Form Factors

Early PCs used the AT form factor and 12in wide motherboards. The sheer size of an AT motherboard caused problems for upgrading PCs and did not allow use of the increasingly popular slimline desktop cases. These problems were largely addressed by the smaller version of the full AT form factor, the Baby AT, introduced in 1989.

Baby AT

The Intel Advanced/ML motherboard, launched in 1996, was designed to solve issues of space and airflow that the Pentium II and AGP graphics cards had caused the preceding LPX form factor. As the first major innovation in form factors in years, it marked the beginning of a new era in motherboard design. Its size and layout are completely different to the BAT format, following a new scheme known as ATX. The dimensions of a standard ATX board are 12in wide by 9.6in long; the mini ATX variant is typically of the order 11.2in by 8.2in.

The ATX design gets round the space and airflow problems by moving the CPU socket and the voltage regulator to the right-hand side of the expansion bus. Extra room is made for the CPU, also  components such as the Flash BIOS, I/O logic and keyboard controller have been scaled down. This means the board need only be half as deep as a full size Baby AT, and there’s no obstruction whatsoever to the expansion slots.


An important innovation was the new specification of power supply for the ATX that can be powered on or off by a signal from the motherboard. This allows  software-controlled power management, as well as shutdown and start-up. A 3.3V output is also provided directly from the power supply. Accessibility of the processor and memory modules is improved dramatically, and relocation of the peripheral connectors allows shorter cables to be used. This also helps reduce electromagnetic interference. The ATX power supply has a side vent that blows air from the outside directly across the processor and memory modules, allowing passive heatsinks to be used in most cases, thereby reducing system noise.

Mini-ATX is simply a smaller version of a full-sized ATX board. On both designs, parallel, Video, and USB ports are located on a double-height I/O shield at the rear. Being soldered directly onto the board generally means no need for cable interconnects to the on-board I/O ports. A consequence of this, however, is that the ATX needs a newly designed case, with correctly positioned cut-outs for the ports, and neither ATX no Mini-ATX boards can be used in AT-style cases.

Mini ATX

Introduced in the late 1990s, the MicroATX is basically a smaller version of Intel’s ATX specification, intended for compact, low-cost consumer systems with limited expansion potential.

Micro ATX

The maximum size of the board is 9.6in square, and its designed to fit into either a standard ATX case or one of the new micro-tower desktop designs. The double-decker I/O shield is the same as that on the ATX design, but there’s only provision for up to four expansion slots as opposed to the seven that ATX allows. The microATX also allows use of a smaller power supply, such as the SFX design, which is reduced in both size and power output.