overheating before and after swapping the logic board and the battery

my iphone 4 started strongly overheating, specifically where the logic board is located, while charging and any kind of use (apart from standby). (similar to this case.) after a month the logic board died completely.

i bought another copy of the iphone 4 (older and with many signs of wear), swapped the board - same issue. hence swapped the battery - same issue.

seemingly it must be some other component that makes the board overheat. at aforementioned thread there's a sugestion form one Linda, that it may be caused by the camera. i will check this.

any other suggestions are welcome. i would love not to test components one by one as well as not to fry another board.

regards

antoni