So it is a fact that Nexus 4 SoC throttles pretty often and even if it does not remove anything to this phone, it is one of its particularity compared to other phones using APQ8064 that are not known to throttle so strongly. So, question is why?
So first of all, what is thermal throttling, and what does it has for consequences for your Nexus 4 ?
Quick explanation is that when a chip (CPU, GPU or in our case a SoC) reach a limit temperature, it will reduce its frequency to reduce heat and temperature. For a more complete description, check Wiki.
Keep in mind that with or without throttling, the Snapdragon S4 in your N4 remains one of the quickest SoC on the market. However, it can affect performances on eavy duty applications like 3D games. Better than a long description, check below graph.
So lets try to find from where this thermal throttling does come from;
After having a look at Ifixit teardown, I was pretty surprised by the low thermal inertia of my phone and the heat concentration under stress situations (mainly located on the right of the back camera). It looked to me pretty strange, especially after seeing following pictures;
Indeed, after “mentally returning” the 2 pictures, you understand that SoC is facing display and should be spread its heat on the whole grey frame.
So I decided to dismount my own Nexus 4, and performed some measurements with a calliper. I found the following;
Which is quite a surprise if you consider that the chips height is almost the same;
In other words, the EMI shield (left) may come in contact with the frame (in the 1mm depth pocket) while the APQ8064/DRAM Stack (centre) may remain isolated (no contact in the 1,5mm pocket). This is pretty sad if you consider that the below frame appears not to be plastic material but to be made of light weighted metallic alloy (Aluminium and/or Magnesium) and could be used as heatsink.
I won’t overflowed you with tons of pictures but if you look carefully at Ifixit teardown you will also notice that motherboard is pretty thermally insulated from the metallic frame, while at the same time it is pretty well thermally coupled to the battery with bronze housings/threads and wide copper band;
(Please note the 2 battery max temperatures, 40 or 60°C?)
So what do all these pictures show? That mainboard may evacuate its heat through the metallic bridge to the battery while it ‘should’ evacuate it through the metallic frame.
After several stress test runs, it seem that Nexus 4 tries to keep its battery below 40°C, so it downclocks from 1512Mhz to 1242Mhz when reaching ~37°C and from 1242Mhz to 1134Mhz when hitting ~39°C, while it may completely shutdown when battery reach 60°C.
So after all these founding I made a small quick tendency study;
So, no need to be a genius to understand what is happening, the “insulated” motherboard spreads its heat to the battery which behaves like a heat capacitor all this through the ‘metallic bridge’. Unfortunately, battery thermal sensor is between heat source and heat capacitor.
So I first tried to apply some thermal paste (a lot! );
Which lead to an obvious result as you can hardly fill a 0.5mm gap with thermal paste;
So as second try I used a small piece of copper to fill the gap (Pre-installation pictures only for showing copper piece dimensions);
I used stability test to stress CPU and observed time before it throttles first @1242Mhz and then @1134Mhz. Please note that I throttling appear to happen when battery sensor reach 37°C. (EDIT: or when SoC reach 60°C)
As a result I obtained following delays before throttling; (EDIT: Other users results added)
Even the thermal ‘pad’ solution, despite its bad efficiency, induces a significant improvement, which gives credits to previous diagnostic/explanation.
Concerning the ‚copper solution, it seems that it solves completely the shutdown problem;
The asymptotic temperature appears to be below 50°C (it was ~41°C after ~15min).
We can also notice that phone temperature surface is more uniform, and hot spots almost disappeared.
So is it ideal solution to solve N4 throttling?
For sure not, because it does not completely remove throttling problem, but at least gives you much more headroom before it happens (especially under real world applications). Not to forget that the battery is deeply glued to the frame so it may collect heat pretty well, but at least battery sensor will now reports a more realistic temperature and not any more battery PCB one.
It also may increase cooling capabilities, as with this mod, you use the complete surface of the phone to cool down (front, back and even sides as frame goes at edges).
In other words, this mod allows you to use your whole phone heat capacity, which increases time before throttling. Using it with undervolting soft mod may bring you the best results.
I hope this could be useful.
Doing the Mod
_ Careful!!! Disassembly Nexus 4 is not difficult, as long as you keep cool, you have proper tools and enough time. Please do not forget to put back the black plastic cover before screwing the motherboard again, however you may damage your screen! (check Jiia posts)
_ Dimensions of the pocket in the aluminium/magnesium frame is about 15mm x 15mm x 0.5mm (with corner rounding ~0.25mm). The APQ8064 chip itself has smaller dimensions, more in the 13mm x 13mm range. So "perfect safe dimensions" would be 14.5mm x 14.5mm x 0.5mm for the pad, if it is a solid (metallic) one. Be careful and try to keep your shim as flat as possible to avoid any stress after montage. For sure you have more flexibility with a soft one (thermal pad, graphite...).
_ Concerning material; if we remain IRL (forget diamonds!) a list of possible material (with decreasing efficiency), may look likes:
graphite foil, Silver/Copper shim, aluminium shim, (most) other metal shim, thermal pad/paste, thermal tape.
Graphite foil: may give the better performances, but may be difficult to supply (not so expensive, but companies sale usually only big quantities).
Copper shim: DIY may not that easy (0.5mm is easy to bend), but you can buy finish products.
Aluminium shim: relative easy to supply and to manufacture (scissor may work), with very close performance than copper.
Other metal shim: may be the easiest to find (ask your local garage mechanic). Lower performance than copper or aluminium but better than pad (steel family). Check thermal conductivity for other possibilities (Bronze, brass, zinc...)
_ Where to find/buy;
Graphite Foil. or on Ebay (careful, Inches)
0.5mm Copper pads on ebay, or on ebay...
0.5mm Aluminium sheet. also on ebay
Thanks to all the regulars and helpful contributors to this post