A SO-DIMM, SODIMM, or small outline dual in-line memory module, is a type of computer memory built using integrated circuits. SO-DIMMs are a smaller alternative to a DIMM, being roughly half the size of regular DIMMs.
SO-DIMMs are often used in systems that have limited space, which include laptops, notebooks, small-footprint personal computers such as those based on Mini-ITX motherboards, high-end upgradable office printers, and networking hardware such as routers and NAS devices.
Most types of SO-DIMMs can be recognized visually by the distinctive notches that “key” them for different applications:
- 100-pin SO-DIMMs (SDR SDRAM) have two notches.
- 144-pin SO-DIMMs (SDR SDRAM) have a single notch near the center.
- 200-pin SO-DIMMs (DDR SDRAM or DDR2 SDRAM) have a single notch nearer to one side. The exact location of this notch varies (read below).
- 204-pin SO-DIMMs (DDR3 SDRAM) have a single notch closer to the center than on 200-pin SO-DIMMs.
- 260-pin SO-DIMMs (DDR4 SDRAM), 69.6 mm (2.74 inches) wide and 30 mm (1.2 inches) high, making them 2 mm (0.079 inches) wider than DDR3 SO-DIMMs, with a single notch after pin 144.
- 260-pin SO-DIMMs (UniDIMM), 69.6 mm (2.74 inches) wide and 30 mm (1.2 inches) or 20 mm (0.79 inches) high, with a single notch placed differently than on DDR3 SO-DIMMs.
Variants of 200-pin SO-DIMMs
The 200—pin SO-DIMM can be of the types DDR and DDR2. In both cases the notch is located at one fifth of the board length (20 pins + notch + 80 pins), but in DDR3 the notch is located slightly closer to the center of the board. These two types of memory are not interchangeable. The different notch locations are designed to prevent cross-installation, aiding at the same time in visually identifying 200-pin SO-DIMM modules.
This variance is designed to prevent coupling of memory and controllers of different DDR generations due to electrical incompatibility. DDR SO-DIMMs operate at 2.5 V while DDR2 SO-DIMMs operate at 1.8 V.
Variants of DDR3 SO-DIMMs
These modules, while using the same physical layout, may differ in the specified voltage – 1.5 or 1.35 volts. The lower voltage modules are sometimes, but not always, identified as DDR3L – see also DDR3 SDRAM. Low voltage modules are compatible with high voltage mainboards, but not vice versa. There are some exceptions to that rule.
200- and 204-pin SO-DIMMs are 67.6 mm (2.66 inches) long and 31.75 mm (1.250 inches) wide, with a maximum total depth of 3.8 mm (0.15 inches).
SO-DIMMs are more or less equal in power and voltage rating to DIMMs, and despite the smaller size of memory modules, SO-DIMM technology does not mandate lower performance compared to larger DIMMs. For example, DDR3 SO-DIMMs provide clock speeds such as 533 MHz (1066 MT/s, PC3-8500), CAS latencies such as 7, and higher capacities such as 4 GB per module.
DDR2 SO-DIMM memory modules commonly have clock speeds from 200 MHz up to 400 MHz (PC2-6400). 204-pin SO-DIMMs can also contain DDR3 SDRAM, with specifications such as PC3-6400, PC3-8500, PC3-10600, PC3-14900 and PC3-17000. 260-pin UniDIMMs may contain either DDR3 or DDR4 SDRAM, depending on their configuration. Some SO-DIMMs also provide ECC functionality; unbuffered ECC SO-DIMMs are called SO-CDIMMs.