Motherboard Form Factors

You’ve probably heard the term motherboard a thousand times, but do you know what it really means and how it relates to the rest of your computer?

The form factor of a motherboard determines the specifications for its general shape and size.

Case and Power Supply

It also specifies what type of case and power supply will be supported, the placement of mounting holes, and the physical layout and organization of the board.




Form factor is especially important if you build your own computer systems and need to ensure that you purchase the correct case and components.

The motherboard form factor describes its general shape, the type of case and power supply it can use, and its physical organization (layout of the motherboard).

Over time, in the computer industry, we have had a number of different motherboard form factors being developed.

AT and Baby AT (Advanced Technology)

In the early days of the computer, the AT and baby AT form factors were the most common motherboard form factors. These two variants differ primarily in width: the older full AT board is 12″ wide.




It is an obsolete motherboard form factor only found in older machines, 386 class or earlier.

One of the major problems with the width of this board (aside from limiting its use in smaller cases) is that a good percentage of the board “overlaps” with the drive bays. This makes installation, troubleshooting and upgrading more difficult.

A Baby AT motherboard is 8.5″ wide and 13″ long. The reduced width means much less overlap in most cases with the drive bays, although there usually is still some overlap at the front of the case.

Baby AT motherboards are distinguished by their shape, and usually by the presence of a single, full-sized keyboard connector soldered onto the board.

The serial and parallel port connectors are almost always attached using cables (ribbons) that go between the physical connectors mounted on the case, and pin “headers” located on the motherboard.

Most of the boards use AT power supplies and the system units tend to be tower casing.

Advantages of the Baby AT Motherboard Design

The size of 8.5” by 10” makes it easier to design smaller desktop PCs
Most of the board is easily accessible for upgrades and expansion

Disadvantages of the Baby AT design

CPU location – with the processor and heat sink in place, it is difficult to fit a long expansion card into one of the expansion slots. This is the main problem encountered with the AT-style motherboard-the CPU can get in the way of the expansion cards.

Motherboard mounting – some system cases are not drilled or punched to support all the mounting holes on a Baby AT mother-board.

Therefore, the front edge of the system board tends to be left unsupported and over time this edge can warp (bend) leading to loose components and expansion cards causing intermittent problems.

Some Improvements of the ATX Motherboard Form Factor

  • Integrated I/O Port Connectors: Baby AT motherboards use headers which stick up from the board, and a cable that goes from them to the physical serial and parallel port connectors mounted on to the case. The ATX has these connectors soldered directly onto the motherboard.
  • Integrated PS/2 Mouse Connector: ATX motherboards have the PS/2 port built into the motherboard.
  • Reduced Drive Bay Interference: Since the board is essentially “rotated” 90 degrees from the baby AT style, there is much less “overlap” between where the board is and where the drives are thus making it easy to access the board, and fewer cooling problems.
  • Reduced Expansion Card Interference: The processor socket/slot and memory sockets are moved from the front of the board to the back right side, near the power supply. This eliminates the clearance problem with baby AT style motherboards and allows full length cards to be used in most (if not all) of the system bus slots.
  • Better Power Supply Connector: The ATX motherboard uses a single 20-pin connector instead of the confusing pair of near-identical 6-pin connectors on the baby AT form factor.
  • “Soft Power” Support:The ATX power supply is turned on and off using signaling from the motherboard, not a physical toggle switch. This allows the PC to be turned on and off under software control, allowing much improved power management.
  • 3.3V Power Support: The ATX style motherboard has support for 3.3V power from the ATX power supply.
  • Improved Design for Upgradability: In part because it is the newest design, the ATX is the choice “for the future”. More than that, its design makes upgrading easier because of more efficient access to the components on the motherboard.

Full-ATX – (12″ wide x 9.6″ deep) / Mini-ATX – (11.2″ wide x 8.2″ deep).

The ATX, Created by Intel in 1995, was developed as an evolution of the Baby AT form factor and was defined to address four areas of improvement:-

  • Enhanced ease of use
  • Better support for current and future I/O
  • Better support for current and future processor technology, and
  • Reduced total system cost.

The ATX is basically a Baby AT rotated 90 degrees and providing a new mounting configuration for the power supply. The processor is relocated away from the expansion slots, (unlike Baby AT) allowing them to hold full length add-in cards.

The longer side of the board is used to host more on-board I/O ports. The ATX power supply, rather than blowing air out of the chassis, as in most Baby AT platforms, provides air-flow through the chassis and across the processor.

motherboard computer

MicroATX Motherboard Form Factor

This form factor was developed as a natural evolution of the ATX form factor to address new market trends and PC technologies. MicroATX supports:

  • Current processor technologies
  • The transition to newer processor technologies
  • AGP high performance graphics solutions
  • Smaller motherboard size
  • Smaller power supply form factor

Flex ATX

This is a subset of MicroATX developed by Intel in 1999. It allows more flexible motherboard design, component positioning and shape. Can be smaller than regular microATX.

  • Supports current socketed processor technologies
  • Smaller motherboard size
  • ATX 2.03 I/O panel
  • Same mounting holes as microATX
  • Socket only processors to keep the size small

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