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 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 IBM's long-standing standard, AT (Advanced Technology), was superseded in 1995 by the current industry standard ATX (Advanced Technology Extended), which still governs the size and design of the motherboard in most modern PCs.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. The E-ATX form factor is not standardized and may vary according to the motherboard manufacturer.
Comparisons
Tabular information
Form factor Originated Date Max. sizeFor 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.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. 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 20 % shorter variant of the ATX form factor. 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 Mini-ATX Intel 1995 or 1996 284 × 208 mm (11.2 × 8.2 in) ATX boards measure 12” x 9.6”, which allows cutting two boards per 24” x 18” panel. In order to fit four boards per panel, Mini-ATX, which measures 11.2" x 8.2", may be used. Estimated to reduce cost of the board by approximately 30%. Removed from the ATX specification in version 2.1 Many so-called ATX boards are actually Mini-ATX. 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. 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.