Would be weird in the actual mass production process if the flash would need to be pre-programmed somehow; one DFU process IMO must be able to do everything needed.
Also if there are two flash chips they need to be installed in a certain order. Not sure of the rationale behind that precisely. I doubt it's a hardware difference.
There's a tool for that (many, many of them), both software-based and physical-based tools that can copy out the SysCFG block and write it into a new chunk of flash.
sounds like a national security threat to me
https://www.solveyourtech.com/uncovering-where-iphones-are-m...
I don't repair/upgrade iPhones or anything, and I'm an EE not a trained technician, but I do need to swap BGAs from time to time and my rework rate for 0.4mm pitch BGAs is not the best. It works say, 3 of 4 times. But compared to colleagues I'm pretty good. But that rate is way too low to run any kind of viable business, I would think.
In this situation you're doing literally the same rework over and over, which helps, and probably have equipment and stencils specific to the job, which helps.
So I'm curious what kind of success rate was achieved if you don't mind sharing.
How can the new chip work if the original chip is milled off completely? I would be surprised if you could read all necessary identifiers through iOS/USB software before milling unless the device was jailbroken and read that way. Seems like a big oversight for Apple not to implement simple countermeasures to make it a little bit harder, or that Apple would undo protections they had back in the iPhone 6S days.