Let's say that I want to rework a SMD chip using the hot-air gun, but few cm away on the PCB there is some component that is thermally sensitive, e.g. an electrolytic.
What's the correct way to proceed in order to avoid damaging that component?
I've seen in some rework videos that a barrier of alu foil is used, sometimes held in place with kapton tape. Is that all that is possible, or there are smarter ways?
Foil and Kapton would be my first choice
there is some component that is thermally sensitive, e.g. an electrolytic.
Electrolytics are not that thermally sensitive.
However, in many places where I have worked, they use a hot plate or pre-heater to heat the whole board up to just below the melting point of solder.
Then they use the heat gun to melt/reflow just the component being reworked.
This minimises the thermal stress of the components around it.
I often use foil placed around the component being worked on and just wrap/form it so it doesn't get blown off. I also use the oven preheat method, it works well too. Another bonus of the foil is that it keeps any tiny SMD parts nearby (i.e. BJTs) from blowing away should their solder happen to melt also.
or there are smarter ways?
use thermo insulator shielding instead of alu foil the thermal not insulator.
there is some component that is thermally sensitive, e.g. an electrolytic.
Electrolytics are not that thermally sensitive.
However, in many places where I have worked, they use a hot plate or pre-heater to heat the whole board up to just below the melting point of solder.
Then they use the heat gun to melt/reflow just the component being reworked.
This minimises the thermal stress of the components around it.
We normally use a pre heater only on thermally "difficult" parts or boards.
Having access to a pro tool like this...
...means I can use the metal bucket like shields in the foreground to put around the component to be reworked. If that doesn't fit because of nearby components in the way, then kapton tape is the "plan B"...
If you're handy with alu foil, maybe you can fold something into shape. It may not be a thermal insulator, it will keep the hot air away from sensitive components.