In computing, the motherboard form factor is the specification of a motherboard – the dimensions, power supply type, location of mounting holes, number of ports on the back panel, etc. 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 been developed and implemented.

Overview of form factors

Comparison of some common motherboard form factors (pen for scale)

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 computer
  • To be interchangeable (in most cases) with different components (in particular CPU and expansion cards) for the purposes of customization and upgrading
  • To distribute power to other circuit boards
  • 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, the introduction of AGP and, more recently, PCI Express have influenced motherboard design. However, the standardized size and layout of motherboards have changed much more slowly and are controlled by their own standards. The list of components required on a motherboard changes far more slowly than the components themselves. For example, north bridge microchips have changed many times since their introduction with many manufacturers bringing out their own versions, but in terms of form factor standards, provisions for north bridges have remained fairly static for many years.

Although it is a slower process, form factors do evolve regularly in response to changing demands. IBM's long-standing standard, AT (Advanced Technology), was superseded in 1995 by the current industry standard ATX (Advanced Technology Extended), which still governs the size and design of the motherboard in most modern PCs. The latest update to the ATX standard was released in 2007. A divergent standard by chipset manufacturer VIA called EPIA (also known as 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 maximum 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 Date Max. size[info 1]
width × depth		 Notes
(typical usage, Market adoption, etc.)
XT IBM 1983 216 × 279 mm
(8.5 × 11 in)		 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 305 × 279–330 mm
(12 × 11–13 in)		 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 216 × 254–330 mm
(8.5 × 10–13 in)		 IBM's 1985 successor to the AT motherboard. Functionally equivalent to the AT, it became popular due to its significantly smaller size.
ATX Intel 1995 305 × 244 mm
(12 × 9.6 in)		 Created by Intel in 1995. As of 2017, it is the most popular form factor for commodity motherboards. Typical size is 9.6 × 12 in although some companies extend that to 10 × 12 in.
SSI CEB SSI ? 305 × 267 mm
(12 × 10.5 in)		 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.
SSI EEB SSI ? 305 × 330 mm
(12 × 13 in)		 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, but SSI EEB motherboards do not.
SSI MEB SSI ? 411 × 330 mm
(16.2 × 13 in)		 Created by the Server System Infrastructure (SSI) forum. Derived from the EEB and ATX specifications.
microATX Intel 1996 244 × 244 mm
(9.6 × 9.6 in)		 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 2017.
Mini-ATX AOpen 2005 150 × 150 mm
(5.9 × 5.9 in)		 Mini-ATX is considerably smaller than Micro-ATX. Mini-ATX motherboards were designed with MoDT (Mobile on Desktop Technology) which adapt mobile CPUs for lower power requirement, less heat generation and better application capability.
FlexATX Intel 1999 228.6 × 190.5 mm max
(9.0 × 7.5 in)		 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 170 × 170 mm max
(6.7 × 6.7 in)		 A small, highly integrated form factor, designed for small devices such as thin clients and set-top boxes.
Nano-ITX VIA 2003 120 × 120 mm
(4.7 × 4.7 in)		 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 × 72 mm max
(3.9 × 2.8 in)
Mobile-ITX VIA 2007 75 × 45 mm
(2.953 × 1.772 in)
Neo-ITX VIA 2012 170 × 85 × 35 mm
(6.69 × 3.33 × 1.38 in)		 Used in the VIA Android PC
Deep Mini-ITX ASRock Rack ? 170 × 208 mm max
(6.7 × 8.2 in)		 Proprietary Mini-ITX variant i.a. for more than two memory slots, designed for chassis with support for Micro-ATX size and above
Mini-STX Intel 2015 147 × 140 mm
(5.79 × 5.51 in)		 Smaller than Mini-ITX, but bigger than the NUC, this board is used in small form factor computers, using a socketed intel core processor and SO-DIMMS.
BTX (Balanced Technology Extended) Intel 2004 325 × 267 mm max
(12.8 × 10.5 in)		 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. The RAM slots and the PCI slots are parallel to each other.

Processor is placed closest to the fan. May contain a CNR board.

MicroBTX (or uBTX) Intel 2004 264 × 267 mm max
(10.4 × 10.5 in)		 MicroBTX (also called uBTX) is a computer motherboard form factor. A microBTX is 10.4 × 10.5 in (264 × 267 mm) and can support up to four expansion slots.
DTX AMD 2007 200 × 244 mm max
(8.0 × 9.6 in)		 DTX backward compatible with ATX cases. Shorter variant, Mini-DTX (mDTX) of 8 × 6.7 inches (203 × 170 mm)
smartModule Digital-Logic ? 66 × 85 mm
(2.60 × 3.35 in)		 Used in embedded systems and single-board computers. Requires a baseboard.
ETX Kontron 1999 95 × 114 mm
(3.74 × 4.49 in)		 Used in embedded systems and single-board computers. Requires a baseboard.
COM Express Basic PICMG 2005 95 × 125 mm
(3.74 × 4.9 in)		 Used in embedded systems and single-board computers. Requires a carrier board.
COM Express Compact PICMG 2005 95 × 95 mm
(3.74 × 3.74 in)		 Used in embedded systems and single-board computers. Requires a carrier board.
COM Express Mini PICMG 2005 55 × 84 mm
(2.17 × 3.31 in)		 Used in embedded systems and single-board computers. Requires a carrier board. Adheres to pin-out Type 10[1]
COM-HPC Size A PICMG 2020 95 × 120 mm
(3.7 × 4.7 in)		 Used in embedded systems. Requires a carrier board. Typically used for COM-HPC Client Type modules.
COM-HPC Size B PICMG 2020 120 × 120 mm
(4.7 × 4.7 in)		 Used in embedded systems. Requires a carrier board. Typically used for COM-HPC Client Type modules.
COM-HPC Size C PICMG 2020 160 × 120 mm
(6.3 × 4.7 in)		 Used in embedded systems. Requires a carrier board. Typically used for COM-HPC Client Type modules with multiple SODIMM memory sockets.
COM-HPC Size D PICMG 2020 160 × 160 mm
(6.3 × 6.3 in)		 Used in embedded systems. Requires a carrier board. Typically used for COM-HPC Server Type modules with 4x full size DIMM memory sockets.
COM-HPC Size E PICMG 2020 200 × 160 mm
(7.9 × 6.3 in)		 Used in embedded systems. Requires a carrier board. Typically used for COM-HPC Server Type modules with 8x full size DIMM memory sockets.
CoreExpress SFF-SIG ? 58 × 65 mm
(2.28 × 2.56 in)		 Used in embedded systems and single-board computers. Requires a carrier board.
Extended ATX (EATX) Un­known ? 305 × 330 mm
(12 × 13 in)		 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.
Enhanced Extended ATX (EEATX) Supermicro ? 347 × 330 mm
(13.68 × 13 in)		 Used in rackmount server systems. Typically used for server-class type motherboards with dual processors and too much circuitry for a standard E.ATX motherboard.
LPX Western Digital ? 229 × 279–330 mm
(9 × 11–13 in)		 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 Western Digital ? 203–229 × 254–279 mm
(8–9 × 10–11 in)		 Used in slimline retail PCs.
PC/104 PC/104 Consortium 1992 97 × 91 mm
(3.8 × 3.6 in)		 Used in embedded systems. AT Bus (ISA) architecture adapted to vibration-tolerant header connectors.
PC/104-Plus PC/104 Consortium 1997 97 × 91 mm
(3.8 × 3.6 in)		 Used in embedded systems. PCI Bus architecture adapted to vibration-tolerant header connectors.
PCI/104-Express PC/104 Consortium 2008 97 × 91 mm
(3.8 × 3.6 in)		 Used in embedded systems.
PCI Express architecture adapted to vibration-tolerant header connectors.
PCIe/104 PC/104 Consortium 2008 97 × 91 mm
(3.8 × 3.6 in)		 Used in embedded systems.
PCI/104-Express without the legacy PCI bus.
NLX Intel 1999 203–229 × 254–345 mm
(8–9 × 10–13.6 in)		 A low-profile design released in 1997. It also incorporated a riser for expansion cards,[2] and never became popular.
UTX TQ-Components 2001 88 × 108 mm
(3.46 × 4.25 in)		 Used in embedded systems and IPCs. Requires a baseboard.
WTX Intel 1998 355.6 × 425.4 mm
(14 × 16.75 in)		 A large design for servers and high-end workstations featuring multiple CPUs and hard drives.
SWTX Supermicro ? 418 × 330 mm
(16.48 × 13 in)		 A proprietary design for servers and high-end workstations featuring multiple CPUs.
HPTX EVGA 2008 345 × 381 mm
(13.6 × 15 in)		 A large design by EVGA currently featured on two motherboards; the eVGA SR2 and SRX. Intended for use with multiple CPUs. Cases require 9 expansion slots to contain this form-factor.
XTX Ampro / Congatec 2005 95 × 114 mm
(3.74 × 4.49 in)		 Used in embedded systems. Requires a base.
  1. ↑ For boards which take expansion slots, the length of the expansion card aligns with the depth of the system board. The case may support cards longer than the depth of the mainboard.

Size variants

List is incomplete

Form factor Originated Date Max. size[lower-alpha 1]
width × depth		 Slots Notes
(typical usage, Market adoption, etc.)
ATX Intel 1995 12 × 9.6 in (305 × 244 mm) 7 Original, successor to AT motherboard
Proprietary, specific to crypto-mining specific motherboards Un­known 2011 12 × 8 in (305 × 203 mm) 3 3 double-slot add-in cards with 1 slots of free space in between
SSI CEB SSI ? 12 × 10.5 in (305 × 267 mm) 7 Compact Electronics Bay
SSI MEB SSI 2011 16.2 × 13 in (411 × 330 mm) 12 Midrange Electronics Bay
SSI EEB SSI ? 12 × 13 in (305 × 330 mm) 7 Enterprise Electronics Bay
SSI TEB SSI ? 12 × 10.5 in (305 × 267 mm) 7 Thin Electronics Bay, for rack-mount, has board component height specification
microATX Intel 1997 9.6 × 9.6 in (244 × 244 mm) 4 Fits in ATX, and EATX cases.
FlexATX Intel 1997 9 × 7.5 in (229 × 191 mm) 3
Extended ATX (standard) Supermicro / Asus ? 12 × 13 in (305 × 330 mm) 7 Screw holes not completely compatible with some ATX cases. Designed for dual CPUs, and quad double slot video cards.
Extended ATX (commonly) Un­known ? 12 × 10.1 in (305 × 257 mm)
12 × 10.4 in (305 × 264 mm)
12 × 10.5 in (305 × 267 mm)
12 × 10.7 in (305 × 272 mm)		 7 ATX pattern screw holes
EE-ATX Supermicro ? 13.68 × 13 in (347 × 330 mm) 7 Enhanced Extended ATX
Ultra ATX Foxconn 2008 14.4 × 9.6 in (366 × 244 mm) 10 Intended for multiple double-slot video cards, and dual CPUs.
XL-ATX EVGA 2009 13.5 × 10.3 in (343 × 262 mm) 9
XL-ATX Gigabyte 2010 13.58 x 10.31 in (345 x 262 mm) 7
XL-ATX MSI 2010 13.6 × 10.4 in (345 × 264 mm) 7
WTX Intel 1998 14 × 16.75 in (356 × 425 mm). 9 Discontinued 2008
Mini-ITX VIA 2001 6.7 x 6.7in (170 × 170 mm). 1 Originally designed for home theatre or other fanless applications
Mini-DTX AMD 2007 8 × 6.7 in (203 × 170 mm) 2 Derived from Mini-ITX and DTX
BTX Intel 2004 12.8 × 10.5 in (325 × 267 mm) 7 Canceled 2006. Also micro, nano, and pico variants. Not generally compatible with ATX mounting.
HPTX EVGA 2010 13.6 × 15 in (345 × 381 mm) 6 Dual processors, 12 RAM slots
SWTX Supermicro 2006 16.48 × 13 in (419 × 330 mm)
and others		 5 Quad processors, not compatible with ATX mounting

Maximum number of expansion card slots

ATX case compatible:

SpecificationNumber
HPTX9
ATX/EATX/SSI EEB/SSI CEB7
MicroATX4
FlexATX3
DTX/Mini-DTX2
Mini-ITX1

Visual examples of different form factors

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.[4]

The 5.75 × 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 drive; 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 small form factor very close in size to an external CD or DVD drive. Mini PCs have proven popular for use as HTPCs.

Examples

See also

Notes

  1. ↑ For boards which take expansion slots, the length of the expansion card aligns with the depth of the system board. The case may support cards longer than the depth of the mainboard.

References

  1. ↑ "Atom module shrinks to nano size". Archived from the original on 2012-05-24. Retrieved 2009-12-30.
  2. ↑ "Form Factors Rev 1.3 :: NLX" Motherboards.org
  3. ↑ "PC/104 Embedded Consortium's History". Archived from the original on 2008-02-11. Retrieved 2008-01-29.
  4. ↑ Angel, Jonathan (2010-02-01). "Open standard defines tiny expansion modules". LinuxDevices.com. Retrieved 2014-03-18.
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