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: 
To add on to the other excellent answers, and to answer your third question: the red glue you see is likely to be some kind of **corner staking** or **underfilling**. After soldering, an adhesive compound is added to mitigate in-the-field failure, particularly when packages are subjected to thermal or physical stresses. It's not intended to improve solderability.

See [the ANSYS website](https://www.ansys.com/blog/bga-and-qfn-failure-mitigation-underfilling-edge-bonds-and-corner-staking-physics-of-failure) for more info.
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 ...