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.
* 1. No, absolutely no glue should be used on BGAs prior to reflow. BGA solder balls collapse slightly during reflow, increasing contact with the pad, and any adhesive would interfere with that.
* 2. The solder paste holds the chip in place prior to the melt, then surface tension during. No need for anything else.
* 3. For backside components, limit the BGA size / weight depending on the solder paste contact area. If you can’t avoid this, you may need to do a 2-pass reflow with higher melt solder on back for pass 1, then lower-melt solder on top for pad 2. This adds cost of course.

You can consider using NSMD pads to increase solder-to-pad contact area (and thus, surface tension) during reflow. NSMD pads have proven more mechanically robust than SMD pads for very fine-pitch BGAs.

Finally, make sure your board planarity is well controlled. Also your package planarity needs to be specified and assured by the vendor.

The glue you're seeing isn't for soldering. It's 'corner staking' applied after the reflow process for improved shock-and-vibe mechanical robustness. It's a reduced type of underfill where adhesive is injected between the corners of the soldered BGA and the board, as opposed to complete underfill which is injected underneath the entire IC.

More here: <https://www.ansys.com/blog/bga-and-qfn-failure-mitigation-underfilling-edge-bonds-and-corner-staking-physics-of-failure>