I have seen videos on youtube with some guys using hot air pistols (or even small propane burner with bunsen flame) to desolder the components from a PCB. The job is done REALLY quick! The components were really hot. They were still handling the components with gloves, many seconds after they fell from the PCB. My question is: can the components survive this kind of treatment.
The critical temperature when (batch) desoldering
solderingtemperature
Related Solutions
If you don't follow the reflow profile properly, there are a few things that can happen
- If your board contains large copper pours, and are not warmed up properly, during reflow, your components can tombstone which is when a resistor or something will literally stand up and then is soldered in place like that. This can be a pain to fix depending on where the component is.
- If you heat up too quickly, you can thermally shock your components that are already placed on the board. While there have been measures taken to reduce this during the design of chips, it is still a problem if procedures aren't followed properly. Thermally shocked components may not function as expected until they are brought to regular conditions although sometimes they will flat-out break.
- Another more obvious problem is that some of your components might not be soldered. Its true that solder paste is metallic although solder paste alone is not a great connection. If you don't reflow the board properly, some components might just not get soldered and will eventually fall off or cause other problems.
With those said, if your board is not the most dense board, like it doesn't have a bunch of decoupling capacitors underneath a processor, I wouldn't worry too much about it. You can easily observe whether something is soldered. It would still be a good idea to get a preheater since the copper pour issue is kind of annoying (You can literally watch it happen while using hot-air rework)
Since you do have a double sided board, then you might want to consider getting thermal adhesive. It is like a little red glue that you put on the components that you reflow first on to the board so that when you flip it over to reflow the other side, the parts don't fall off.
The easiest and less destructive way would be to turn your board upside-down, and heat the chip area from the PCB side. Use some powerful heat gun instead of a tiny hot-air pencil. As soon as the solder temperature gets to the soften/melting point (which you don't know and shouldn't guess), the chip will fall down by itself, maybe with a little help.
You can't afford re-balling, plus you will need to develop a PCB interposer for the BGA with breakout pins, etc. Best way would be to use a very thin magnet wire, AWG36 to AWG40, and break the connection to some DIP-40 socket, or something. Solder 60/40 would be just fine, if you have a 10X stereo stand and fine-tip soldering iron.
ADDITION: Here is your other option without soldering the chip back:
You just need to clean your eMMC thoroughly.
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Best Answer
tl;dr : It depends.
I think you are referring to the method of heating things up with a hot air gun, and then shaking the pcb so everything falls of, or a variation of this.
First of all, remember that the process of actually soldering SMT PCBs is quite similar. Stuff goes into a reflow oven, heats up until solder paste melts to liquid solder, then cools down.
But of course, this is a much more uncontrolled thing when using a heat gun. It will depend on:
For the last point, you can look at a datasheet of a specific part, especially at the reflow profile. Controlling temperature and exposure time is next to impossible with a heatgun, and when the parts are sensitive to being heated up while containing humidity, they are toast.
So it all depends and is mostly luck and statistics. When doing it by people with some experience in the process, the yield of functional parts can be way over 80%.