Page 340 - A Practical Guide from Design Planning to Manufacturing

310 Chapter Ten

double every 6 years. The processor industry has seen exactly this kind of
increase with the first microprocessor having 16 leads in 1971 and high
performance processors in 2005 having lead counts between 500 and 1000.
Some of these new leads are data leads, which carry information into
or out of the processor. A wider data bus allows higher data bandwidths
but at the cost of adding more leads to the package. Many more of the addi-
tional leads are power leads, which supply current to power the processor.
In modern processors, it is common for more than half of the leads to be
used simply to power the processor. To draw large amounts of power at low
voltages requires very large currents, which would melt a single lead. If
the maximum safe current per lead were half an ampere, then a proces-
sor able to draw 100 Awould require 200 leads for current flowing into the
processor and another 200 for current flowing out of the processor. Rapid
increases in power have therefore contributed to the rapid increase in the
number of leads required.
Leads are typically placed on the package in three basic configurations,
dual in-line, quad, and grid array packages (Fig. 10-3 and Table 10-1).
For low lead counts, the leads can be placed along only two edges of
the package, so called a dual in-line package (DIP). If more leads are
needed, they can be placed on four sides, making a quad package. For
very high lead counts, grid array packages use multiple rows of leads
on all four sides. The spacing between leads (lead pitch) is commonly
between 1 and 0.4 mm. For a given lead count, smaller pitches allow a
smaller package but require a more expensive PCB with traces that
match the tighter pitch.
The number and configuration of leads for a processor package must
be chosen very carefully. These choices will determine the number of data
leads through which the processor communicates, the number of power
leads through which it draws power, and which motherboards will be able
to use the processor. Compatibility with existing motherboards is a huge
advantage for new processors that are able to use the same package as

Die Die Die

Dual in-line Quad package Grid array
Figure 10-3 Lead configurations.