard.
This placement guideline is meant to minimize the effect of localized hot spots from
baseboard components.
Note: Testing an active heatsink with a variable speed fan can be done in a thermal chamber
to capture the worst-case thermal environment scenarios. Otherwise, when doing a
bench top test at room temperature, the fan regulation prevents the heatsink from
operating at its maximum capability. To characterize the heatsink capability in the
worst-case environment in these conditions, it is then necessary to disable the fan
regulation and power the fan directly, based on guidance from the fan supplier.
28
Thermal and Mechanical Design Guidelines
Advertisement
Table of Contents
