I'm building an Arduino drone. In this case, I just want to ask all of you here: Can I directly solder from BLDC motor wires to ESC30A board?
Technically, yes. Then it's up to Your skills.
You should be able to as long as you have a good, preferably temperature controlled soldering iron with an appropriately sized tip.
You might have to put a lot of heat into thick wires.
Just beware of melting the insulation, particularly if the insulation is PVC.
All the above plus............ you WILL find it next to impossible.
The heat will transfer from your iron into the board giving one of three results.
Iron not large enough and not enough additional flux and if 60/40 solder not used to tin the board, it will simply cook the board.
Too much heat applied for too long, again, without extra flux, will cook the board and the wiring.
Not enough heat will quite possibly lift the pcb tracks and again, destroy the board.
Far better to heat sink the existing 3 cables and splice into them one at a time. Usually 100mm or so of cable to play with but if done correctly, best approach.
Or,,,,,,, use half decent 2 screw type tunnel connector and some heatshrink.(zero solder)
Add: Exploring to other forums, soldering directly to the ESC board is okay as long as I can do that "perfectly". (the same as what @Railroader and @JohnLincoln said). This is the answer I need.
Some said that, by directly soldering to ESC board will reduce resistance, etc. But, the point of this issue is HOW to do it perfectly, otherwise will destroy ESC board 
thanks again for your input.
please give me a link of image of the thing, please.
By tinning the stranded wires before trying to attach to the board. That way there are NO stray wires and the strands do not suck up all the solder you are using to make the attachment. You can avoid melting PVC insulation by using an aluminum heat sink clamp right next to the insulation.
-- the same as --
I'll try, thanks, since I have one ESC left for this experiments 