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.
What others have said. We use a small under-board preheater (my technician says that is essential) so minimal airflow is needed on top using a hot air pencil. We just bought one of those inexpensive reflow ovens (under USD$500), which with some controller modifications can be quite good for doing small board runs. I saw one in use an an OSHWA meeting a couple years ago and was quite impressed. It has actual profiles and you can add thermocouples etc to do it right but on a small scale. For double sided boards surface tension should hold the upside-down parts on but you can of course use a slightly lower temp solder for the second side. Also you may find a local assembly house that can do small runs for you, and they should even have x-ray inspection for BGAs. There's such a place in Salt Lake City.