OK, I wrote a little script to stress the CPU, and at the same time to display the core speeds and temps.
stress -c 4 -i 4 -m 2 &
sudo watch -n 1 \
cat /sys/devices/system/cpu/cpu*/cpufreq/cpuinfo_cur_freq /sys/devices/virtual/thermal/thermal_zone*/temp
With the bare board sitting on my desk it runs about 800MHz or slower at 78 or 79 degrees. I have a 8GB swap area mounted on the SDCard so this will affect performance of stress compared to a board with no swap area.
Note: the board I am using is latest production and it has an EMI shield over the CPU so it is not easy to connect a heatsink directly to the CPU. Connecting to the EMI shield won’t do a lot of good because of the small air gap between the EMI shield and the CPU. Even without the EMI shield, adding a heatsink on the top of the board will block attachment of mezzanine cards. Fortunately the 410c board was designed to allow attachment of a heatsink even with the EMI shield in place.
Look at the bottom of the 410c board you will see a gold square underneath the CPU. This is intended for the attachment of the heatsink. It makes very good thermal contact to the CPU chip through the copper layers inside the board, and the 760 balls under the 410 chip.
You will need a 25x25mm x 5mm thick thermal conductive pad, (Digikey part number 1168-1557-ND), four 4mm tall standoffs M3 (Digikey part number 952-2006-ND), four 4-40 screws about 1/2" long (Digikey part number H843-ND self-tapping, or you can use regular 4-40 screws if you have a machine tap handy), a drill with a 3/32" drill bit, and a heatsink ( http://www.heatsinkusa.com/2-079/ 2.079" Wide Extruded Aluminum Heatsink. Choose options, select 4" long, then in the special instructions box ask for it to be cut to 3.3 inches long).
Place the 410c board on top of the heatsink, mark the locations of the 4 mounting holes, drill all four holes 3/32" diameter, tap the holes by driving in the screws (I actually used a machine tap for this operation, I didn’t try it with the self tapping screws). Place the thermal pad onto the 410c over the gold rectangle, place the 4 standoffs onto the heatsink over each tapped hole, place the 410c over the standoffs, and install the four screws. Since the standoffs are 4mm tall, and the thermal pad is 5mm tall it will get crushed 1mm and make good thermal contact.
Results: the bare board runs at 800Mhz or lower without the heatsink, and at 1.1GHz with the heatsink (no fan). Your results will vary depending on the ambient air temperature and the local air currents. Your actual compute load will make even more temperature-speed differences.
Full Disclosure: I am an employee of Qualcomm Canada and any opinions expressed in this or any other post do not necessarily reflect the opinions of my employer.
