Smaller Than microATX

There are several motherboard form factors smaller than microATX: FlexATX, DTX, Mini-DTX and Mini-ITX. They are all designed for "half-length" (175mm=6.9" long) or shorter PCI cards, but DTX and Mini-DTX retain microATX's large vertical clearance. FlexATX reduces the clearance to 58mm (2.3"). (Mini-ITX does not appear to specify vertical heights.) DTX is as deep as microATX.

Motherboard electronics have proven to fit onto Mini-ITX motherboards (170mm x 170mm, 6.7"x6.7"), although barely. Mini-ITX systems tend to use soldered CPUs, laptop RAM, single-channel RAM, and/or a smaller I/O panel. Most of them are intended for industrial use; they appear to use 6-layer PCBs rather than 4-layer PCBs common in consumer motherboards, which is acceptable in a relatively high-price niche market. Mini-DTX boards tend to add only a single-lane (x1) PCIe slot compared with Mini-ITX, because with an x16 slot they would hardly gain usable motherboard space, and do not seem to be available on the market (as of November 2007).

FlexATX

The FlexATX form factor seems to be the most attractive existing motherboard form factor for small PCs using desktop components. (Even smaller form factors are appropriate in combination with all-laptop components.) With 229mm x 191mm (9"x7.5") FlexATX is less wide and deep than microATX, it lowers the vertical clearance, and it provides more usable motherboard space than Mini-ITX and Mini-DTX. It allows for up to three expansion slots in a dedicated FlexATX case, or two expansion slots and even more motherboard space. (In a microATX case, an additional slot at the left edge is usable.)

Form Factor Comparison: Usable Motherboard Areas

Size of motherboard areas that allow 30.5mm (1.2") high parts or higher: Motherboard size minus PCI slot area ("area B"), I/O panel area (roughly 6.2"x0.75"), and other areas specified for very low parts height (FlexATX "area E"). Mounting holes are not counted. (Sizes are rounded to two digits.)

Usable Area; Height ≥1.2"	Slots	usable sq in	usable cm2
Mini-ITX	1	37	240
Mini-DTX	2	37	240
FlexATX	4*	43	280
FlexATX	3	48	310
DTX	2	60	390
microATX	4	55 [64]*	350 [410]

Note about FlexATX: One of the FlexATX rows assumes 4 expansion slots, the other one only 3 slots, adding 7"x0.8" of usable motherboard space. Note also that while a microATX case will accommodate up to 4 slots, a dedicated FlexATX case may not accommodate a card in the slot at the left edge. (That is, a FlexATX board with 3 slots may only provide 2 usable PCI slots when used in a FlexATX case.)

Note about microATX: There is a large part of "area B" (under expansion cards) which is usable for very low motherboard parts but not for a northbridge heat sink, memory slots or tall connectors. This area is about 9.4 sq in = 60cm² and included in the bracketed sizes.

Why PCs Are Big

Height

The main reason why even Small Form Factor PCs are either tall or deep is that the popular microATX motherboard form factor requires that parts surrounding the CPU on the motherboard can be 71mm (2.8") tall and that there must be 76mm-89mm (3"-3.5") of clearance above the motherboard in those areas. This means that for reduced height the optical and hard disk drives must be arranged in front of this area, or for reduced depth they must be arranged very high above the motherboard.

Slots

The second reason for relatively large cases is that large PCI cards require both a tall case and also low motherboard electronics under where PCI cards may be installed. Although many SFF PCs opt for low-profile PCI slots which reduce both the height and the length of PCI cards, the microATX form factor requires very low parts height in areas near the front of the motherboard where low-profile PCI cards will not reach.

Width & Depth

Third, the smallest common retail motherboard form factor is microATX, and almost all cases are designed to fit a 244mm x 244mm (9.6"x9.6") motherboard. For a small desktop PC using a modern chipset, this appears to be larger than necessary. Also, microATX has space for 4 expansion slots, but many SFF PCs are built with at most one of them in use.

With a target depth of a small case of less than 250mm (10"), the components cannot be placed in front of one another — they need to be placed side by side and/or stacked vertically. Since the largest components have a depth of 175mm (7") or less, plus connectors, the microATX motherboard depth becomes the limiting factor for the case depth.