You cannot trust the motherboard box. Use an unfinished wood bread/cutting board, or a large piece of plain, brown cardboard.
The problem with motherboard boxes (graphics card boxes too) is no telling what metals (including lead) and other conductive ingredients/materials may be in the inks used to print the graphics on those motherboard boxes. So best not to take any chances.
If the inside of the motherboard box is plain white, you can use that - but a large flat piece of plain brown cardboard, or unfinished (no paint or stain) wood is better.
***
Edit add: I see a reply to above was deleted. I am assuming it was thought to be of low quality as it may have appeared to make light of my comment about motherboard boxes.
To clarify further, below was my reply.
Sadly it is true. Well, not the boxes themselves, but the inks used to print the graphics on them.
With voltages running through motherboard circuits being of such tiny potentials, it only takes very little getting drawn off through a parallel circuit established by a couple lead points on the powered motherboard, making contact with the graphics inks. This can influence that circuit, one way or another.
And for higher potentials (such as found in a defective PSU) it can, in extreme cases, lead to a hair raising shock, smoke, fire or worse!
So, like I said, best not to take any chances.