it's not spesific board when use that global search
Actually, it is. When you use the search option, it searches inside the forum section you are (child boards included).
Anyway, since the in-forum search system is not really the best, you'd better learn to use a search engine in an efficient way; you can search the key words you want to look for followed by :bitcointalk: so that it only shows results inside the forum. This will be way faster, and most likely get more relevant results than using the forum search
