Thermal Control

Airflow

• Airflow in the case when building a custom PC is extremely important, especially in compact (SFF) builds.
• Normally the front fans should blow fresh air into the case, and the rear fans should pull hot air out of the case. This may not always be possible (for instance SSUPD Meshlicious case has no space for a rear fan).
• The fans should never press directly against the panels of the case, even mesh panels like in the Meshlicious case. In that case the airflow will be very bad. If this is the case, either install a slimmer fan (10-15mm between panel and fan is enough) or use washer / joining nuts.

Fans

• As a general rule, bigger fans should be quieter. Thus it's best to take the biggest available fan that fits into your case / CPU cooler. Then you can check RPM and decibel (noise) levels.
• When trying to reduce the noise of the PC, you have to take into account ALL fans that will be present. If you forget some, you may end up with a poor result. For instance, in addition to the usual CPU cooler fan and additional case fans, the PSU (Power Supply Unit) has a fan. The motherboard can also have a VRM heatsink and fan. I am not sure what exactly is the VRM heatsink fan but with the Asus - ROG Strix Z590-I motherboard, this fan could go up to 10K RPM and was responsible for 95% of the noise of the computer when it triggered. Hopefully, this fan could be disabled (or at least controlled) in the BIOS so the issue was solved in this case.
• Be careful not to disable too much the fans - a slow rotation speed can get the overall temperature very high, and this starts to throttle CPU and produce a lot of issues.
• Fans should be connected to the motherboard. The newer standard is to use 4 pin connectors with PWM (Pulse Width Modulation).
• Motherboards may not have enough fan headers. There are cables like the Noctua NA-SYC2 that will solve this issue though. You can also use the pump header for a fan if you only use a CPU cooler fan - it will work, although some fan headers do not allow for correct control of the fans. With the Asus - ROG Strix Z590-I motherboard for instance, a fan connected to the AIO pump fan header will apparently always run at full speed.
• When connecting the fans / building the PC, note what fan is connected to which motherboard header. It can be useful for troubleshooting / fan configuration in the BIOS.

CPU & CPU coolers

• Pay close attention to the TDP (Thermal Design Power) of a CPU. The higher it is, the hotter it will run, which will create many issues and make it harder to get a stable and quiet build.
• For instance the Intel i7 11700K processor that I have was probably a very poor choice. It has a theoretical TDP of 125W (apparently needs even more), and run extremely hot. The 11700F variant (without integrated GPU) has a TDP of only 65W, it might have been a better choice.
• A bigger CPU cooler is better but be very careful of the dimensions of your cooler as it needs to fit in your case. A compatibility site like Pangoly should always be used before buying.
• The orientation / direction of the CPU cooler is critically important. It needs to fit of course, but ideally it should not be over the VRM and M2 areas (see M2 section on this page).
• When applying thermal paste, don't apply too much! The goal is not to have the paste spread out everywhere in the CPU die, but to apply a small pea-size dot in the center and let the cooler spread out the paste.
• When removing old paste, use isopropyl alcool to clean everything.
• Normally the CPU cooler fan should blow air into the cooler, so should be in intake mode, not exhaust mode.

M2 NVME SSD drives

• It's very important to keep those drives at a low temperature; high temperatures throttle them and create many issues.
• Basically the best is to use thermal pads both on the top AND bottom of the drive (so two thermal pads) when you install the drive.
• Additionally, try to position the CPU cooler away from the M2 drive area as much as possible. On a SFF build with the SSUPD Meshlicious case, the M2 drive was getting extremely hot (70 to 85 degrees) when the Big Shuriken CPU cooler was over the M2 area. Moving the cooler 180 degrees fixed the issue and the M2 drive went to temperatures under 70 degrees. This created other issues though as the cooler was pressuring the RAM sticks, but it's a different story.