Motherboard form factor

Pictorial comparison of some common computer form factors.

In mongo9, a form factor specifies the physical dimensions of major system components. Specifically, in the IBM PC compatible industry, standard form factors ensure that parts are interchangeable across competing vendors and generations of technology, while in enterprise computing, form factors ensure that server modules fit into existing rackmount systems. Traditionally, the most significant specification is for that of the motherboard, which generally dictates the overall size of the case. Small form factors have succeeded in shrinking the size of motherboards, but further reduction in overall size is hampered by current power supply technology.

Overview of form factors

A PC motherboard is the main circuit board within a typical desktop computer, laptop or server. Its main functions are as follows:

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

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 standardized size and layout of motherboard have changed much more slowly, and are controlled by their own standards. The list of components a motherboard must include changes far more slowly than the components themselves. For example, north bridge controllers have changed many times since their 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 The original PC standard (AT) was superseded in 1995 by the current industry standard ATX, which still dictates the size and design of the motherboard in most modern PCs.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.

Comparisons

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	AOpen 2005	5.9" × 5.9" 150 mm × 150 mm				Mini-ATX is slightly smaller than Mini-ITX. Mini-ATX motherboards were design with MoDT (Mobile on Desktop Technology) which adapt mobile CPU for lower power requirement, less heat generation and batter application capability.
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				Targeted at smart digital entertainment devices such as PVRs, set-top boxes, media centers and Car PCs, and thin devices.
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, according to Intel the layout has better cooling. BTX Boards are flipped in comparison to ATX Boards, so a BTX or MicroBTX Board needs a BTX case, while an ATX style board fits in an ATX case.
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.
nanoETXexpress	Kontron	55 x 84 mm				Used in embedded systems and single board computers. Requires a carrier board.
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 card. 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.

Graphical comparison of physical sizes

This image compares the sizes of common form factors to ISO 216 paper sizes (e.g. A4) (Sizes are in mm):

Visual examples of different form factors

• Different form factors
• 
  ATX
  (Abit KT7)
• 
  mini-ITX
  (VIA EPIA 5000AG)
• 
  Pico-ITX
  (VIA EPIA PX10000G)

PC/104 and EBX

PC/104 is an embedded computer standard which defines both a form factor and computer bus. PC/104 is intended for embedded computing environments. Single board computers built to this form factor are often sold by COTS vendors, which benefits users who want a customized rugged system, without months of design and paper work.

The PC/104 form factor was standardized by the PC/104 Consortium in 1992.[3] An IEEE standard corresponding to PC/104 was drafted as IEEE P996.1, but never ratified.

The 5.75 x 8.0 in. Embedded Board eXpandable (EBX) specification, which was derived from Ampro's proprietary Little Board form-factor, resulted from a collaboration between Ampro and Motorola Computer Group.

As compared with PC/104 modules, these larger (but still reasonably embeddable) SBCs tend to have everything of a full PC on them, including application oriented interfaces like audio, analog, or digital I/O in many cases. Also it's much easier to fit Pentium CPUs -- whereas it's a tight squeeze (or expensive) to do so on a PC/104 SBC. Typically, EBX SBCs contain: the CPU; upgradeable RAM subassemblies (e.g. DIMM); Flash memory for solid state disk; multiple USB, serial, and parallel ports; onboard expansion via a PC/104 module stack; off-board expansion via ISA and/or PCI buses (from the PC/104 connectors); networking interface (typically Ethernet); and video (typically CRT, LCD, and TV).

Mini PC

Mini PC is a PC form factor very close in size to an external CD or DVD drive.

Examples

• AOpen XC mini
• Apple's Mac mini

References

• The official Intel Form factors website containing form factor descriptions

See also

• Small form factor
• Hard-disk-drive form factors