In 1984, at a time when the idea of the computer becoming a household item was considered laughable (computers were simply too expensive in the 1980s), IBM introduced the Personal Computer/AT (Advanced Technology), and the AT form factor along with it. By specifying everything exactly, from the motherboard’s dimensions and the maximum height of the installed components to the spacing of the motherboard mounting points and the standardized power supply, IBM enabled third-party companies to develop computer hardware that was compatible with its Personal Computer/AT. This proved to be a smart decision, contributing greatly to the speedy adoption of the PC. A short while later, the Baby-AT form factor was introduced as well, allowing more compact systems while still using cases that followed the AT specification.

In 1996, technological innovations made it necessary to introduce a new form factor, called ATX (Advanced Technology Extended), which dominates the PC market to this day. Designed by Intel, ATX represented a kind of evolution of the Baby-AT standard and focused on simpler handling, improved support for all current and upcoming interfaces (at the time) such as USB, and better support for current and future processor technologies. One of the changes that came with the ATX standard was the integration of the external connectors in the connector shield, although motherboard makers are free to change the layout within that area. This step also helped reduce electromagnetic emissions, since the connectors for the parallel and serial interface were integrated right on the motherboard instead of requiring cables leading to the back. On the whole, an ATX motherboard is basically a Baby AT board rotated by 90° and a relocated processor socket. The new layout freed up space for more expansion slots and simplified the use of capacitors and voltage regulation modules that were indispensable for the new generation of processors.

The IDE and floppy connectors on the circuit board moved closer to their respective drive bays, which allowed short ribbon cables. This step permitted the use of IDE drives using the ATA/ATAPI-4 and ATA/ATAPI-5 standards (Advanced Technology Attachment) with a maximum data transfer rate of 33.3 MB/s and 66.6 MB/s respectively. Unlike the AT standard, the ATX specification also called for a standardized power supply design, with the result that cables, connectors and switches were standardized as well. Obviously, this finally brought easy and safe upgradability for power supplies as well, which had previously been more a matter of luck, due to compatibility issues.

The primary concern when designing the new ATX standard-compliant case was to ensure the best possible ventilation of the computer’s core components, e.g. the CPU and the graphics card. Thus, the ATX specification calls for the power supply to vent some of the warm air out of the case. In order to achieve this, the power supply needed to be located near the processors socket. Additional fans located at strategic places inside the case were to improve airflow even further. Despite these measures, Intel still managed to come very close to the thermal limits of the ATX specification when it introduced the Pentium 4 Processor.

Due to this processor line’s high thermal profile, ventilation became an ever greater concern, leading Intel to attempt to improve cooling by partitioning the case into thermal zones and channelling airflow more efficiently. Thus, Intel’s BTX form factor was born in 2003 – only to be buried again in 2007, when the company stopped supporting this format. The explanation is simple – the thermal profile of current Core 2 Duo systems no longer poses a problem to the cooling capabilities of the ATX specification. Also, many of the innovations of BTX also found their way into newer ATX cases, alleviating some of the other problems. Another reason for the very limited success of the BTX form factor was that its layout had been designed with Intel CPUs in mind. Implementing it for the AMD platform with its integrated memory controller proved very problematic. This was one of the main reasons AMD never supported the BTX standard.