Screw of the mainboard broken. what can I do? (repasting thermalpaste)

There are different options depending on the screw damage. I would advise you to share some photos of this problem with us - perhaps we will be able to figure out a better solution than Dremel or drill.

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But answering your question, yes, Dremel can help.

If the head is stripped, use Dremel cutting wheel to make a slot in the center of the head and remove it with a regular slotted screwdriver.

Like here: https://www.youtube.com/watch?v=DD7jYf8A...

Or if the head is completely broken, or part of the broken screw is stuck inside, you can try to drill it out using the drill bit for metal, just keep the following in mind:

• choose an extremely small drill bit for metal
• drill the screw in the center
• apply power and pressure very slowly, don’t turn the drill too fast and don’t press too hard
• stop drilling in some time and try removing the screw with pliers