Page 339 - A Practical Guide from Design Planning to Manufacturing

Microprocessor Packaging 309

but also the motherboards the processor can be used with. The most
important package design choices are:
1. Number and configuration of leads
2. Lead type
3. Substrate type
4. Die attach method
5. Use of decoupling capacitors
6. Use of heat spreader

Many of these choices are independent of one another, and over time
almost every possible combination has been tried. The result is what
seems like an overwhelming number of different types of packages (each
with its own acronym). The zoo of modern packages is understood more
easily by looking at each of these design choices separately. The most obvi-
ous characteristic of any package is the number and configuration of leads.

Number and configuration of leads
In 1965, Gordon Moore first proposed his law predicting the growth over
time of the maximum number of components on an integrated circuit. Five
years earlier, E.F. Rent of IBM was investigating the number of inter-
connections required between discrete circuits. He found that the number
of interconnections required by a circuit is a function of the total number
of components. This relationship came to be known as Rent’s Rule. 1

Rent’s rule:
R
Num leads ∝ (num components)
As logic gates are added to a processor to increase performance or add
new functionality, the processor consumes data more quickly. Additional
leads are required to prevent the processor’s performance from being
unduly limited by its ability to communicate. The interaction of Moore’s
law and Rent’s rule has been the biggest driver in the evolution of proces-
sor packages. If the number of components on a microprocessor is increas-
ing exponentially, we should also expect an exponential increase in the
number of leads. The value of the Rent’s exponent R varies for different
types of integrated circuits but is usually estimated for microprocessors
2
as about 0.5. If the number of logic transistors on a microprocessor
doubles about every 3 years, we should expect the number of leads to

1
Landman and Russo, “Pin versus Block Relationship.”
2
Bakoglu, Circuits, Interconnections, and Packaging, 418.