Motherboard Form Factors

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).

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 nominally 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 design

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

Disadvantages of the Baby AT design

1. 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.
2. 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.

ATX and Mini ATX

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. 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.

Some improvements of the ATX motherboard form factors:-

• 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.

MicroATX

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

FlexATX

A subset of microATX developed by Intel in 1999. 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

LPX and Mini LPX (Low Profile casing eXtended)

The LPX motherboard form factors are designed to be used in small Slimline or "low profile" cases typically found on these sorts of desktop systems. The primary design goal behind the LPX form factor is reducing space usage (and cost).

The most distinguishing feature is the riser card that is used to hold expansion slots. The riser card of the LPX motherboard form factor is situated at the center of the motherboard. Expansion cards plug into the riser card; usually, a maximum of just three. This means that the expansion cards are parallel to the plane of the motherboard. This allows the height of the case to be greatly reduced, since the height of the expansion cards is the main reason full-sized desktop cases are as tall as they are. The problem is that you are limited to only two or three expansion slots!

While the LPX form factor can be used by a manufacturer to save money and space in the construction of a custom product, these systems suffer from non-standardization, poor expandability, poor upgradability, poor cooling and difficulty of use for the do-it-yourself.

NLX (New Low profile eXtended)

The need for a modern, small motherboard standard has lead to the development of the new NLX form factor. In many ways, NLX is similar to LPX. Also like ATX, the NLX standard was developed by Intel Corporation in 1998.

NLX still uses the same general design as LPX, with a smaller motherboard and a riser card for expansion cards. The riser card is pushed to one extreme edge of the motherboard.

NLX makes the following main changes:-

• Revised design to support larger memory modules and modern DIMM memory packaging.
• Support for the newest processor technologies, including the new Pentium II using SEC packaging.
• Support for AGP video cards.
• Better thermal characteristics, to support modern CPUs that run hotter than old ones.
• More optimal location of CPU on the board to allow easier access and better cooling.
• More flexibility in how the motherboard can be set up and configured.
• Enhanced design features, such as the ability to mount the motherboard so it can slide in or out of the system case easily.
• Support for desktop and tower cases.