Cooling

Small desktop PCs need some attention to cooling. They need fans because they are built around desktop components and to accommodate at least some low-profile expansion cards. On the other hand, they do not require the several large fans of a high-performance, high-power PC. The lower the power consumption of the CPU and the whole system, the less cooling it needs.

The 80-92mm fans in SFX and TFX power supplies help cool the entire case. In addition, a fan may be needed in front of the expansion slots if a graphics card is installed, or even just for better cooling of the northbridge and other components.

Even a mid-power CPU (up to about 65W) needs special consideration in a small case. Low-profile designs with an optical drive above the CPU (about the only place where one will fit in a case less than 10 inches deep) have less than the 3 inches (76mm) of clearance above the motherboard that microATX requires and thus do not fit stock coolers.

One possibility is to remove the fan from the stock heat sink and mount it nearby, or to mount a smaller 40-50mm fan near or directly behind the heat sink. Another is to use a very low-profile heat sink and mount a fan on top of it, with air guides to ensure that the fan draws cool air.

Small PC Power Supplies

Power supply form factors are surprisingly fragmented. There are several very different shapes and sizes that are called "SFX", and significant variations in the sizes of TFX and FlexATX power supplies. Some commercial systems use custom power supplies.

The more promising power supply form factors seem to be:

• "SFX12V-J" which is 63.5mm x 125mm x 100mm (WxDxH 2.5"x4.9"x3.9") with a large 80mm-92mm side fan. This fits well in a case that also places HDDs and/or a memory card reader vertically and on one side.
• TFX which is 70mm x 175mm x 85mm (2.8"x6.9"x3.4") with a 60mm-80mm side fan. This is a better fit for lower-profile cases that accommodate low-profile expansion slots but not a vertical HDD.
• FlexATX which is 40.5mm x 150mm x 81.5mm (1.6"x6"x3.2") with a 40mm fan at the inner face for very small cases.
• Mini power supplies that plug into the motherboard power connector and have a single-voltage input. However, for best usability, there should be another, 12V-only power supply in the case, and there appear to be no standard form factors for that.
  • These mini power supplies are just one step removed from integrating all electronics for generating the motherboard voltages from a single-voltage 12V supply, which would simplify system design and distribute heat sources. It would require additional motherboard real estate for more on-board voltage regulators but replace the large 24-pin power connector with a much smaller one.

A side fan seems desirable, to draw air across the PCI slots or across the CPU depending on the placement. A side fan also has a larger diameter than an end fan, reducing fan speed and noise.

In terms of power, some commercially available SFF PCs have power supplies with as little as 150W of output.

Some 220W-270W FlexATX power supplies are "80 plus" certified. Higher efficiency significantly reduces cooling requirements. For example, for 180W output:

• 60% efficient: 300W input, 120W waste heat to dissipate
• 80% efficient: 225W input, 45W waste heat to dissipate

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.

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.

Desktop Components

The attraction of using desktop components as much as possible (as opposed to laptop components) lies in their lower cost, larger selection, higher speed and/or capacity, and earlier availability of new features. For example, for the hard disk drive (HDD) a large capacity at low cost is highly desired, and high speed helps with system and application startup and memory paging.

For the optical disk drive (usually a DVD burner), which is the largest single piece of equipment in a small PC but tends to be used relatively little and tends to be relatively inexpensive, using a slim-line version is the most tempting, and the most effective in reducing overall size. An exception to the low usage and cost is a Home Theater PC which may want to be equipped with the latest technology, meaning HD DVD or BD right now. Even LightScribe disk labeling is hard to get in a slim drive. Aside from cost, this is a size vs. features trade-off: A desktop optical drive is possible for small desktop PCs with 8-10 liters of volume but not for smaller ones.

Small desktop PCs should be able to use desktop CPUs and desktop memory, unlike mini PCs.

Size and Shape of a VCR

Here is a picture of a VCR. It's about 14.19" wide, 9.25" deep and 3.625" high (360mm x 235mm x 92mm), which comes out to a volume of about 7.8 liters (a tad over 2 gallons).

A PC with desktop components might have to be a little taller or wider, but not by much.

Wanting to Build or Buy a Small PC

I have always liked small but affordable computers. In the age of dinosaurs, I had an Atari Mega ST, a slim pizza box of a computer that served as the monitor stand and had no wasted space once I put a hard disk inside of it.

A few months ago, I wanted to build a small PC: Buy a barebones system, motherboard, CPU etc. and put them together. Building it was to be part of the fun, aside from getting a useful computer out of it. I started window shopping and looking online for cases and motherboards, parts and specs.

I found a lot of systems and cases that are bigger than I like, even what are called "Small Form Factor" (SFF) systems. They are either quite tall, about 8 inches (20cm) or more, and have a shape that I am not very fond of, or they are slim and look slick from the front, but are then usually 16 inches (40cm) deep. If your desk is 26 inches deep, then that's a lot.

There are a couple of slim, less deep PCs available from big manufacturers which come close, but they all have some fatal flaw, and it's not possible to just opt for a different motherboard because they use non-standard sizes.

And then there is the Mac Mini which has inspired "mini PCs" of similar size. They are built entirely with laptop components, which makes them more expensive, and they have fewer features. The power brick and an external memory card reader add back some of the size, and add to the cable mess.

There should be room (and a market) for small desktop PCs that are fairly full-featured, use mostly desktop components, and are sized between SFF PCs and mini PCs. They would be less than 10 inches (25cm) deep to fit on short desks, on kitchen counters and into bookshelves. They would have an overall volume of less than 10 liters (2.5 gallons), and all components would be integrated into the case (including the power supply and at least a minimal memory card reader).

A typical VCR has a volume of around 8 liters (2 gallons) or less. A PC with the size and shape of a VCR fits well into entertainment systems and should be attractive as a Home Theater PC.

By the way, I did build a PC recently, with one of the smallest microATX cases on the market, and it's indeed a little taller, deeper and more voluminous than I like. It measures about 13.5 liters (3.5 gallons).

I would like to share my thoughts and would love to hear from others who are interested in buying or designing a small desktop PC.