In monitoring this thread I've seen a lot of activity suggesting its more than a few phones, but, apparently not enough to warrant action on their part. The heating approach I used is still working two months later. This definitely points to a manufacturing related issue. I won't bore you with repeating my earlier observations/posts with manufacturing process variations, but the combination of these variations and firmware changes could be causing the issue.
None of us have exposure to the necessary data to come to root cause. My past experience with semiconductor issues would suggest that this data is not easily obtained as it will be obtained from IC trace codes affiliated with failed devices, contract manufacturers & their associated manufacturing runs, the IC packaging industry & it's product characterization s, & finally the specifics of the firmware update. W/O apple taking enough of the failed phones back to get a statistical set of data they themselves will not get to root cause.