The invention relates to an electrical connector assembly. The electrical connector assembly includes a main circuit board having a through hole, a processor, and an auxiliary circuit board. The processor includes a chip and a substrate. The chip is electrically connected to the substrate and located in the through hole. The substrate is at least partially located in the through hole. The auxiliary circuit board has a transitional connecting surface. A first conducting region and a second conducting region electrically connected to each other are disposed on the transitional connecting surface. The first conducting region is electrically connected to the substrate, and the second conducting region is electrically connected to the main circuit board.