Small form factor
||This article needs additional citations for verification. (July 2008)|
A small form factor (SFF) is a computer form factor designed to minimize the volume of a desktop computer. For comparison purposes, the size of an SFF case is usually measured in litres. SFFs are available in a variety of sizes and shapes, including shoeboxes, cubes, and book-sized PCs. Their smaller and often lighter construction has made them popular as home theater PCs and as gaming computers for attending LAN parties. Manufacturers also emphasize the aesthetic and ergonomic design of SFFs since users are more likely to place them on top of a desk or carry them around.
Small form factors do not include computing devices that have traditionally been small, such as embedded or mobile systems. However, "small form factor" lacks a normative definition and is consequently open to interpretation and misuse. Manufacturers often provide definitions that serve the interests of their products.
The acronym SFF. originally stood for "Shuttle Form Factor," describing shoebox-sized personal computers with two expansion slots. The meaning of SFF evolved to include other, similar PC designs from brands such as AOpen and First International Computer, with the word "Small" replacing the word "Shuttle".
Small form factor computers are generally designed to support the same features as modern desktop computers, but in a smaller space. Most accept standard x86 microprocessors, standard DIMM memory modules, standard 3.5 inch hard disks, and standard 5.25 inch optical drives.
However, the small size of SFF cases may limit expansion options; many commercial offerings provide only one 3.5" drive bay and one or two 5.25" external bays. Standard CPU heatsinks do not always fit inside an SFF computer, so some manufacturers provide custom cooling systems. Though limited to one or two expansion cards, a few have the space for 3/4-length cards such as the GeForce GTX-295  Most SFF computers use highly integrated motherboards containing many on-board peripherals, reducing the need for expansion cards.
Even if labeled "SFF," cube-style cases that support full-sized (PS2 form factor) power supplies actually have a microATX form factor. True SFF systems use TFX or smaller power supplies, and some require a notebook-PC style external "power brick."
Some SFF computers even include compact components designed for mobile computers, such as notebook optical drives, notebook memory modules, notebook processors, and external AC adapters, rather than the internal power supply units found in full-size desktop computers.
SFF Types 
Many SFF computers have a cubic shape. Smaller models are typically sold as barebones units, including a case, motherboard, and power supply designed to fit together. The motherboard lies flat against the base of the case. Upgrade options may be limited by the non-standard motherboards, cramped interior space, and power and airflow concerns. The Power Mac G4 Cube, released in 2000, and the Shuttle XPC are good examples of this design. MSI and Asus produce similar designs. The Xi3 Modular Computer is an example of a cube computer with a little more upgrade possibilities.
AOpen Inc. produced a stackable S120 case, allowing the user to stack up to four components vertically or horizontally. These layers can be for add-on cards, optical drives, and hard drives, using either internal power supplies or external AC adapter power sources. After the S120, AOpen made more small form factor cases for systems with Micro ATX and Mini-ITX.
Until recently, SFF cases were usually sold as barebones units (case, power supply, and motherboard) to system integraters and home-based builders. In 2005, Apple Inc. introduced its Mac Mini (volume of 1.4 L, excluding external power brick). Later in the same year, the first AOpen mini PC MP915 (renamed to XC mini in 2007 since “mini PC” could not be registered as a trademark), was announced. The size of the XC mini series PC — 165(W) × 50(H) × 165(D) mm — makes it one of the smallest desktop PC systems (1.3L volume). It was criticized for looking like the Apple Mac Mini, but Apple has not taken action on this subject. In February 2007, AOpen redesigned the case of the mini PC MP945 series..
Since 2006, major OEM PC brands such as HP and Dell have begun to sell fully assembled SFF systems. These are often described as bookshelf units since they resemble a miniature tower case small enough to fit on a bookshelf. The HP Slimline series and Dell C521 (volume 1.65 L) are good examples of this trend. The Maxdata Favorite 300XS is another mini computer. The HP Slimline uses a non-standard motherboard that is very similar in size to Mini-ITX.
In addition to its industrial use, the extremely small Mini-ITX motherboard form factor has also been incorporated into SFF computers. These are often extremely compact, incorporating low-power components such as the VIA C3 processors. The Travla C134 is an example of this design; it is somewhat larger than the Mac mini (7×10×2" vs 6.5×6.5×2"), and barely bigger than a standard 5.25" optical drive.
Beginning in 2007, several other companies have released other very small computers that besides a small size, focus on a low price, and extremely high power efficiency (typically 10 W or below in use). These include the Zonbu, fit-PC, Linutop, and A9home. With the release of Intel ATOM CPU, AOpen also made Nettop systems: the uBox series with model LE200 and LE210. The uBox series equips a dual core Intel Atom 270/330 processor, single channel DDR-II 533/667 memory, Intel 945GC+ICH7 chipset, three SATA connectors and 5.1channel high definition audio output.
Home theatre boxes 
Essentially a bookshelf-style case lying on its side, a miniature HTPC replicates the look of other smaller-than-rack-sized home theatre components such as a DVR or mini audio receiver. The front panel interface is emphasized, with the optical disc drive rotated relative to the case in order to maintain horizontal mounting, and more motherboard port connectors (such as for USB) are routed to the front panel.
A Computer-on-module (COM) is a complete computer built on a single circuit board. They are often used as embedded systems due to their small physical size and low power consumption. Gumstix  is one example of a manufacturer of COMs.
Ultra-Small Form Factor 
Each model of Dell's Optiplex line of computers typically includes an Ultra-Small Form Factor (USFF) chassis option. In the Core 2 era, these machines used 3.5" desktop hard drives and external power supplies, like the Optiplex 745 and 755. More recent units use 2.5" laptop hard drives and have integrated power supplies, like the Optiplex 990 USFF. The compact size comes at the cost of restricted expandability, as USFF models have no PCI or PCIe slots and may have limited CPU and memory options.
See also 
- Joe Rybicki (May 2007). "The Incredible Shrinking Game Machine! Part One: The Small Form-Factor PC". Games for Windows: The Official Magazine (6): 92–96. "Yes, early small form-factor machines (let's just call 'em SFFs) had some issues. OK, a lot of issues. Designed for the gimpiest casual user, these mini-PCs didn't offer many options in the way of upgrades or power. The cases were often too small to fit a full-size videocard [...] Still, two very specific users saw the SFFs' potential. First, home theater enthusiasts realized that these pint-sized PCs made for ideal media center hubs, and second, LAN partygoers naturally glommed on to them for the portability factor. [...] we could see the SFF market gaining a lot more momentum. Until then, enjoy being the first on your block to squeeze a full-size tower's worth of top-level gear into a shoebox-size package."
- SilverStone Technology Co., Ltd. What is SFF (SG03) ?
- Assembly And BIOS : Shuttle’s SX58H7 Ultra-Portable Core i7 Platform
- "List of Small Form Factors," PC/104 and Small Form Factors, 2008
- S120 spec: http://global.aopen.com/products_detail.aspx?auno=2367
- HP and Compaq Desktop PCs - Motherboard Specifications, PTGV-DM (Onyx2)
- uBox news: http://global.aopen.com/news_detail.aspx?auno=10309&ntype=Product%20News
- Gumstix Inc