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Motherboard Form Factors

In order to view the stunningly gorgeous (yeah, right) ASCII drawings below, you need to use a proportional font (such as Courier or Courier New) as your second font. Otherwise, it'll be a jumbled mess. I didn't do line drawings because I have no talent, and this web page is a free personal home page with very limited server space.

Much of the information about the different form factor types comes from the Intel ATX Specification. The Hardware Book also has pinouts of various power connectors. And some good tidbits of info can be found on Ars Technica's Case and Cooling Fetish FAQ.

Each letter represents a different mounting hole for the various board types. The 'mounting holes' are the holes used for either the plastic spacers or the metal risers. The listed types include ATX, Mini-ATX, Micro-ATX, Baby-AT, and Full-AT. Neither LPX nor NPX are listed because of their unusual riser card. See The PC Guide's Form Factors page for more information. And please note that Mini-ATX and Micro-ATX are not the same thing. More info on the Micro-ATX form factor can be found on the Form Factor's website.

Be aware that the motherboard in your current computer may actually not be included here. Especially if you bought one of the mass-market OEM machines (such as Packard Bell, Compaq, IBM, etc.) that used a proprietary (non-standard or LPX) form factor. The only upgrades for cases such as those is to buy another proprietary form factor motherboard from the same manufacturer (assuming they sell anymore like yours - don't bet on it) or get a new case along with a new motherboard.

Note that it is entirely possible to have a system case whose mounting holes line up exactly with one of the standard form factors, but still not be able to use it with a standard size motherboard because the I/O ports on the back of the mainboard don't align with the cutouts in back of the case. Shuttle has a page showing the numerous different types of ATX backplates. And Antec has a more recent listing.

All of the motherboards below are oriented so that the I/O connectors (keyboard, etc.) would be on the right side down near the bottom.

All types: (in inches)

    3.35  1.4 1.45     6.1
  /------/---/---/-------------/
  |      |   |   |             |
+--------------------------------+ 
| N      K   T   G             A |---/-
|                                |   |3.1
|                R               |   |
|                S             B |---/-
|                                |   |1.8
|                                |   |
| O      L   U   H             C |---/-
|                                |   |2.8
|                                |   |
|   P              I           D |---/-
|                                |+  |
|                                ||  |3.4
|            V   W           X   |+  |
| Q      M       J           F E |---/-
+--------------------------------+
  | |    |       | |           |
  /-/----/-------/-/-----------/
 0.9 2.45  2.85  0.9    5.2

ATX:
       +-------------------------+ 
       | K       G             A |---/-
       |                         |   |
       |                         |   |
       |                       b |   |4.9
       |                         |   |
       |                         |   |
       | L       H             C |---/-
       |                         |+  |
       |                         ||  |
       |                         ||  |
       |                         ||  |6.2
       |                         ||  |
       |                         |+  |
       | M       J           F   |---/-
       +-------------------------+
         |       |           | |
         /-------/-----------/-/
           2.85      5.2     0.9

Mini ATX:
           +---------------------+ 
           | T   G             A |---/-
           |                     |   |
           |                     |   |
           |                   b |   |4.9
           |                     |   |
           |                     |   |
           | U   H             C |---/-
           |                     |+  |
           |                     ||  |
           |                     ||  |5.4
           |                     ||  |
           |                     |+  |
           | V   W           X   |---/-
           +---------------------+
             |   |           | |
             /---/-----------/-/
             1.45     5.2    0.9

Micro-ATX:
       +-------------------------+ 
       |         R               |---/-0.8
       |         S             B |---/-
       |                         |   |1.8
       |                         |   |
       | L       H             C |---/-
       |                         |+  |
       |                         ||  |
       |                         ||  |
       |                         ||  |6.2
       |                         ||  |
       |                         |+  |
       | M       J           F   |---/-
       +-------------------------+
         |       |           | |
         /-------/-----------/-/
           2.85      5.2     0.9

Baby AT:
+--------------------------------+ 
| N              G             A |---/-
|                                |   |3.1
|                                |   |
|                              B |---/-
|                                |   |1.8
|                                |   |
| O              H             C |---/-
|                                |+  |2.8
|                                |+  |
|   P              I           D |---/-
+--------------------------------+
  | |            | |           |
  /-/------------/-/-----------/
  0.9     5.3    0.9    5.2

Full AT:
+--------------------------------+ 
| N              G             A |---/-
|                                |   |3.1
|                                |   |
|                              B |---/-
|                                |   |1.8
|                                |   |
| O              H             C |---/-
|                                |   |
|                                |   |
|                                |   |
|                                |+  |6.2
|                                ||  |
|                                |+  |
| Q              J             E |---/-
+--------------------------------+
  |              |             |
  /--------------/-------------/
        6.2            6.1

• Hole 'A' is elongated (oval). And its center may vary as much as 0.5".
  
  
• Hole 'N' on the Baby-AT and Full-AT motherboards does not line up perfectly with holes 'A' and 'G'. It is 0.25" closer to the edge of the motherboard than the other two holes.
  
  
• Hole 'B', which was required on previous specifications, is considered optional by the ATX 2.01 spec for ATX and Mini-ATX chassis.
  
  
• Hole 'F', which was optional on previous specifications, is required by ATX spec 2.01 for ATX chassis.
  
  
• ATX motherboards have a PS/2 mouse and PS/2 keyboard connector built into the board. The serial ports, parallel port, and any USB ports are also built in. The AT board only has the 5-pin AT keyboard connector built in. The rest attach with cables to various headers on the mainboard and are usually mounted to an expansion slot plate on the back of the computer.
  
  
• The 20-pin ATX connector is keyed so it can only be inserted into the socket one way. It is sometimes difficult to fully seat the ATX power connector. Try not to flex the board too much when inserting it.
  
  
• The momentary button on the front of the case that powers on the system is supposed to be connected to the motherboard. Unlike AT-style systems, it isn't directly connected to the power supply. Because the ATX motherboard is not physically disconnected from the P/S by the power switch, the motherboard can enter power down mode and be brought back up in a short amount of time. The main shutoff switch is almost always in the back of the power supply, although this feature is becoming more and more scarce with newer units. But holding down the front power switch for a few seconds will also shut off the machine (assuming things are setup that way in the BIOS). Some BIOSes allow an ATX momentary switch to act like an AT power switch and immediately shut off the computer. But the ATX power supply still needs to be attached to the motherboard and not directly to the On/Off switch.
  
  
• Do not insert any components while the ATX power supply is still plugged in. The ATX P/S constantly supplies juice to the motherboard, and components can be damaged if upgrading while the P/S is still plugged in.
  
  
• Beware of ATX power supplies that are not ATX 2.01 compliant. Some newer motherboards won't power up if the P/S doesn't conform. Some power supplies will also not come on if there is no draw from a floppy or hard drive in addition to the motherboard. This may vary from P/S to P/S. Although I believe this only applies to AT power supplies. And, no, I don't know why I stuck this observation on the ATX page. :-P
  
  
• There is a new ATX power supply specification (0.9) that drastically increases the amperage spec for the 3.3 volt line on the power supply. I believe the old spec was for 14 amps for a 300 watt supply. It is now 28 amps for a 300 watt supply because of the possibility of running two amperage hungry chips in a dual configuration.
  
  
• According to the ATX specification, the fan on ATX power supply is intended to blow in over the CPU to allow for cooling. This isn't because this is necessarily the best solution for cooling a hot processor. It's mostly a cost saving measure. In most of the older Pentium systems, Intel recommends a dedicated fan in the front blowing in from the bottom of the case (if it's a Mini/Mid/Tower) and the power supply fan at the top back of the case blowing out. A decent airflow should develop. At least that's the idea. But you'll have to figure out what the best thermal solution for your particular situation is. Because what seems like a logical solution to a cooling problem (such as adding an additional fan), can actually have unintended consequences. See FloTherm for an example. In the end, a heatsink with the ATX power supply blowing over the heatsink fins may be all you need.
  
  
• While ATX is a standard, there are many different styles of I/O backplates available. Be sure that you get the right one to fit the port connectors on the back of your mainboard. See Shuttle's page of backplates for some of the variants.
  
  
• Some cases (like my InWin Q500) can take many different styles of boards. Some can only take one or two. An ATX case which can accept AT motherboards should have a backplate for the AT keyboard's 5-pin DIN connector to stick through.
  
  
• Most Pentium-II based systems (even ones from OEMs) are ATX. Motherboards based on the 440EX chipset come almost exclusively in the Micro-ATX format, though there may be a few exceptions.
  
  
• Micro-ATX specifies up to 4 expansion slots (usually 2 PCI and 2 ISA). MicroATX also allows for a smaller power supply (SFX) that has the same motherboard connector as an ATX P/S, but takes up less space.
  
  
• Holes 'R' and 'S' on the Micro-ATX board are not standard on regular ATX cases, and the front left of the motherboard may not be supported at all. Using a standoff with adhesive on the bottom or a rubber bumper to support the board is suggested. Although the bumper will only help support ISA cards when they're inserted, and not when they're removed.
  
  
• In order to help with feeding newer more powerful CPUs their required amperage draws, a new ATX power supply standard: ATX12V has been developed. This new power supply lessens the required amperage rating on the 3.3v line and transfers much of the amperage draw to an auxilliary 4-pin plug. This extra plug is required alongside the traditional 20-pin ATX power plug for Pentium 4 motherboards. While the ATX12V standard is intended to be backwards compatible, it may pose some problems for other CPUs that draw a good deal of amperage, but whose boards don't use the new plug. This includes some of the very high power draw Athlon processors, and in particular dual Athlon systems. These may require a different power supply altogether. So changing between Intel and AMD systems using the same P/S may now be more difficult. Beware of this possible snafu. AMD has a list of compatible power supplies on their website.
  
  
• There may be an auxilliary connector (P2) on your ATX power supply that resembles an old AT-style 6-pin plug. This is used on some server motherboards. If your board has a plug for this power connector, use it. Don't confuse this P2 connector with the 22-pin P2 connector for WTX power supplies.
  
  
• There may be an extra 6-pin plug (Aux - 2x3 configuration) on your ATX power supply. This is for fan monitoring, IEEE-1394 (Firewire) power supply, and remote 3.3v sense.