Most of the logic board covers in the iPhone 4 can be removed without heating. They are sort of clamped over a frame which is soldered to the board.
You can use for example fine tweezers with a fine tip to remove the covers off the frame.
This way, you can access most if not all areas on the logic board (except for one small part).
If that's not enough, and you still need to remove a shield (on this or any phone), I think 320°C is too hot and you risk damaging/displacing lots of components and/or damaging the solder underneath some of the major chips.
I would use something like 260 degrees, full air speed, without nozzle, while covering all surrounding areas with polyamide (Kapton) tape. If you've got the hang of it, you can skip the polyamide tape, but I strongly recommend it for starters/newbies.
While heating, have the tip of the tweezers inserted underneath the shield in any opening you can find, while staying away from tiny components under there, and keep a steady but very light pressure (lever up) on the shield, this way once the solder melts, the shield will pop up.
If done well, it should be over in 45 seconds.
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