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 main reason for using staking or underfill is to 1) reduce the stress on the BGA solder joints caused by CTE differences between the package and the board, 2) reduce the possibility of the part detaching from the board during a high shock (depth charge near a submarine) or vibration (rocket launch) event and 3) in the case of underfill, provide a better thermal path from the package to the board.

As others have said, you want the part to be able to move a bit during the solder reflow operation, so any staking or underfill is done after soldering and after initial testing shows that the unit works properly.

Whether staking or underfill is needed, and the specific material used, can only be determined after a rigorous structural (for staking) and thermal (for underfill) analysis has been performed.

Staking is preferred to underfill because of the difficulty/impossibility of reworking a BGA that has been underfilled.

**Edit 1**

Note that on double side SMT boards that go through a solder reflow operation, it is usual practice to use a small bit of adhesive under components to hold them in place, especially those on the underside of the board. However this adhesive is not meant to serve a structural or thermal need.