SIMM chips
Single in-line memory modules (SIMMs) come in 30-pin and 72-pin variants. The best way to identify a SIMM chip is by a notch next to the contacts on one side. The 72-pin SIMM has a notch among the contacts in the middle. Used a lot on older computers, Figure A shows a 30-pin, 3.5-inch SIMM chip (note the tin contacts).
Figure A

The newer 72-pin, 4.25-inch SIMM chip is shown in Figure B (note the gold contacts).
Figure B

Chip contacts are either gold- or tin-plated. I recommend chips with gold contacts because they don’t corrode. Also, never insert tin-plated chips into gold-plated sockets. Different metals will causecorrosion.

DIMM chips
Dual in-line memory modules (DIMMs) have 168 pins and transfer data 64 bits at a time. DIMMs can easily be identified because they:
Have no bottom-corner notch (like SIMMs have).
Have semicircular holes on both sides.
Have two notches among the contacts.
Are installed perpendicular to the memory socket, unlike 72-pin SIMMs that are installed at a slight angle.

Figure C is a 168-pin, 5.25-inch DIMM chip.
Figure C

I use these more than any. They are used in 66mhz through 133mhz systems and are usually backward compatible.

SO-DIMM chips
Small outline dual in-line memory modules (SO-DIMMs) are mostly used in notebook computers. SO-DIMM chips are smaller than standard DIMMs and come in 72-pin and 144-pin varieties.

Figure D shows a 144-pin, 2.66-inch SO-DIMM chip.
Figure D

Figure E shows a 72-pin, 2.35-inch SO-DIMM chip.
Figure E

The best way to identify an SO-DIMM is by the chip’s thinness. The 72-pin chip also has a notch on the side, while the 144-pin chip has a notch among the contacts.

SDRAM and DRAM chips
Synchronous DRAM (SDRAM) is a generic term for different types of dynamic random access memory (DRAM), the most common form of RAM used for personal computers. DRAM chips store each bit of data in a storage cell comprised of a capacitor and a transistor. Because capacitors quickly lose their electric charge, DRAM chips must be recharged every few milliseconds to maintain their data. SDRAM chips use interleaving and burst-mode technologies to synchronize with the clock speed the CPU is optimized for, thus accelerating memory retrieval.

Figure F shows a 168-pin SDRAM chip.
Figure F

SDRAM chips look similar to DIMM chips but have fewer large processor modules.

RIMM and SO-RIMM chips
Developed by Kingston Technology Company in conjunction with Intel and Rambus, Inc., RIMM is the trademarked name for a Direct Rambus in-line memory module. RIMM chips are similar to DIMMs but have 184 pins and transfer data in 16-bit chunks. SO-RIMM chips, which are 160-pin variants, are also available for small form factor devices.

RIMMs are faster than DIMMs but generate more heat. As such, heat spreaders made of aluminum are required to prevent overheating. On RIMM chips, heat spreaders cover most of the chip’s exterior, while SO-RIMM chips have narrower spreaders.

Figure G shows a 184-pin RIMM chip.
Figure G

Figure H shows a 160-pin SO-RIMM chip.
Figure H

You can see the heat spreader forming an upside-down U shape on the chip. These spreaders can be used to identify RIMM or SO-RIMM chips.



Summing it up:
This article should help you identifyu many common types of RAM. Remember to look for notches, the number of pins, the chip’s length, and the presence or absence of heat shields when trying to identify a particular chip.

This year, a new type of memory called DDR has shown up in the new PC’s. Read about it here.