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A CLOSER LOOK
Memory comes in a variety of sizes and shapes. In general, it looks like a flat green
stick with little black cubes on it. Obviously, there's a lot more to memory than that.
The illustration below shows a typical memory module and points out some of its
most important features.
WHAT MEMORY LOOKS LIKE
A closer look at a 168-pin SDRAM DIMM.
PCB(PRINTED CIRCUIT BOARD)
The green board that all the memory chips sit on is actually made up of several
layers. Each layer contains traces and circuitry, which facilitate the movement of
data. In general, higher quality memory modules use PCBs with more layers. The
more layers a PCB has, the more space there is between traces. The more space
there is between traces, the lesser the chance of noise interference. This makes the
module much more reliable.
DRAM (DYNAMIC RANDOM ACCESS MEMORY)
DRAM is the most common form of RAM. It's called "dynamic" RAM because it can
only hold data for a short period of time and must be refreshed periodically. Most
memory chips have black or chrome coating, or packaging, to protect their
circuitry. The following section titled "Chip Packaging" shows pictures of chips
housed in different types of chip packages.
CONTACT FINGERS
The contact fingers, sometimes referred to as "connectors" or "leads," plug into the
memory socket on the system board, enabling information to travel from the system
board to the memory module and back. On some memory modules, these leads are
plated with tin while on others, the leads are made of gold. To learn more about the
type of metal on the contacts, refer to the section titled, "Tin versus Gold" on page 67.
INTERNAL TRACE LAYER
The magnifying glass shows a layer of the PCB stripped away to reveal the traces
etched in the board. Traces are like roads the data travels on. The width and
curvature of these traces as well as the distance between them affect both the
speed and the reliability of the overall module. Experienced designers arrange, or
"lay out," the traces to maximize speed and reliability and minimize interference.
CHIP PACKAGING
The term "chip packaging" refers to the material coating around the actual silicon.
Today's most common packaging is called TSOP (Thin Small Outline Package).
Some earlier chip designs used DIP (Dual In-line Package) packaging and SOJ
(Small Outline J-lead). Newer chips, such as RDRAM use CSP (Chip Scale Package).
Take a look at the different chip packages below, so you can see how they differ.
DIP (DUAL IN-LINE PACKAGE)
When it was common for memory to be installed directly on the computer's system
board, the DIP-style DRAM package was extremely popular. DIPs are through-hole
components, which means they install in holes extending into the surface of the
PCB. They can be soldered in place or installed in sockets.
SOJ (SMALL OUTLINE J-LEAD)
SOJ packages got their name because the pins coming out of the chip are shaped
like the letter "J". SOJs are surface-mount components - that is, they mount directly
onto the surface of the PCB.
TSOP (THIN SMALL OUTLINE PACKAGE)
TSOP packaging, another surface-mount design, got its name because the package
was much thinner than the SOJ design. TSOPs were first used to make thin credit
card modules for notebook computers.
CSP (CHIP SCALE PACKAGE)
Unlike DIP, SOJ, and TSOP packaging, CSP packaging doesn't use pins to connect the
chip to the board. Instead, electrical connections to the board are through a BGA (Ball
Grid Array) on the underside of the package. RDRAM (Rambus DRAM) chips utilize
this type of packaging.
CHIP STACKING
For some higher capacity modules, it is necessary to stack chips on top of
one another to fit them all on the PCB. Chips can be "stacked" either internally or
externally. "Externally" stacked chip arrangements are visible, whereas "internally"
stacked chip arrangements are not.
Example of externally stacked chips.
WHERE MEMORY COMES FROM
MAKING THE CHIP
Amazing but true: memory starts out as common beach sand. Sand contains silicon,
which is the primary component in the manufacture of semiconductors, or "chips."
Silicon is extracted from sand, melted, pulled, cut, ground, and polished into silicon
wafers. During the chip-making process, intricate circuit patterns are imprinted on the
chips through a variety of techniques. Once this is complete, the chips are tested and
die-cut. The good chips are separated out and proceed through a stage called "bonding":
this process establishes connections between the chip and the gold or tin leads, or pins.
Once the chips are bonded, they're packaged in hermetically sealed plastic or ceramic
casings. After inspection, they're ready for sale.
MAKING THE MEMORY MODULE
This is where memory module manufacturers enter the picture. There are three
major components that make up a memory module: the memory chips, PCB, and
other "on-board" elements such as resistors and capacitors. Design engineers use
CAD (computer aided design) programs to design the PCB. Building a high-quality
board requires careful consideration of the placement and the trace length of every
signal line. The basic process of PCB manufacture is very similar to that of the
memory chips. Masking, layering, and etching techniques create copper traces on
the surface of the board. After the PCB is produced, the module is ready for assembly.
Automated systems perform surface-mount and through-hole assembly of the
components onto the PCB. The attachment is made with solder paste, which is then
heated and cooled to form a permanent bond. Modules that pass inspection are
packaged and shipped for installation into a computer.
WHERE MEMORY GOES IN THE COMPUTER
Originally, memory chips were connected directly to the computer's motherboard
or system board. But then space on the board became an issue. The
solution was to solder memory chips to a small modular circuit board - that is, a
removable module that inserts into a socket on the motherboard. This module
design was called a SIMM (single in-line memory module), and it saved a lot of
space on the motherboard. For example, a set of four SIMMs might contain a total
of 80 memory chips and take up about 9 square inches of surface area on the
motherboard. Those same 80 chips installed flat on the motherboard would take
up more than 21 square inches on the motherboard.
These days, almost all memory comes in the form of memory modules and is
installed in sockets located on the system motherboard. Memory sockets are easy
to spot because they are normally the only sockets of their size on the board.
Because it's critical to a computer's performance for information to travel quickly
between memory and the processor(s), the sockets for memory are typically located
near the CPU.
Examples of where memory can be installed.
MEMORY BANKS AND BANK SCHEMAS
Memory in a computer is usually designed and arranged in memory banks. A memory
bank is a group of sockets or modules that make up one logical unit. So, memory
sockets that are physically arranged in rows may be part of one bank or divided
into different banks. Most computer systems have two or more memory banks -
usually called bank A, bank B, and so on. And each system has rules or conventions
on how memory banks should be filled. For example, some computer systems
require all the sockets in one bank to be filled with the same capacity module.
Some computers require the first bank to house the highest capacity modules. If
the configuration rules aren't followed, the computer may not start up or it may not
recognize all the memory in the system.
You can usually find the memory configuration rules specific to your computer
system in the computer's system manual. You can also use what's called a memory
configurator. Most third-party memory manufacturers offer free memory configu-rators
available in printed form, or accessible electronically via the Web. Memory
configurators allow you to look up your computer and find the part numbers and
special memory configuration rules that apply to your system.
Kingston Technology's memory configurator includes "bank schema" drawings for
different computer systems (a bank schema drawing depicts the sockets in the
system), along with special instructions that list any unusual configuration rules
that apply to a the systems. For more information on this, see "What kind of memory
is compatible with my system?" on page 74 and "How to Read a Bank Schema" on
page 75.
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