I haven't watched the video yet, but my question is wouldn't it make more sense to add the heatsink later to show how well it cools it off? As I understand it heat will continue to build up under load, so...
It wouldn't cool off. Temperature would cool down for a few seconds until the heatsink is warm as well.
A heatsink spreads the heat, it adds more surface where the heat can spread, so you won't have hot spots.
Temperature would rise slower, until the surroundings are all warmed up.
See this as a room with closed windows and a heater:
The room is the heatsink, the heater the SoC, the window a fan.
The smaller the room will need about 5 minutes to heat up.
The larger room needs about 10 minutes to heat up.
But they won't cool off until someone opens the window (= switches the fan on).
As a heatsink is exposed to fresh air, it cools down a tiny bit - but not so much to in a mobile phone sized case that it wouldn't step down after a while.
You'd need a fan to actively replace the warmed air with fresh one.
So the only REAL test would be:
1. Run the test with a heatsink on.
2. Let the board cool down completely and redo the same thing without heatsink.
Removing / adding the heatsink WHILE the board is running is no proper test.
Wow, scary. My 150W GTX560 GFX card heats up to ~75°C max at full load, pretty warm but nothing to worry about, even in a Mini-ITX case. But 78°C into an tiny, passive cooled Handheld device? Thats just insane.
That's nothing scary, it's the same with the Pyra and all other modern SoCs.
They are not meant to run with full strength for a longer period of time.
Also, keep in mind that the GPU Temperature sensor is INTERNAL. The real temperature of the SoC will be lower.
