Short microATX Motherboards

I found that we need not wait for the industry to adopt the FlexATX form factor (although that would be nice), or anything else "new": There is already a number of microATX (µATX) motherboards available which are shorter (less deep) than the standard allows as a maximum. Most of them are still 9.6 inches (244mm) wide, but they are only between 7.2 and 8.3 inches deep (180-210mm). (For non-US readers: 210mm is the width of A4 paper.)

This is somewhat larger than FlexATX (9"x7.5"), but a motherboard depth of 8.25 inches or less allows for a nice case around 9 inches deep. With good, current board availability, and with many of the boards being rather affordable, this seems to be the best bet for building a small desktop PC. (Short of going the Mini-ITX route.)

I window-shopped at a couple of online retailers and put together a page with links and some details for Short µATX Motherboards.

Why Did FlexATX Flop?

There are full-featured, modern FlexATX motherboards: The Kontron KT965/FLEX supports current Intel desktop CPUs, four DIMMs, four expansion slots, and lots of I/O. The Tyan Toledo i965R S5180 supports two DIMMs and two slots. Kontron and Tyan also offer a few other FlexATX server and embedded boards. However, much of what a web search turns up are yesteryear's out-of-stock models.

So FlexATX did not flop entirely, it seems to have enjoyed some popularity and then faded into a niche. What happened? Did CPUs become too hot? Did Intel de-emphasize FlexATX when it created BTX? Did consumers not care enough about size? Or do those who do care about the size of a PC simply choose a laptop?

Does anyone have any insights into the rise and fall of FlexATX?

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.