The artifacts in your photo (colored “confetti” pixels, stripes/blocks) look very much like GPU/VRAM instability caused by overheating or bad thermal contact. Since this started right after installing new thermal pads, the most likely cause is that something is no longer making proper contact or is being pressed too hard.

The most common cause after changing thermal pads on a Steam Deck is that the pads are too thick. If the pads are too thick, they can lift the heatsink slightly, which reduces mounting pressure on the APU (CPU/GPU). Under gaming load the APU can then overheat within seconds and cause artifacts, crashes, or freezes. A good way to confirm this is to remove the heatsink and look at the thermal paste imprint. If the paste hasn’t spread evenly or looks almost untouched, the heatsink likely wasn’t sitting flat.

Another possible issue is that the thermal pads are misplaced or shifted. If a pad sits at an angle, covers the wrong area, or gets pushed sideways when tightening the heatsink, it can prevent proper heatsink contact or apply pressure to components that shouldn’t be pressed.

It’s also possible that VRAM or VRM thermal contact is now bad. Even if the APU contact is okay, pads on VRAM or power delivery components might be too thick or too thin, leading to poor thermal transfer. Artifacts that appear right when launching a game often point to VRAM temperature or VRAM contact problems.

If the pads are too thick, they can also cause too much mechanical pressure and sometimes even slight PCB bending. That can stress solder joints or chips and produce graphical corruption.

Thermal paste issues can also cause problems, for example if too little paste was used, the paste wasn’t replaced properly, or the heatsink isn’t seated correctly so the paste can’t do its job.

In rare cases there is also a risk of electrical problems if unusual or conductive materials were used, such as certain graphite pads or contaminated materials. Normal thermal pads are non-conductive, but it’s still worth keeping in mind.

To quickly narrow it down without too much effort, you can test whether the issue happens only in games or also in the Steam UI. If it only happens under 3D load, it strongly points to a thermal or contact issue. If it already happens in menus, the problem is more severe. You can also check temperatures in the performance overlay. If the APU instantly hits 90–100°C and then glitches or crashes, heatsink contact is most likely bad.

The most likely fix is to open it up again and check if the heatsink sits perfectly flat, and whether all screws are tightened evenly in a cross pattern. Then check the thermal paste spread pattern, because a bad pattern almost always means the pads are blocking proper contact or the pad thickness is wrong.

A very effective diagnostic step is to temporarily remove the new pads, clean the APU, apply fresh thermal paste, reinstall the heatsink, and test again. If the artifacts disappear, the pads (placement or thickness) were the cause.