Kapton Tape! I have seen video's mentioning heat resistant tape but I couldn't find the right stuff. A search on 'Kapton Tape' brings it up and can now order some. Thanks for mentioning it by its name.
You are welcome, it took me a while to find too (was a large one for a 3D printer bed initially). A small one helped me a lot with the replacement of a failed dc/dc chip (nasty QFN with power pad) on an AVR Dragon (a lot of stuff around especially yellow tantalum capacitors). Don't even need to stick correctly, basically shielding from hot air and intensive IR thermal ray. I needed it to protect during desoldering and during resoldering with the new part using solder paste.
I used a set of ordinary tweezers although these were a little too pliable but I seemed to manage OK. I had some proper SMD rework ones on order from China and these came the other day. On the old board I had an adjacent capacitor rupture due to the heat so I was a bit wary, but I was able to remove various transistors and even ICs without any physical damage. Hopefully once I can prevent this as shown in the video by masking with the Kapton tape.
Tweezers are okay bad or good it's your confort which change, I do have a set of really decent one (not costing too much), but I dream of the incredible ones I tried once (40€ per tweezers huh I didn't have such a good brand) .
However, my advice is not to pull away the component you are desoldering, that's why I put more or less the gravity force on it to push in order to get it sliding away, that way, it will move only when it's hot enough and I have no risk of taking away the pcb trace (which is earier because it's hotter). Sometimes you can't do that if there isn't enough room / too much soldering around.
Regarding the heat setting on the 858D, what is the best temperature to set, and which nozzle to use?
Is higher better as its faster?
I will answer with what can be qualified as a "une réponse de Normand" in French : hot enough, but not too much, fast enough but not too fast.
Temp / Heat : start not too high (but at least melting point of lead-free solder, somewhere between 220°C and 270°C if my memory is okay)
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keep in mind that if you are too shy, you will harm more than protect, because you will heat far longer to get nothing, that's worse than a bit too high the right amount of time-
keep in mind that the heating will decrease dramatically with the increase of the distance between your nozzle and your board/part/pins, to really see how much, take an apple (fruit not device) and try to "burn/cook" the outside, you will see how close and how high temp you need to be. Hot air more or less heat surface not deep. I first did that for fun but it helped me a lot to understand the heating process and find suitable settings for hot air SMD rework- You can take an eye on some datasheet especially from the parts you desolder / solder, you will find the various reflow temperature and limits, generally there is a really huge margin (when it's written, you may find that in separate docs). For example, I suggest you look a bit inside the TI doc
http://www.ti.com/lit/an/spraby1/spraby1.pdf, it will help you to understand how much and for how long a device can handle soldering temp (pre-great then 260°C reflow for 30s for some parts, that's a lot, previously I was afraid to get that high that long, but well that's what the components are designed to handle in the oven.
That's what I've mostly done, soldering a really thin package with under power pad or removing IC and replacing it (harder because you really need to not fry the new part, in your case you can cook the older transistor no problem).
In oven reflow soldering you choose the reflow profile, depending on the most sensitive part and your soldering paste (which may be changed if the most sensitive part need a lower reflow temp. That's a big science, that I barely know.
Airflow : -
I learned by trying and feeling how it's going, the idea is you need the biggest possible airflow to heat faster and wider, but... you want your airflow to be low enough to prevent your resistor, capacitor and parts around to fly away (really it's what's happening) if you heat a bit around by accident (especially true when you solder the replacement IC in case it's a nasty QFN or SSOP with a power pad).
- If it's not melting, and the temperature is appropriate (more or less), it's often because the airflow it a bit too low (I'm generally not using really high flow). I would choose a Nozzle almost big enough to cover your transistor but not much bigger to prevent from heating too much around (but you can "preheat" around a bit by blowing hot air from a bigger distance hanging around, it can lower the thermal constraints). However in your case it's really not an issue, on another thread we end up saying that at the end of the day except for BGA rework it's more or less useless to have an under quartz heater or to preheat the board.
I hope my feedback can help you, but it seems that I do more by instinct and observation on this one, this is knowledge I've yet to consolidate and improve to be able to transmit properly.