Motherboards and Form Factors

What a motherboard does

The motherboard (also mainboard or system board) is the main printed circuit board of a computer. Every other component plugs into it or connects to it with a cable. It does three jobs:

• Connecting: copper traces in its layers carry signals between the CPU, memory, slots and ports.
• Powering: it takes power from the power supply and converts it, through its voltage regulator modules (VRMs), into the low, precise voltages the CPU and memory need.
• Starting: it carries the firmware chip that tests the hardware and starts the operating system.

Because the motherboard decides which CPU, memory and expansion cards can be used, it is the first thing to check in almost every upgrade question. See Choosing and Upgrading Components.

Form factors

A form factor is a standard for the size, shape and mounting of a component, so that parts from different makers fit together. For motherboards it fixes the board dimensions, the positions of the mounting holes and the location of the rear input/output (I/O) panel, and so decides which case the board fits in.

ATX (Advanced Technology Extended), introduced by Intel in 1995, is the standard desktop form factor. It defines the full-size board, the case layout and the power supply connectors that almost all desktop PCs still use. Smaller variants keep the same mounting holes where they overlap, so a smaller board fits in a larger case.

A case lists the form factors it supports, for example "ATX, microATX, mini-ITX". E-ATX is used loosely by manufacturers, so check the actual board width against the case specifications before buying. Laptops, all-in-ones and many brand-name office PCs use proprietary boards that follow no public form factor at all; a replacement must come from the manufacturer.

The chipset

The chipset is the set of chips on the motherboard that manages communication between the CPU and the rest of the system. On most current boards it is a single chip (some high-end AMD boards chain two) under a small heat sink. Its model (for example a B-, H-, X- or Z-series name) decides much of what a board can do: how many USB and SATA ports, how many extra PCI Express (PCIe) lanes, and whether the CPU and memory may be overclocked.

Northbridge and southbridge

Older boards had two chips. The northbridge, close to the CPU, handled the fast parts: the memory controller and the graphics slot, connected to the CPU over the front-side bus. The southbridge handled the slower I/O: USB, SATA, audio, the older PCI slots and the firmware chip.

Over time the northbridge's jobs moved into the CPU itself. Today the memory controller and the lanes for the main graphics slot are part of the processor, which is faster and uses less power. What is left is essentially the old southbridge: Intel calls it the Platform Controller Hub (PCH), and AMD simply the chipset. It connects to the CPU over one dedicated link, so all devices behind the chipset share that link's bandwidth.

CPU sockets

A CPU socket is the mount on the motherboard that holds the processor and connects its hundreds or thousands of contacts to the board. There are three designs:

• Land grid array (LGA): the pins are in the socket on the motherboard; the CPU has flat contact pads (lands). A lever-operated load plate presses the CPU onto the pins. Used by Intel desktop CPUs and by AMD's mainstream desktop CPUs since AM5 (its workstation and server CPUs earlier).
• Pin grid array (PGA): the pins are on the CPU; the socket has holes, and a lever clamps the pins. Used by older AMD desktop sockets such as AM4.
• Ball grid array (BGA): the CPU is soldered directly to the board with tiny solder balls. Used in almost all laptops, tablets and phones; the CPU cannot be replaced.

The socket must match the CPU exactly: a socket name such as LGA 1700 or AM5 names a specific physical and electrical design. A matching socket is necessary but not enough. The chipset and the firmware must also support that CPU generation, and a board made before a CPU was released may need a firmware update before it will start with it. The manufacturer's CPU support list for the board is the authoritative answer.

Slots and storage connectors

Memory slots

Desktop boards have two or four slots for dual inline memory modules (DIMMs); workstation and server boards have more. The slots are grouped in channels, and the manual (often also colored slots) says which slots to fill first so that the modules run in dual channel. A slot only accepts its own memory generation: a DDR5 slot does not take DDR4. See Memory.

Expansion slots

An expansion slot is a connector on the motherboard for an expansion card, such as a graphics card or a network card, that adds functions the board does not have. Today that means PCIe slots in sizes x1, x4, x8 and x16, the number giving the lanes (each lane is one pair of wires to send and one pair to receive). Two rules explain most confusion:

• A card fits in a slot of its own size or larger: an x1 card works in an x16 slot, and uses one lane.
• A slot's physical size is not always its electrical width. A full-length x16 slot lower on the board is often wired for only four lanes, printed on the board as "x16 (x4 mode)" or similar.

Lanes are a limited budget. The CPU provides a fixed number, the chipset some more, and boards distribute them over slots with switches: fill one slot and another gets fewer lanes or switches off. A worked example with a fictitious board:

cpu_lanes = 24   # lanes this (fictitious) platform's CPU provides to slots

devices = [("Graphics card", 16), ("M.2 slot 1 (NVMe SSD)", 4), ("M.2 slot 2 (NVMe SSD)", 4)]

used = 0
for name, lanes in devices:
    used += lanes
    print(f"{name:<22} x{lanes:<2}  lanes used: {used:>2} of {cpu_lanes}")

extra = ("M.2 slot 3 (NVMe SSD)", 4)
if used + extra[1] > cpu_lanes:
    print(f"{extra[0]}: no free CPU lanes; the board must share or use chipset lanes")

Graphics card          x16  lanes used: 16 of 24
M.2 slot 1 (NVMe SSD)  x4   lanes used: 20 of 24
M.2 slot 2 (NVMe SSD)  x4   lanes used: 24 of 24
M.2 slot 3 (NVMe SSD): no free CPU lanes; the board must share or use chipset lanes

Real boards solve this in their own way, and the manual documents it in a table or footnotes such as "SATA6G_5 is disabled when M.2_3 is occupied". Read that table before adding a drive. PCIe generations and speeds are covered in Expansion Cards and Graphics.

M.2 slots

M.2 is a small form factor for expansion cards that plug into an edge connector on the motherboard and lie flat, held by one screw. It is used mostly for solid-state drives (SSDs) and for Wi-Fi and Bluetooth cards. M.2 describes only the shape and the connector; what travels over it can differ:

• Size: a four- or five-digit code gives width and length in millimeters. 2280 is 22 mm wide and 80 mm long, the most common SSD size; 2242 and 2230 are shorter cards for small laptops and handheld devices. The board has screw positions (a standoff) for the lengths it supports.
• Key: notches in the edge connector. An M key slot carries up to four PCIe lanes and is the slot for NVMe SSDs; a B key slot carries SATA or up to two PCIe lanes; an E key slot is for wireless cards. SATA M.2 SSDs usually have both notches (B+M) so they physically fit in either slot.
• Protocol: an M.2 SSD is either a SATA drive or a Non-Volatile Memory Express (NVMe) drive over PCIe. NVMe is several times faster. Not every M.2 slot supports both.

SATA ports

Serial ATA (SATA) ports are small L-shaped 7-pin connectors, usually on the right edge of the board, for 2.5-inch SSDs, 3.5-inch hard drives and optical drives. Current SATA runs at up to 6 Gbit/s. The drive gets its data through this cable and its power through a separate 15-pin SATA power connector from the power supply.

Power connectors

Two connectors from the power supply feed the motherboard:

• The 24-pin ATX main connector supplies 3.3 V, 5 V and 12 V to the board. Older boards used 20 pins, which is why many power supplies have a 20+4-pin plug that can be split.
• The CPU power connector (EPS12V; the 4-pin version is ATX12V) near the CPU socket supplies 12 V to the CPU's voltage regulators. It has 4 or 8 pins; boards for power-hungry CPUs may have an 8-pin plus a 4-pin or a second 8-pin connector.

The CPU's 8-pin EPS connector and a graphics card's 8-pin PCIe connector look alike but are keyed differently and are not interchangeable; forcing one into the other can damage hardware. Graphics card power and wattage are covered in Power Supplies and Power.

Headers

Headers are groups of pins on the board for cables from the case and accessories.

• Front panel: the case's power switch, reset switch, power LED and drive activity LED. The switches work either way round; the light-emitting diodes (LEDs) only light with the right polarity (+ and −). The pin map is printed on the board and in the manual.
• USB: a 9-pin header for two front USB 2.0 ports, a larger 19-pin header for USB 3.x ports, and on newer boards a small separate header for a front USB-C port.
• Fans: 3-pin headers control fan speed through the voltage, 4-pin headers through a pulse-width modulation (PWM) signal. Connect the CPU cooler to the header labeled CPU_FAN: many boards warn or refuse to boot when no fan is detected there.
• Red-green-blue (RGB) lighting: 4-pin 12 V RGB headers and 3-pin 5 V addressable RGB (ARGB) headers. They are not compatible: plugging 5 V lighting into a 12 V header can destroy it.
• Front audio, and on some boards a header for a plug-in Trusted Platform Module (TPM).

The CMOS battery

The CMOS battery is a coin cell on the motherboard, almost always a 3 V CR2032 lithium cell, that keeps the real-time clock running and preserves firmware settings while the computer is unplugged. The name comes from the small complementary metal-oxide-semiconductor (CMOS) memory in which older boards stored their settings. Current boards keep most settings in flash memory, but the clock still needs the battery.

It lasts for years, but when it runs empty the symptoms are typical:

• The date and time reset to a default (often the board's manufacturing date) after the computer has been unplugged.
• Messages at startup such as "CMOS checksum error", "CMOS battery low" or a prompt to press F1 to continue.
• Firmware settings such as boot order or enabled virtualization are lost.
• Secondary effects from the wrong time: websites give certificate errors and the computer cannot authenticate to a domain until the clock is corrected.

Replacing it is a routine repair: power off, unplug, ground yourself, and swap the cell. Removing the battery (or using the clear-CMOS jumper or button) is also the standard way to reset the firmware to its defaults, for example after a bad overclock or a forgotten firmware password on many desktop boards. Settings are covered in BIOS, UEFI and the Boot Process; safe handling of the board in Safety and Handling.

The rear I/O panel

The rear I/O panel is the row of ports on the back of the board that reaches out through an opening in the case. Typically:

• USB ports, both USB-A and USB-C.
• Video outputs (HDMI, DisplayPort): these only work with a CPU that has integrated graphics.
• An RJ45 Ethernet port for the built-in network adapter.
• Audio jacks, sometimes an optical audio output.
• On some boards: Wi-Fi antenna connectors, a button to flash the firmware without a CPU, a clear-CMOS button, or a PS/2 port for an old keyboard or mouse.

A metal I/O shield fits in the case opening around these ports; on many boards it is attached already, on others you install it in the case before the board. You can identify a board without opening the case:

Get-CimInstance Win32_BaseBoard | Select-Object Manufacturer, Product

Manufacturer   Product
------------   -------
Example Corp.  B650M-EXAMPLE

The output here is illustrative. On Linux, sudo dmidecode -t baseboard gives the same information. With the model name you can find the manual, the CPU support list and the lane table.