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.
Personally, I'm intimidated by the whole idea of BGA rework with hobby grade equipment, and really wouldn't do it.

But, no, if I were doing it, I would be very hesitant to glue the chip in place. Surface mount soldering relies on letting the surface tension of melted solder being able to pull the chip into alignment. Gluing would prevent that from happening.

The issue is that with hot air, you may have problems getting all the balls molten at the same time. I really wouldn't even consider personally trying it without using a board heater to bring the temp just below the eutectic point, and then using hot air to nudge the region of the board over melting.

Note that facilities that do BGA rework often have xray devices to check the results. Certainly, you would need to be able to tolerate errors, and be able to run a functionality test to verify correct placement.