[Dr_C2]

Procedure in outline:

1/ preparation for 64 pin IC removal
- open APB
- disconnect ribbon cable to LED/button PCB by un-soldering. (take care not to damage tracks.

2/ Microcontroller removal
- avoid using a heat gun as this might dislodge nearby SMD components.
- instead flood all four sides of the microcontroller with fresh solder and sequentially add heat to each side using a soldering iron using a sequential circular motion. As the heat builds up the microcontroller becomes free and can be lifted by tweezers.
- check that no excess solder has splashed onto any nearby components.

3/ Fitment of a replacement microcontroller.
- clean the 64 landing pads and add fresh solder. This should create a ‘domed’ solder surface over each pad.
- very carefully position the replacement microcontroller and then spot solder a couple of diametrically opposite pins.
- check for ‘perfect’ alignment of pins over their corresponding pads.
- then using a point tipped soldering iron heat each pin in turn to create a perfect solder bond between the pin and the underlying solder dome.
- repeat until all 64 pins have been resoldered.
- visually check there are no solder bridges 

4/ continuity check
- make sure there is electrical continuity between each pin of the microcontroller and a nearby component which is know to track back to that pin.

5/ re-attach the ribbon cable (in my case I replaced with an FFC cable).

6/ program the microcontroller with firmware using a PICkit on the ICSP 4 pin interface.

7/ if necessary re-program the APB to firmware v1.009 using a correctly wired USB-to-RS485 cable.

Overall conclusion:

The microcontroller inside an APB can be replaced to bring back full functionality when the microcontroller has failed in some way. However, the repair procedure is quite challenging with lots of opportunities for things, potentially, to go wrong. When taking extreme care at each of the above steps it can be done successfully :)

Not for the faint hearted!

c