Updated: Mar 9, 2020
Thermal Probe Installation
Before I began my investigation, I had to install a thermal probe above the SoC. I couldn't install it directly on the SoC, so I chose to locate it above the pink thermal pad. SoC = Probe T2
The second probe, Probe T1, was left hanging freely in the air to monitor the ambient temperature, which fluctuates wildly this time of year (spring)... Air = Probe T1.
Upon booting into Android, you could see that the Amlogic S912 SoC was already about 5 degrees celsius hotter than ambient.
Fan-Off, YouTube Playback, Sealed DIY Case
My first test was to keep the fan turned off, and playback some YouTube videos via Google Chrome. Many of our TV Box users like to keep the fan turned off (or sometimes they buy a VIM2 without a fan, with just the heatsink).
Passive cooling is the quietest method of cooling, as it doesn't involve spinning motors, forced convection, and the buffeting of high speed air...
For this test, not only was the fan turned off, but I had completely sealed-up the DIY Case. This replicates the experience of TV box users who are also concerned with having a good-looking, aesthetically pleasing TV box at home.
Temperatures reached 60C (degrees celsius) after 30 minutes of YouTube playback, at an ambient air temperature of 31C. The DIY Case felt warm to the touch, however not alarmingly so. The DIY Case has a hole at the top for warm air to rise through.
Fan-On, YouTube Playback, Sealed DIY Case
Next, I turned the fan on to the lowest speed, with the DIY Case completely sealed. Temperatures cooled down to an acceptable 39C, at an ambient temperature of 30C.
Fan-On, CPU Stress Test, Sealed DIY Case
Logically, the next test was to leave the fan on and run a 8-core CPU stress test, which would be more comprehensive than just watching YouTube videos.
After 30 minutes, temperatures had stabilised around 42.7C, at an ambient temperature of 25C. The DIY Case felt slightly warm at this point, and the quiet hiss of out-rushing air could be heard. It's possible that at 30C, SoC temperatures could be closer to 45C.
Fan-On, CPU Stress Test, Opened DIY Case
Opening the DIY Case lowered the SoC temperature to 38C, with the heatsink temperature at 36C (see picture, where I had stuck the probe into the heatsink). For intensive tasks, you may still want to open up the case. In the next section, let's explore another option.
Fred21's Modification, Fan-On, YouTube Playback
A user on the Khadas Forum, Fred21, recommended placing a piece of stiff paper behind the heatsink, and then leaving a small gap between the top-cover and bottom-case for better ventilation. Here are the results of his modification - Fred21's Forum Post.
With the lid slightly propped up, and the thick paper behind the heatsink, temperatures dropped to about 37C during Android YouTube playback, at an ambient temperature of 27C.
Well done Fred21!
Tone Board, New VIM2, Fan Off, YouTube Playback
Finally, to end-off this blog post, lets get to the thermal-arch-enemy; stacking a Tone Board on top of a VIM2, inside a DIY Case, with the fan turned off, whilst playing YouTube videos.
After about 30 minutes of playback, temperatures stabilised at 62C, at an ambient of 28C. It's likely that with other ROMs (e.g. CoreELEC, Volumio), you could see temperatures stabilise at 70C. With the fan-on, temperatures dipped back down to the high 40s.
In conclusion, with the Tone Board mounted, you're better off keeping the 3705 Cooling Fan set to speed 1 (lowest setting), to keep everything reasonably cool with the least noise. And maybe add a sheet of paper behind the heatsink to let the hot-air out quicker too (Fred21's Modification).
Quick Video Summary
YouTube video of a quick 1-minute summary of the different testing phases, which you can also watch on your VIM2. Pardon the 1080P resolution, I did not have my 4K recording gear with me.
Thanks for reading! Khadas Team