Overview of form factors

Pictorial comparison of some common computer form factors.

A PC motherboard is the main circuit board within a typical desktop computer, laptop or server. It has a number of functions of which the main ones are:

• As a central backbone to which all other modular parts (CPU, RAM, hard drives etc) can be attached as required to create a modern computer
• To accept (on many motherboards) different components (in particular CPU and PCI cards) for the purposes of customization.
• To distribute power to many of the PC components
• To electronically co-ordinate the operation of these, and interface all of these with one another.

As new generations of components have been developed, the standards of motherboards have changed too - for example with AGP being introduced, and more recently PCI Express. However the basic standardized size and layout of motherboard have changed much more slowly, and are controlled by their own standards. This is helped by the fact that in many ways, the list of components a motherboard must include changes far slower than the components themselves. For example, north bridge controllers have changed many times since their original introduction, with many manufacturers bringing out their own versions, but in terms of form factor standards, the requirement to allow for a north bridge has remained fairly static for many years.

Although it is a slower process, form factors do evolve regularly in response to changing demands. The original PC standard (AT) was superseded in 1995 by the current industry standard ATX, which still dictates the size and design of the motherboard in most modern PCs. The latest update to the ATX standard was released in 2004. A divergent standard by chipset manufacturer VIA called EPIA (aka -ITX, and not to be confused with EPIC) is based upon smaller form factors and its own standards.

Differences between form factors are most apparent in terms of their intended market sector, and involve variations in size, design compromises and typical features. Most modern computers have very similar requirements, so form factor differences tend to be based upon subsets and supersets of these. For example, a desktop computer may require more sockets for maximal flexibility and many optional connectors and other features on-board, whereas a computer to be used in a multimedia system may need to be optimized for heat and size, with additional plug-in cards being less common. The smallest motherboards may sacrifice CPU flexibility in favor of a fixed manufacturer's choice.

[edit] Comparisons

[edit] Tabular information

Form factor	Originated	Max. size	Typical feature-set (compared to ATX)	Typical CPU flexibility	Power handling	Notes (Typical usage, Market adoption, etc)
XT	IBM 1983	8.5 × 11" 216 × 279 mm				Obsolete - see Industry Standard Architecture. The IBM Personal Computer XT was the successor to the original IBM PC, its first home computer. As the specifications were open, many clone motherboards were produced and it became a de facto standard.
AT (Advanced Technology)	IBM 1984	12 × 11"–13" 305 × 279–330 mm				Obsolete - see Industry Standard Architecture. Created by IBM for the IBM Personal Computer/AT, an Intel 80286 machine. Also known as Full AT, it was popular during the era of the Intel 80386 microprocessor. Superseded by ATX.
Baby-AT	IBM 1985	8.5" × 10"–13" 216 mm × 254-330 mm				IBM's 1985 successor to the AT motherboard. Functionally equivalent to the AT, it became popular due to its significantly smaller size.
ATX	Intel 1996	12" × 9.6" 305 mm × 244 mm				Created by Intel in 1995. As of 2007, it is the most popular form factor for commodity motherboards. Typical size is 9.6x12" although some companies extend that to 10x12".
SSI CEB	SSI	12" × 10.5" 305 mm × 267 mm				Created by the Server System Infrastructure (SSI) forum. Derived from the EEB and ATX specifications. This means that SSI CEB motherboards have the same mounting holes and the same IO connector area as ATX motherboards.
microATX	1996	9.6" × 9.6" 244 mm × 244 mm				A smaller variant of the ATX form factor (about 25% shorter). Compatible with most ATX cases, but has fewer slots than ATX, for a smaller power supply unit. Very popular for desktop and small form factor computers as of 2007.
Mini-ATX	Intel	11.2" × 8.2" 284 mm × 208 mm
FlexATX	Intel 1999	9.0" x 7.5" 228.6 × 190.5 mm max.				A subset of microATX developed by Intel in 1999. Allows more flexible motherboard design, component positioning and shape. Can be smaller than regular microATX.
Mini-ITX	VIA 2001	6.7" × 6.7" 170 mm × 170 mm max.			100W max	A small, highly-integrated form factor, designed for small devices such as thin clients and set-top boxes.
Nano-ITX	VIA 2003	4.7" × 4.7" 120 mm × 120 mm
Pico-ITX	VIA 2007	100 mm × 72 mm max.
Mobile-ITX	VIA 2007	2.953"× 1.772" 75 mm × 45 mm
BTX (Balanced Technology Extended)	Intel 2004	12.8" × 10.5" 325 mm × 267 mm max.				A standard proposed by Intel as a successor to ATX in the early 2000s.
MicroBTX (or uBTX)	Intel 2004	10.4" × 10.5" 264 mm × 267 mm max.
PicoBTX	Intel 2004	8.0" × 10.5" 203 mm × 267 mm max.
DTX	AMD 2007	200 mm × 244 mm max.
Mini-DTX	AMD 2007	200 mm × 170 mm max.
smartModule	Digital-Logic	66 x 85 mm				Used in embedded systems and single board computers. Requires a baseboard.
ETX	Kontron	95 x 114 mm				Used in embedded systems and single board computers. Requires a baseboard.
Extended ATX (EATX)	?	12" × 13" 305mm × 330 mm				Used in rackmount server systems. Typically used for server-class type motherboards with dual processors and too much circuitry for a standard ATX motherboard. The mounting hole pattern for the upper portion of the board matches ATX.
LPX	?	9" × 11"–13" 229 mm × 279–330 mm				Based on a design by Western Digital, it allowed smaller cases than the AT standard, by putting the expansion card slots on a riser.[1] Used in slimline retail PCs. LPX was never standardized and generally only used by large OEMs.
Mini-LPX	?	8"–9" × 10"–11" 203–229 mm × 254–279 mm				Used in slimline retail PCs
PC/104	PC/104 Consortium 1992	3.8" × 3.6"				Used in embedded systems AT Bus architecture adapted to vibration-tolerant header connectors
PC104plus	PC/104 Consortium 1997	3.8" × 3.6"				Used in embedded systems PCI Bus architecture adapted to vibration-tolerant header connectors
PCI/104Express	PC/104 Consortium 2008	3.8" × 3.6"				Used in embedded systems PCI Express architecture adapted to vibration-tolerant header connectors
NLX	Intel 1999	8"–9" × 10"-13.6" 203–229 mm × 254–345 mm				A low-profile design released in 1997. It also incorporated a riser for expansion cards, and never became popular.
UTX	TQ-Components 2001	88 x 108 mm				Used in embedded systems and IPCs. Requires a baseboard.
WTX	Intel 1998	14" × 16.75" 355.6 mm × 425.4 mm				A large design for servers and high-end workstations featuring multiple CPUs and hard drives.
XTX	2005	95 x 114 mm				Used in embedded systems - requires a baseboard.