Question: I have often see ball grid array (BGA) chips, mostly those from CPUs or GPUs, being glued around in the corners with some red glue or to the perimeter with a translucent one.

Having to manually solder BGA chips using hot air, should I glue the chips to the board before heating?

In their answers to a quite similar question about soldering small electronic parts (but not specifically BGA chips), some users mention that glue can cause additional problems when it is not applied properly: [adhesive glue before soldering](https://electronics.stackexchange.com/questions/71682/adhesive-glue-before-soldering)

Not having an assistant, such soldering process remains currently challenging for me, as I hold the hot air gun (from a rework station) in one hand and the tweezers in the other one.

Without using glue, I see at least three difficulties:

* positioning and aligning the chips precisely
* maintaining the surface of the chips parallel to thus of the PCB when bringing them to it
* remaining stable during the soldering, without false move nor trembling

So, my questions are:

1. Is the use of glue recommended, given the context?
2. Are there possibly alternative compounds to help keeping the chips into place during the soldering, like a kind of "butter" that would progressively melt when reaching high temperatures?
3. Are there other improvements that I can make to facilitate the soldering of the BGA chips?

Answer: 
What you see around the corners is most probably not glue, and certainly not put there to hold the chip in place during automated assembly.

Some SMD components need to be glued down after the soldering, as in case of a PCB with components on both sides, when you flip it upside down to assemble the other side some components might fall when the solder is liquid. This happen in case the surface tension of the molten solder is not enough to hold them in place.

That said, it is possible that it is just a sealing compound that is put there to avoid moisture penetrating below the chip, especially for chips that might get hot, such as a CPU or GPU. Moisture can penetrate below the part, and diffuse inside the chip itself, depending on the package technology, and when the chip heats up the water can become steam, and crack certain parts of the chip. To avoid this, you bake the parts before soldering, you solder, and then shut the sides to avoid any moisture ingress.

How can you solder your BGAs is not a single-answer question. This entirely depends on what pitch we are dealing with, PCB thickness, PCB status (new/used) and equipment you have.

A good recipe is to use a very (very very very) thin amount of really thin (as in runny) flux on the PCB, place the part, hot air & pray. You might need a hot plate if the PCB is particularly thick/big, and if it is small enough you can get away with the hot plate only.

And absolutely, categorically, no glue.
The "red" glue you are seeing is an SMT red glue and it's a certain type of temperature-set adhesive. Normally most assembly houses will not be using these adhesives, as surface tension will position the components correctly. However that is the "theory" ... In practice and depending on the circumstance, it may be required or used sometimes.
I know for sure that hand soldering small BGAs based on the common idea of "surface tension will center it naturally - don't worry" is pie in the sky ! If the hot-air gun blows too strongly, as they often do, even on minimum setting, you will quickly blow the part away, quicker then you can sneeze ! In this case, some sort of manual positioning on your prototype PCB might be the answer ... its not easy to say what type of glue works best (if you really had to go that way), but the temperature sensitive red glue is used because it hardends very quickly over 130 - 150 Celcius, so there's a reason for it being used in some cases ...
Having said that, bigger BGA components, say with over 160 BGA balls you hopefully don't need any adhesive at all as the surface tension etc will do the job once the solder is melting ...