A photo leaked on social media (via @L0vetodream)shows the possible logic board belonging to iPhone 12, although it is not clear which model it may correspond to. Based on what is printed, it is manufactured by Samsung.
The logic board does not provide many clues, but from the photo, it is understood that it has a more elongated design than the more compact ones seen on the iPhone 11. This board was produced in the 40th week of 2019, that is to say in early October. There are still at least four months before a new iPhone is unveiled.