Form factor

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. Whilst this remains a common form factor, there have been several improvements since. All designs are open standards and as such don't require certification. A consequence is that there can be some quite wide variation in design detail between different manufacturers' motherboards.

BAT

The Baby AT (BAT) format reduced the dimensions of the motherboard to a typical 9in wide by 10in long, and BAT motherboards are generally characterised by their shape, an AT-style keyboard connector soldered to the board and serial and parallel port connectors which are attached using cables between the physical ports mounted on the system case and corresponding connectors located on the motherboard.

With the BAT design the processor socket is located at the front of the motherboard, and full-length expansion cards are intended to extend over it. This means that removing the processor requires the removal of some or all expansion cards first. Problems were exacerbated by the increasing speeds of Pentium-class processors. System cooling relied on the AT power supply blowing air out of the chassis enclosure and, due to the distance between the power supply and the CPU, an additional chassis fan or active heatsink became a necessity to maintain good airflow across the CPU. AT power supplies only provide 12V and 5V outputs to the motherboard, requiring additional regulators on the motherboard if 3.3V components (PCI cards or CPUs) are used. Sometimes a second heatsink was also required on these voltage regulators and together the various additional heat dissipation components caused serious obstruction for expansion slots.

Some BAT designs allow the use of either AT or ATX power supplies, and some ATX cases might allow the use of a Baby-AT motherboard.

LPX

The LPX format is a specialised variant of the Baby-AT used in low profile desktop systems and is a loose specification with a variety of proprietary implementations.

Expansion slots are located on a central riser card, allowing cards to be mounted horizontally. However, this arrangement can make it difficult to remove the motherboard, and the more complex engineering required adds to system costs. As the riser card prevents good airflow within the system case, additional chassis fans are almost always needed.

ATX

The Intel Advanced/ML motherboard, launched in 1996, was designed to solve these issues and 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 problem by moving the CPU socket and the voltage regulator to the right-hand side of the expansion bus. Room is made for the CPU by making the card slightly wider, and shrinking or integrating components such as the Flash BIOS, I/O logic and keyboard controller. This means the board need only be half as deep as a full size Baby AT, and there's no obstruction whatsoever to the six expansion slots (two ISA, one ISA/PCI, three PCI).