I'm assuming (but do not really know) that the original Macintosh used a two layer PCB. So my question is, when did the Macs start using more complicated boards with four (or more) layers?
For those that do not know, a PCB (Printed Circuit Board) is typically stacked in layers with copper traces separating them. Two layer boards (top and bottom) are the simplest (not counting single layer boards) and were widely used back in the dawn of computers (even today, really). So stacking more layers in a PCB helps reduce the overall size of the board. Which saves money but complicates the design.