Hi Alplily, I use a heating pad for my seedling orchids. I do find that the way it works is I wet the fabric mat on the pad and when pad heats up the water evaporates creating a warm humid environment. This is 'trapped' by a humidity dome like those for seedling propagation so that the moisture and heat don't dissipate as quickly.
As usual make sure you buy one from a reputable source, use carefully and as per instructions. I always worry about these things and heaters etc etc... don't want to mess around with electrical stuff and cause fires etc etc...
here's a couple of pics of the set up with my Paph randsii seedlings.