Rheem burnt control board

Came home to a burning smell and no heat. Went down to furnace and didn’t have any apparent smell until I opened the cabinet and saw this. I think the unit is like 15-20 years old and was planning on replacing anyway soon due to my A/C having the old refrigerant. Any idea what might have caused this. Looks like the main source of the damage originated from the bottom middle of the board.

Came home to a burning smell and no heat. Went down to furnace and didn’t have any apparent smell until I opened the cabinet and saw this. I think the unit is like 15-20 years old and was planning on replacing anyway soon due to my A/C having the old refrigerant. Any idea what might have caused this. Looks like the main source of the damage originated from the bottom middle of the board.