ATX

With the need for a more integrated form factor which defined standard locations for the keyboard, mouse, I/O, and video connectors, in the mid 1990's the ATX form factor was introduced. The ATX form factor brought about many chances in the computer. Since the expansion slots were put onto separate riser cards that plugged into the motherboard, the overall size of the computer and its case was reduced. The ATX form factor specified changes to the motherboard, along with the case and power supply. Some of the design specification improvements of the ATX form factor included a single 20-pin connector for the power supply, a power supply to blow air into the case instead of out for better air flow, less overlap between the motherboard and drive bays, and integrated I/O Port connectors soldered directly onto the motherboard. The ATX form factor was an overall better design for upgrading.
micro-ATXMicroATX followed the ATX form factor and offered the same benefits but improved the overall system design costs through a reduction in the physical size of the motherboard. This was done by reducing the number of I/O slots supported on the board. The microATX form factor also provided more I/O space at the rear and reduced emissions from using integrated I/O connectors.