Form Factor

Advanced Technology (AT) and Baby AT

AT and baby AT form factors were popular till 1997 and they differ on width. The AT motherboard is 12 inches wide by 13 inches long and baby AT is 9 inches wide by 10 inches long.

Both have processor and memory socket toward the front. Long expansion cards extended over them, which made removing the processor difficult and additional chassis fan or active heat sink became a necessity to maintain good airflow across the CPU. AT SMPS gives them 12V and 5V. Additional regulators are needed for 3.3V cards (PCI) or processors are used. 5-pin DIN keyboard connector soldered to the board and serial and parallel port connectors which are attached using cables

This design was widespread due to no clearance and cooling issues but with faster CPUs, needed more cooling so, a better motherboard design was needed.

Advanced Technology Extended (ATX) and Mini ATX

It was developed to solve the problems of earlier design. The ATX form factor size is 12 inches wide by 9.6 inches long. A mini ATX is typically 11.2 inches wide by 8.2 inches long. It has no obstruction for expansion slots and relocation of the peripheral connectors to allow shorter cables. ATX SMPS has side vent to blows air from outside directly across the processor and memory modules, allowing passive heat sinks to be used in most cases, thereby reducing system noise. The ATX has following advantages:

Earlier cables for ports were used but in ATX they are integrated into motherboard.
The ATX power connector can be plugged into the motherboard in one direction only.
ATX have PS/2 mouse ports integrated into motherboard but no PS/2 port in earlier designs
ATX gives easy access to parts than earlier design and more space for additional components.
ATX supports 3.3V power from ATX SMPS for newer processors.
ATX SMPS can turn itself off from a signal from the motherboard so, power management utility to operating systems is offered against physically turning PC off with the power button.
In ATX CPU is closer to SMPS, so better air circulation and cooling.
ATX uses smaller, six-pin mini-DIN keyboard connector or PS/2 connector identical to the mouse PS/2 connector but AT use a five-pin DIN keyboard connector.

Micro-ATX

It was made for a smaller and less expensive motherboard. It offers all features of ATX and

introduced in December 1997. Its size is 9.6 inches x 9.6 inches with fewer expansion slots.