ATX、E-ATX、SSI-EEBの互換性について解説

Key Takeaways

• Standard ATX is 305 × 244 mm, while the SSI EEB specification allows a maximum baseboard size of 305 × 330 mm.
• “E-ATX” on a motherboard specification sheet does not guarantee one fixed board width or one predictable chassis fit.
• Two boards sold as E-ATX can differ by more than 50 mm in depth.
• A motherboard fitting inside the chassis does not prove that its standoffs, cable openings, DIMM clearance, PCIe layout, or power connectors will work.
• For OEM, workstation, industrial, and server projects, the safest compatibility check uses the exact motherboard model and mechanical drawing—not the form-factor label alone.

Motherboard compatibility looks simple until someone has to build the system.

ATX. E-ATX. SSI-EEB.

Three labels. Three boxes on a specification sheet. Easy, right?

Not really.

For a consumer PC build, getting this wrong may mean returning a case. For a system integrator ordering 200 chassis, an industrial computer manufacturer freezing a production BOM, or an OEM shipping servers internationally, the same mistake can trigger new sheet-metal tooling, revised motherboard trays, blocked cable routes, delayed assembly, and a very uncomfortable conversation with purchasing.

That is why Motherboard Form Factor Compatibility needs to be treated as a mechanical engineering question rather than a marketing-label question.

And nowhere is that more obvious than with E-ATX vs SSI-EEB.

ATX Gives Us a Reliable Starting Point

ATX is the easy one.

The SSI Forum’s Enterprise Electronics Bay specification defines an ATX baseboard as 305 × 244 mm, or 12 × 9.6 inches. The same SSI document explains that the EEB design evolved from ATX.

That standardized ATX footprint gives chassis engineers a reasonably predictable starting point for:

• board outline
• rear I/O position
• PCIe slot relationship
• motherboard tray design
• standard mounting locations

This predictability is why ATX support is relatively straightforward to communicate.

Once the board gets wider, things become messy fast.

If you are already comparing ATX vs E-ATX as part of a broader rackmount architecture decision, do not isolate the motherboard from the rest of the system. Rack height, expansion cards, cooling, power, drive cages, and cable routing all compete for the same internal volume.

ATX, E-ATX and SSI-EEB Size Comparison

The first number is not the problem.

The second one is.

That extra motherboard depth pushes the board edge farther into the chassis, exactly where designers often place cable cutouts, drive cages, side fans, fan walls, front-access connectors, cable covers, or reinforcement structures.

A motherboard can therefore be technically “inside the box” and still be unusable.

Why E-ATX Is the Label I Trust the Least

Here is where the industry creates its own confusion.

ASUS currently lists its ROG Maximus Z890 Extreme as an E-ATX motherboard measuring 12 × 10.9 inches, or 30.5 × 27.7 cm on the official ASUS specifications page.

Now look at a server board.

GIGABYTE lists the MZ33-AR0 as E-ATX, but its official specification gives a size of 305 × 330 mm. That board supports AMD EPYC processors, 24 DIMM slots, and four PCIe Gen5 x16 expansion slots.

Same E-ATX label.

ASUS: 277 mm deep.

GIGABYTE: 330 mm deep.

That is a 53 mm difference.

Fifty-three millimeters is not a rounding error in chassis engineering. It can be the entire cable-management channel.

This is why E-ATX Case Compatibility should never be reduced to a yes/no field in a purchasing spreadsheet. When reviewing E-ATX Case Compatibility for an actual project, ask for the maximum supported board dimensions and the tray drawing.

“Supports E-ATX” is only the beginning of the conversation.

What Exactly Is SSI-EEB?

SSI-EEB comes from the Server System Infrastructure Forum’s Enterprise Electronics Bay specification for servers and workstations.

The specification describes the EEB baseboard as an ATX-derived design stretched to a maximum 305 × 330 mm, or 12 × 13 inches. Interestingly, the document itself notes that this size is sometimes called “extended ATX” or “full ATX.”

There is your naming problem.

A 12 × 13-inch server board may be described in one context as SSI-EEB and in another as E-ATX.

But matching outer dimensions still do not settle the installation question.

When evaluating a SSI-EEB Motherboard for an industrial or server platform, the chassis needs enough board area, but it also needs the correct support structure around that board.

The label tells you the neighborhood.

It does not give you the house number.

The Mounting-Hole Problem Buyers Discover Too Late

This is the part that causes real-world pain.

The SSI specification explicitly states that a compliant baseboard is not required to use every available mounting location. It also defines alternative mounting positions where legacy locations cannot be used.

For rack-mounted designs, the standard says board and chassis developers may need to communicate about which mounting locations will actually be used because implementing removable standoffs can be difficult. The chassis requirements also warn that fixed, non-removable attachment points can create interference problems.

That is not a minor detail.

Imagine approving 100 rackmount enclosures because the specification says “12 × 13 motherboard support.”

The boards arrive.

The PCB fits.

Then assembly discovers that several support points do not align.

Now what?

Drill the tray manually? Add press-in standoffs? Create an adapter plate? Rework finished chassis? Leave unsupported board sections floating?

None of those are attractive production options.

For serious SSI-EEB Case Compatibility work, we prefer to validate the actual board drawing during the engineering stage. That is far safer than discovering the mounting pattern after painted production chassis are sitting on the assembly floor.

I Saw Exactly This Failure in the Wild

Recently, I was browsing an r/buildapc discussion and came across a builder using a Supermicro X11DPH-T を持つ。 Lian Li Odyssey X.

On paper, the match looked difficult to mess up.

The builder described the motherboard as E-ATX/SSI-EEB and said the case supposedly supported that type of board.

Then installation started.

Only a few standoffs lined up.

Some motherboard holes were reportedly three or four inches away from the nearest chassis standoff. You can read the original Reddit discussion about the motherboard standoffs.

That story sticks with me because it captures the mistake perfectly.

The buyer checked the category.

He did not validate the interface.

The motherboard could enter the chassis. That did not mean the chassis properly supported the motherboard.

As a chassis supplier, this is why I never want a project brief that says only:

“Motherboard: E-ATX.”

Send the model number.

Better yet, send the PDF drawing.

My Unpopular View: “E-ATX Compatible” Is Not a Professional Specification

Here is the position I take with buyers:

“E-ATX compatible” is not enough information to approve a production chassis.

That may sound overly strict.

It is not.

Current manufacturer specifications already show that boards carrying the E-ATX name can have dramatically different dimensions. Add mounting locations, cable exits, power connectors, DIMM latches, PCIe slots, risers, fan walls, drive cages, and service access, and the label starts telling you surprisingly little about the completed assembly.

For a one-off gaming PC, people improvise.

For a workstation, industrial computer, GPU server, storage appliance, or product that must ship repeatedly under a controlled BOM, improvisation is bad production engineering.

The question should not be:

Does this chassis support E-ATX?

The better question is:

Does this chassis support this exact motherboard, with this exact mounting pattern, connectors, expansion cards, cabling, cooling hardware, and service envelope?

That small change in wording prevents a lot of expensive mistakes.

Physical Fit Is Only Layer One

Even after the mounting holes match, we are not finished.

A complete compatibility review should check several mechanical zones.

1. Right-Edge Connector Access

As boards become deeper, their right edge can move directly over cable openings or sheet-metal structures.

Typical problem areas include:

• 24-pin motherboard power
• USB headers
• SATA connectors
• front-panel headers
• fan headers
• MCIO or SlimSAS connectors
• side-facing power connectors

A connector can be physically present yet impossible to cable after installation.

That still counts as incompatible.

2. DIMM Installation and Removal

Large workstation and server boards may place DIMM sockets close to the outer PCB edge.

The SSI specification specifically discusses clearance near the board edge because DIMM latches may extend beyond the baseboard footprint when opened. Chassis designers therefore need to preserve insertion and extraction room.

This becomes especially annoying during field service.

A server that requires motherboard removal just to replace memory is not designed for pleasant maintenance.

3. PCIe Slot Alignment

Expansion-slot geometry must match the rear chassis openings.

That gets harder when the system uses:

• GPU risers
• bifurcation boards
• horizontal expansion
• full-height accelerators
• double-width GPUs
• NICs or DPUs
• storage controllers

A motherboard may mount correctly while the intended expansion architecture does not.

4. CPU and EPS Power Clearance

Server and workstation boards often place CPU power connectors near the upper or forward edge of the PCB.

Now add:

• ファンウォール
• large heatsinks
• radiator structures
• cable bends
• drive backplanes
• chassis reinforcement beams

You can lose access quickly.

5. Chassis Depth

Board width and chassis depth are different measurements, but they interact.

A 330 mm motherboard inside a shallow enclosure leaves less room in front of the PCB for fans, storage, backplanes, connectors, and cable bend radius.

That is why SSI-EEB Dimensions should be evaluated against the complete internal chassis layout, not only the sheet-metal outer dimensions. The linked 4U platform, for example, is published for motherboards up to the 12 × 13-inch class, but exact component fit still needs project-level confirmation.

Millimeters matter.

A lot.

E-ATX vs SSI-EEB: What Actually Changes for the Chassis Engineer?

From the chassis side, the biggest mistake is assuming that a form-factor label automatically defines every mechanical interface.

It does not.

For each project, we want to verify:

This is Motherboard Form Factor Compatibility in practical terms.

Not a logo.

An interface map.

The RFQ Should Name the Motherboard

For OEM and ODM projects, motherboard information should be frozen early.

A useful chassis RFQ should provide:

Motherboard manufacturer and exact model
Not just ATX or E-ATX.

PCB dimensions
Use manufacturer dimensions, preferably in millimeters.

Mechanical drawing
Include mounting-hole coordinates when available.

Rear I/O configuration
Standard shield, custom opening, fixed ports, or server-specific I/O.

Expansion architecture
Direct PCIe cards, risers, GPUs, NICs, storage controllers, or switching boards.

CPU and memory configuration
Especially important for high DIMM counts and tall heatsinks.

Power connectors
Include connector location and cable exit direction.

Expected production revision
A sample motherboard is useful, but revision-controlled drawings are better.

This is also why custom projects should be engineered around the complete component stack. The site’s custom server chassis fabrication process, for example, begins with system components, dimensions, cooling and connector requirements before production drawings are released.

That sequence makes sense.

Freeze the electronics.

Then freeze the metal around them.

Do Not Let Procurement Turn Compatibility Into a Checkbox

Procurement teams love standardized fields.

I understand why.

ATX: yes.
E-ATX: yes.
SSI-EEB: yes.

Clean spreadsheet.

Dangerous assumption.

For sourcing, those fields should be treated as preliminary filters. Engineering approval should come from the actual motherboard-to-chassis fit review.

A better supplier question would be:

“Please confirm compatibility with motherboard model XYZ-123, drawing revision B, including all required standoffs, rear I/O, PCIe slots, EPS connectors, DIMM service clearance, and cable-routing access.”

That question is harder to answer.

Good.

Hard questions catch expensive problems while they are still cheap to fix.

A Simple Compatibility Approval Process

Before approving a server or workstation chassis, run the project through this sequence:

1. Identify the exact motherboard model.
2. Confirm the published PCB dimensions.
3. Obtain the mechanical drawing.
4. Overlay the mounting-hole pattern on the chassis tray.
5. Confirm rear I/O alignment.
6. Map PCIe slots and risers.
7. Check DIMM and cooler keep-out areas.
8. Check every motherboard power and data connector.
9. Install planned GPUs and expansion cards in CAD or a physical prototype.
10. Verify fan walls, backplanes, drive cages, cables, and service access.
11. Build one production-representative prototype.
12. Freeze the approved motherboard and chassis drawing revisions together.

Do that, and ATX, E-ATX, and SSI-EEB stop being confusing labels.

They become controlled engineering inputs.

That is where they belong.

よくある質問

Is SSI-EEB the same as E-ATX?

Short answer: No. The names can overlap in real product listings, but you should not assume they guarantee identical mechanical compatibility.

The SSI EEB specification defines a maximum 305 × 330 mm baseboard and documented mounting locations. Current manufacturers also use “E-ATX” for boards with different depths, so exact dimensions and mounting patterns must be checked.

What are the standard SSI-EEB dimensions?

Short answer: The SSI EEB specification defines a maximum baseboard size of 305 × 330 mm, or 12 × 13 inches.

Smaller board outlines are possible under the specification. Chassis compatibility still depends on mounting locations, keep-out zones, connector positions, and the rest of the installed hardware.

What size is a standard ATX motherboard?

Short answer: Standard ATX measures 305 × 244 mm, or 12 × 9.6 inches.

This smaller and more predictable footprint is why ATX compatibility is generally easier for chassis manufacturers to define than the broader range of products marketed as E-ATX.

Why do some E-ATX motherboards have different dimensions?

Short answer: Manufacturers use the E-ATX label on boards with different PCB depths, so the name does not always describe one identical physical envelope.

For example, ASUS publishes a 305 × 277 mm E-ATX board, while GIGABYTE publishes a 305 × 330 mm server board under the same E-ATX label.

Will an E-ATX case always fit an SSI-EEB motherboard?

Short answer: No. Physical board space alone does not guarantee SSI-EEB support.

Verify the motherboard dimensions, mounting-hole locations, rear I/O, PCIe alignment, DIMM clearance, power connectors, cable paths, fan structures, and chassis tray before approving the combination.

Can a motherboard fit inside a case but still be incompatible?

Short answer: Yes. A board can physically enter the chassis while mounting holes, connectors, expansion slots, or service clearances do not align.

This is one of the most common mistakes when compatibility is judged by outer dimensions alone.

What should I send a chassis supplier to confirm motherboard compatibility?

Short answer: Send the exact motherboard model, dimensions, mechanical drawing, mounting pattern, I/O layout, expansion configuration, cooling hardware, and power-connector locations.

For custom server projects, a complete BOM and production-representative sample can make the validation process much more reliable.

What is the safest way to specify E-ATX compatibility in an RFQ?

Short answer: Specify the exact motherboard rather than writing only “E-ATX compatible.”

Include the manufacturer, model number, board revision, dimensions, drawing, standoff locations, PCIe configuration, connector access, and any required service clearances. That converts a vague label into a testable engineering requirement.