[General Topics] wwortel:
Improving CPU cooling revisited

Inspired by the post of aveemashfaq, post #47, I decided to open my fairly new Hi10 Air (prod. Aug 2019) to study and possibly improve the CPU/SoC' cooling.

Alert, Alert: opening the tablet has a significant risk of cracking the touch screen. A replacement touch screen would cost around 20 EUR but while this would work in older Hi10 versions, the newer ones have the touch screen directly laminated onto the LCD screen so as to reduce reflections and improve contrast. Indeed the optical quality of the screen is superb.

But separating the two screens without damaging the LCD is a completely different story. So, only do this _improved cooling_ exercise with extreme care not to crack the screen and when you deem the cooling insufficient!

How I did the opening: Start with removing the 4 little screws (torx 5). In landscape orientation separate carefully the top, starting from the corners, and then the sides of the black plastic frame from the aluminium cover. I used some old credit cards as tool. Keep the separation minimal as to bend the screen as little as possible. The back cover and screen seen from the side now form a V shape with very small opening at the top. Then release the bottom side by pulling the screen upwards, that is almost in the plane of the back cover. The USB keyboard connector in the bottom center prevents pulling the screen perpendicular from the back cover. That is why you need to pull upwards after separating the other 3 sides. When closing the system hold it upside down so the power and volume button insert stays in its position. Shift what is the bottom side upwards into its position and then compress all sides. Before handling the CPU cover, pull the battery connector and do not forget to connect it again before closing the tablet.

This is what I did to improve CPU cooling:

I.s.o. applying an heat spreader glued onto the CPU, I decided to use the original thin sheet metal cover of the CPU/Memory area to fixate a heat spreader but to also maintain electrical shielding and not to create an additional current path from the SoC substrate to ground. That area carries many radio frequency signals and the sheet metal basically keeps the CPU inside a Faraday Cage. The inside of the cover has from factory a thin insulating foil layer. Measuring the thickness of the little slab of heat conducting material that Chuwi puts there I concluded that a piece of 20 x 18 x 1 mm3 of copper or brass would replace it fine. The larger slab of heat conducting material on the outside of the sheet metal cover according to my measurements does not, or barely, touch the aluminium back cover. So I have put another piece of 20 x 18 x 1 mm3 copper on the outside of the sheet metal cover to secure thermal contact to the back cover. And on top of that the larger original slab of whitish heat conducting material. Between the inside copper and the CPU I applied a bit of silver based heat conduction paste as used in any PC. With the chosen dimensions the rest of the sheet metal cover just slightly presses the inside copper spreader onto the CPU, acting a minimal bit as a leaf spring and of course fixating it laterally. The two copper heat spreaders I glued onto opposite sides of the sheet metal cover using a bit of 2 component glue and a clamp to press them against the sheet metal while the glue hardened. Of course they are centered on the position of the CPU. See the photos.

The result is a sandwich of layers with most of the distance in copper:

SOC silicon substrate

heat conductive paste (very thin)

copper spreader (1 mm)

2 component glue (very thin)

original insulating film (very thin)

original sheet metal cover (thin)

2 component glue (very thin)

copper spreader (1 mm)

original heat conductive slab

aluminium back cover

Use the lm-sensors package and 'sensors' command to see CPU core temperatures. Here I cannot get the four z8350 cores above 65 C under full load using a stress test programme while the tablet is still open but with the copper spreaders. After closing the tablet, ambient temp ~22 C, and the command:

stress --cpu 8 --vm 8 --vm-bytes 1GB --io 8 --hdd 8 --hdd-bytes 1GB --timeout 60

'sensors' gives

Core 0: +48.0°C (high = +90.0°C, crit = +90.0°C)

Core 1: +49.0°C (high = +90.0°C, crit = +90.0°C)

Core 2: +49.0°C (high = +90.0°C, crit = +90.0°C)

Core 3: +47.0°C (high = +90.0°C, crit = +90.0°C)

This intervention obviously violates the guarantee conditions and is totally at your own risk. As said, there is a fair risk of cracking the screen.