Before I solder very small electronic parts on PCB board, I want to find some needle point sized injector to inject tiny glue fix the parts, then do the soldering. Some manufacturers used that, but I can not find what it is. Can anybody help?
Honestly, it is more trouble than it's worth if you are hand soldering. I wouldn't bother. It is a lot of work to get the glue and part on properly, and if done improperly it will prevent you from soldering the part correctly.
On the other hand, if you are not using glue and are using a simple hot-air soldering station, then the surface tension of the solder will align the part perfectly for you.
Some manufacturers do use glue, but this is rare and getting more rare. Manufacturers avoid using glue unless they absolutely have to, and glue can't be used on many PCBs. And keep in mind that they are using glue with robots, not with hand soldering.
As for where to buy the glue, or what glue to buy, I really don't know. Nobody uses it by hand, so I don't know if they even make a needle point type injector for use by hand.
I wouldn't recommend using glue to fix the components in place before soldering, it could just get messy, and the steady hand you need to apply the glue correctly could just as well be used to solder the component on!
A technique I normally use is to put a bit of solder on the pad first, then hold the component down with a pair of tweezers onto the pad. Then apply the soldering iron to the component - the solder will melt and make the connection to the component.
That's normally enough to hold the component in place and you can solder the rest of the component in. I've done 0805 resistors and capacitors using this method, and SOT-23 packages. Works like a charm