Page 334 - A Practical Guide from Design Planning to Manufacturing

304 Chapter Ten

the salts common on everyone’s skin could quickly contaminate the cir-
cuitry. This was true for even the earliest microprocessors, so all proces-
sors require some type of packaging.
The package first and foremost provides the chip protection from phys-
ical damage and chemical contamination. The package must do this while
still providing electrical connections (leads) from the outside world to the
chip. The interconnections on printed circuit boards (PCBs) are far
larger than semiconductor chips, and the package must bridge this gap
in scale. The package leads connect to the tight-pitched circuits of the
processor die and spread apart to connect to the larger board traces.
For early packages, protection from the environment and simple con-
nections between the die and the board were all that was required.
Modern packages have grown far more complex as processor perform-
ance and power have increased. To allow for very high-frequency trans-
mission over the data leads, their electrical characteristics (resistance,
capacitance, and inductance) must be tightly controlled. Hundreds of
leads may be required to deliver power to the processor, and all the
power delivered will generate heat, which must be removed from the die
quickly enough to prevent damage from extreme temperatures.
Because of its impact on data bandwidth, as well as on power in and
power out limits, packaging has a large impact on the performance and
reliability of a microprocessor. Package materials and assembly costs
also contribute significantly to overall product cost. This means that
there is no perfect package. Designers must balance the package cost
and performance in choosing the best package for their product. The dif-
ferent needs of different processor products have led to a wide range of
different packaging choices.

Package Hierarchy

A consumer cannot buy a bare silicon chip. Any piece of electronics
inevitably has packaging to connect different components, and most
products will have multiple levels of packaging for multiple chips.
Packaging lets multiple die work together as part of a single product.
Almost always a PCB makes the final level of connections. A personal
computer would typically include a number of PCBs, the largest being
called the motherboard. Architecture determines the processor’s soft-
ware compatibility, but the package dictates with what motherboards
a processor may be used. Figure 10-1 shows a PCB and some ways of
connecting components.
The most straightforward way of adding a silicon die to a circuit board
is to simply solder it directly to the board. This is called direct chip
attach (DCA) or just chip on board (COB). This is the highest per-
formance solution, adds the least possible weight, and takes up the