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Help me replace a QFP100 MCU with another MCU mounted on an interposer PCB
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DC1MC:
@JDubU I've verified, I have the flash version on both units, that doesn't matter if they are replaced, what matters is that one unit is still accessible via serial port and if I could get compatible replacement chips I intend to read and transfer the firmware.

Original idea was to replace the crappy MCU with an STM32F or similar and then indeed write the fw from scratch, is actually not such a big deal to drive a parallel LCD and a couple of encoders and switches. Was a big deal in the '90s, when the tools were pretty spartan, but not nowadays.  As the last resort I will hook at the data cable a Nucleo 767ZI and use it to drive the RF part, it will loose any portability, but still better than scrap them.

A nice Italian parts seller is asking for 130EUR per whole front panel, or so he says, not a bad price if you want to recover a 14yr old radio, but not worth spending if they don't take the the damaged one for refurbishing to offset the cost.

 Cheers,
 DC1MC
Ian.M:
0.5 mm pitch is viable as a BGA, and you can get low temperature alloy BGA solder balls, normally used for reballing.  It therefore should be possible to make a transposer board with a QFN footprint top-side and on the bottom side 0.5mm pitch pads just inside its perimeter, sized to land on the inner end of the existing QFP pads.  Depending on the decoupling requirements and the difference between the transposer board size and the QFN MCU footprint you may not have room for top-side decoupling where you need them so may need to resort to decoupling within the interposer thickness which could be done with a two PCB interposer, with the lower one a narrow frame with its center milled out to clear caps on the underside of the upper one.   Assemble the interposer by normal reflow using standard solder paste, ball it with low temperature balls and reflow it onto the front panel board's QFP footprint.   The combination of low temperature balls, and the QFP pads extending past the interposer edge should make it reflowable without cooking it or even melting the standard temperature solder it was assembled with.. 

However developing and debugging the interposer design and practising its assembly and final reflow is a *LOT* of work + significant expense for the multilayer interposer.  The interposer is probably tougher to design and construct than a complete front panel board replacement, + salvaging and refitting the existing controls and displays.   If you are scared of that, the interposer idea is almost certainly non-viable for you.

Dead bugging is *<EXPLETIVE>* tedious, but with decent magnification, a steady hand and working methodically, has an excellent chance of success.  Its also minimum risk as you'd have to be really unlucky and ham-handed to lift pads on the existing footprint.  IMHO lay off the caffeine for 24H, plan it out, and sit down at the bench and have a go at dead bugging it, with a checklist of connections sorted in order from shortest to longest, and continuity checking them as you go and again on completion. 
JDubU:

--- Quote from: DC1MC on May 09, 2020, 03:24:27 pm ---@JDubU I've verified, I have the flash version on both units, that doesn't matter if they are replaced, what matters is that one unit is still accessible via serial port and if I could get compatible replacement chips I intend to read and transfer the firmware.

--- End quote ---

Don't know if this company is legitimate:
http://www.odysseyelectronics.net/ps.htm
http://www.odysseyelectronics.net/results.html?part=HD6432345FA+&Submit1=Start+Search&search_db=true&user=odyssey
langwadt:
if there is enough space around the part maybe some 0.5mm pitch FFC connectors and flex cables to a board with a different MCU
DC1MC:
Just in case someone want to assist me redesigning the board or for other ideas, here are a couple of pictures just to see the size and situation, I can reliably say that every important component (sockets, LCD and so on ) is recoverable, also many MCU traces have vias that may help with connecting some wires, there are something like 30 or so unaccounted :(
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