[Galaxy S II General] MaRtYy01:
I decided to run Cpu Throttling Test to measure how is the stock thermal performance of the phone and I noticed that it is rather lackluster. After about a minute and half of full cpu load(100 threads option in the program) the processor starts jumping around 800mhz and 1.2ghz, if the load is for a longer amount of time it would likely throttle down to 800mhz completely and keep in mind that this is with only the heat of the cpu, if you are doing something else that is stressing the gpu and storage as well it would throttle harder. Nowadays as you know galaxy s2 is often running on full load due to modern operation systems and apps being pretty heavy.

I had a look at pictures of the motherboard of the phone and instantly noticed a significant design flaw which is what causes the throttling-there is no heatsink over the soc at all. On the pic attached you can see the soc with another fairly big chip next to it(probably the baseband, idk).

I had some copper heatsinks left from a previous project so I decided to try adding them on top of the naked chips with some thermal paste. The size of the copper plates is 15x15mm, thickness 0.3mm.

So I disassembled the phone, removed the circle sticker from the soc, cleaned it and the chip next to it with some isopropyl alcohol then applied a tiny amount of thermal paste to the chips with a plastic spudger. Just a very thin coat. The next step is cleaning the copper heatsinks with isopropyl alcohol and sticking them on top of the chips. You only need to apply thermal paste to the chips, don't bother applying it to the copper heatsinks as well as if you put too much thermal paste and stick the chips it will likely cause a mess and get on the resistors and other components.

After that I applied a small amount of thermal paste to the plastic mid frame where chips would usually fit. Before that I cleaned the spot with a little isopropyl alcohol. I used a picture from ifixit teardown and drew with blue color on the spots where you should put the paste.

Well that is the mod. Now reassemble the phone.

Please keep track of the two little black screws that are securing the mainboard in place and be cautious around your volume and power button flex cables, as they are usually glued with weak adhesive to the plastic frame, so if you pull out the mainboard without wiggling them out of the adhesive first you might accidentaly rip them out or damage them.

And now, for the results. As last time, I did a full cpu load stress test with 100 threads. This time I waited the full 15 minutes and behold, the cpu didn't drop below 1.2ghz at all! This is insane improvement in thermal performance. The program says that the cpu thottled to 91% of the max performance, however that reading is due to the small inconsistency of the ''GIPS'' performance which is due to the difficulty of the load, rather than actual thermal throttling. As for the soc temperature, my rom doesn't seem to read it, I only get the battery temperature which was 35C after the 15 minute stress test. The 50C reading from the program is also wrong, it probably just lists it like that because it can't find a temperature reading for the cpu.

In reality even on stock speeds I feel that the phone runs smoother now. However, there is definitely plenty of room for overclocking now, easily to 1.4-1.6ghz. Unfortunately I'm using @rINanDO 's lineage os 14.1 with emulated storage(android 7.1.2) and there is no kernel with cpu overclock for that version, so I can't test that myself. Although I believe such a kernel will be very useful because this android version runs faster than android 9+ while still supporting nearly all apps.

So to sum it up, the benefits of this mod:

-thermal thottling completely removed

-lower temperatures mean less stress on the silicon and longer device lifespan, which is important as this model is already 11 years old

-plenty of thermal headroom for overclocking and significantly increased performance even on stock speeds

And the requirements for the mod:

-a screwdriver

-thermal paste, preferably non conductive

-two 15x15mm 0.3mm thickness copper heatsinks-I got mine from Aliexpress, a pack of 10 costed me 1.25$. They also come with a generic thermal grease but I don't recommend using it. I guess it will do the job if you don't have any other thermal paste.

-some isopropyl alcohol is also recommended for cleaning the surfaces, for better contact

-maybe a disassembly tutorial, anyway disassembling this phone is fairly easy. If you have no experience take your time

Unfortunately I didn't take pictures while doing the mod, so I hope the generic ifixit disassembly photos and my explanation are enough. The mod is pretty easy and took me around 30 minutes.