Another indirect effect was caused by the complexity of the printed circuit boards (PCBs), or modules. At the original 25 MHz design speed, impedance in the ground wiring proved to be a serious problem, demanding that the PCBs be as small as possible. As their complexity grew, the PCBs had to add more and more layers in order to avoid growing larger. Eventually, they reached 15-layers deep, which proved to be well beyond the capabilities of draftsmen. The design was ultimately completed using new automated design tools provided by a subcontractor, and the complete design required two years of computer time on a Burroughs mainframe. This was a major step forward in computer aided design, and by the mid-1970s such tools were commonplace.