This piqued my interest because i’m dealing with long-running corrosion issues on my ComPilot1 at the moment.
It looks quite similar to the ComPilot1, although there’s a few differences that I note, primarily the radio side (top shielding can by the antenna) is different, and that the STM32F103 part is a BGA part with the solder balls/connections on the underside of the chip as opposed to the TQFP package I have in my ComPilot1 where the chip has legs and you can visibly see and access the solder joints.
Given the STM32F103 is the BGA version here I suspect if you’re gonna have further troubles, it’ll be liquid or other residues remaining underneath the chip and bridging the solder balls that affix the chip to the board - it’ll wick up inside the gap between the chip and the PCB quite nicely but end up being a pain to get out. If it’s giving more hassle after what you’ve already tried, it might be an idea to flood the undersides of both the STM32F103 and the CSR radio with isopropyl, but then use some compressed air to blow out both corrosion debris, and the alcohol from the tiny gap between the BGA package and the PCB.
The proper fix in a case of having corrosion/debris underneath the BGA chips would be to use a hot-air pencil to remove the chips, clean up the board and the chips, then re-attach them to the board, but that’s quite an involved process - chips with legs are much nicer to deal with because often some alcohol and a fiberglass pencil could remove residues bridging the legs! I’d probably not attempt a BGA repair myself as my hands just aren’t steady enough these days, and BGA rework is often quite hard!
Something I really wish Phonak had bothered with on these (especially where there’s BGA parts!) is a conformal coating over the PCB assembly, although with the pads for the DAI socket and the switches, it would definitely be a pain to do as the coating would cover the pads for the socket and wick-up into the switches making them quite useless. However a conformal coating would help prevent liquid ingress underneath the BGA parts as well as protect legs of adjacent components from corroding if exposed to ambient moisture such as sweat. I’m posting off my replacement ComPilot1 to a friend later this week to get the board conformal-coated to see if that actually helps preserve longevity as I identified a lot of corrosion inside my old one just from sweat the last time I opened it up to see why it was sulking. A conformal coating would have probably helped both of our cases!
I hope your efforts so far have saved it, after all what you’ve done so far is all quite correct - but if not do pay attention to getting IPA and some air flushed into the gap between the BGA parts and the PCB to rinse them out, and good luck!