A DIMM (dual in-line memory module) is a double SIMM (single in-line memory module). Like a SIMM, a DIMM is a module that contains one or several random access memory (RAM) chips on a small circuit board with pins that connect it to the computer motherboard.
A SIMM typically has a 32 data bit (36 bits counting parity bits) path to the computer that requires a 72-pin connector. For synchronous dynamic RAM (SDRAM) chips, which have a 64 data bit connection to the computer, SIMMs must be installed in in-line pairs, since each one supports a 32-bit path. A single DIMM can be used instead. Originally, a DIMM had a 168-pin connector to support 64-bit data transfer.
As faster dynamic random access memory (DRAM) was developed, DIMM circuit boards evolved. Modern DIMMs based on double data rate fourth generation (DDR4) SDRAM chips use 288-pin connectors to attach to the computer motherboard to enable the increase in data throughput. As clock speeds of the RAM chips increased, the 64-bit path handled increasing amounts of data.
Another evolution in DIMMs is the use of cooling fins or structures attached directly to the DIMM. The increase in chip density in typical 8 gigabyte (GB) or 16 GB DIMMS, and the increase in clock speed, led to an increase in heat production. This was made worse by the fact that DIMMs based on DDR4 RAM chips can be produced in capacities up to 64 GB.
Cooling structures on the DIMM help vent that heat into the computer enclosure and away from the motherboard and CPU.
While the standard DIMM is in the form of a rectangular stick approximately 5.5 inches in length, the small outline dual in-line memory module (SO-DIMM) is about half that size at 2.74 inches long. Both types of DIMMs are most commonly 1.2 inches tall, but both are made in a very low profile (VLP) format that is only 0.8 inches tall.
SO-DIMM is mainly used for portable computing devices such as laptops and tablets. It differs from standard DIMM in that the DDR4 SO-DIMM has 260 pins versus 288 pins for the DRR4 DIMM.
Standard DIMMs are used in PCs and servers. The VLP DIMM was developed to meet the space requirements of blade servers.
DIMM trends and future outlook
The next type of memory that will be used in a DIMM is DDR5. In March 2017, the microelectronics standards organization, the JEDEC Solid State Technology Association, announced that it would publish the DDR5 specifications in 2018. Data transfer rates are expected to be double those of DDR4, which currently tops out at 25 gigabytes per second (GBps). JEDEC also stated that DDR5 will lower power consumption from DDR4 for an equivalent amount of RAM.
Other types of memory could make DDR5 obsolete shortly after it becomes generally available. At the same time it announced the timeframe for DDR5, JEDEC announced that it is continuing to work on the latest standards for nonvolatile DIMM (NVDIMM), or nonvolatile memory used as system memory, like SDRAM.
Intel has released the first product in its Optane line, a nonvolatile memory system based on a technology known as phase-change memory. Intel's Optane may see enough adoption to possibly become the de facto technology before any NVDIMM based on the JEDEC standard hits the market.