The question of DIMM vs. SO-DIMM yields some clear answers, especially as it relates to size. A proper comparison also considers the effect that CAMM will have on the market.
SO-DIMM stands for small outline dual in-line memory module, so it tracks that a comparison with the standard DIMM largely comes down to size.
Breaking the meaning down further, dual refers to the pins on the front side of the module that have different signals from the pins on the back. In the 1980s, both sides had the same signal. In-line simply means that the pins are presented in a single row, rather than two or three.
In comparing DIMM vs. SO-DIMM, use varies. Nearly all PCs use unbuffered DIMMs, and servers don't use SO-DIMMs, so this article is about unbuffered DIMMs.
Size breakdown
The biggest difference between the DIMM and SO-DIMM formats is size. A standard DDR5 DIMM is normally 133.35 mm x 30 mm with 288 pins. A DDR5 SO-DIMM is half as long, at 67.6 mm x 30 mm, with 262 pins. The odd lengths stem from the original DIMM specification of 133.35 mm or 5.25 inches.
Both are also offered in a very-low-profile format with a height of 20 mm, or two-thirds the standard height of 30 mm.
The SO-DIMM's smaller size makes it difficult to fit more than eight DRAM chip packages on each side. As a result, SO-DIMMs don't support parity or error correction code. Parity and ECC, though, are uncommon in PCs.
Electrically, the two are nearly identical. The pin count difference is from a reduction in the number of pins reserved for future use (15) and extra power and ground pins (11). There's no performance advantage of one over the other.
The Kingston SO-DIMM, in front, is about half the length of the two standard DIMMs behind.
DIMM, SO-DIMM uses
DIMMs are the main way to add memory to PCs and servers. When a manufacturer produces either type of computer, it sells models at varying price points by offering different memory sizes. It can add DIMMs into empty slots for a larger memory configuration or leave some slots empty for a smaller memory arrangement. This approach depends on the user's needs.
DIMMs can be built with larger memory sizes than SO-DIMMs because of their larger physical dimensions.
Users who want to improve the performance of their PC or server can do the same thing. They can buy a DIMM either from the computer maker or another vendor and add it to one of the empty slots in the computer, or they can remove a smaller DIMM and replace it with one that has more memory.
Most servers use standard DIMMs, which were the first type of DIMM. They can be built with larger memory sizes than SO-DIMMs because of their larger physical dimensions. SO-DIMMs were created to make it easier to fit replaceable DRAM in a laptop or notebook. Space is limited in these systems, so the SO-DIMM's smaller form factor is a better fit for these portable devices.
Interchangeability
You can't plug a DIMM into a SO-DIMM socket and vice versa due to the length and pin configuration differences.
If you tried to slip a SO-DIMM into a DIMM socket, the DIMM socket's wider key would prevent the installation to avoid a problem.
DIMM vs. SO-DIMM vs. CAMM
SO-DIMMs are poised for replacement by compression-attached memory modules, which were introduced in 2024. CAMMs use a different form factor and connect to the motherboard with a compression connector rather than the card edge connector in DIMMs and SO-DIMMs.
The CAMM interface supports higher memory capacities by fitting more DRAM chips, offers faster speeds and has a lower profile than SO-DIMMs without adding significantly to the system's cost.
Jim Handy is a semiconductor and SSD analyst at Objective Analysis in Los Gatos, Calif.
