Page 173 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

154 Chapter Fourteen

Die is flipped over, internal wiring layers that
attached to the connect the pads on the
substrate, then
upper surface with pads (or
pins) on the lower surface.
The pads (or pins) on the
lower surface (the side that
actually connects to the
circuit board) will be spaced
much father apart-rela-
substrate (each tively speaking-than the
pad has a ball of pads that connect to the die.
solder attached.)
At some stage the pack-
Figure 14-1 7. A solder bump bonded age will have to be attached
ball grid array packaging technique to a circuit board. In one
technique known as a bull

grid array (BGA) , the package has an array of pads on its bottom surface, and a
small ball of solder is attached to each of these pads. Each pad on the package
will have a corresponding pad on the circuit board, and heat is used to melt
the solder balls and form good electrical connections between the package and
the board.
Modern packaging technologies are extremely sophisticated. For example,
by 2002, some ball grid arrays had pins spaced 0.3 mm (one third of a milli-
meter) apart! In the case of chip-scale packages (CSP), the package is barely
larger than the die itself. In the early 1990s, some specialist applications
began to employ a technique known as die stacking, in which several bare die
are stacked on top of each other to form a sandwich. The die are connected
together and then packaged as a single entity.
As was previously noted, there are a wide variety of integrated packaging
styles. There are also many different ways in which the die can be connected
to the package. We will introduce a few more of these techniques in future
chapters. lo

10 Additional packaging styles and alternative mounting strategies are presented in the
discussions on circuit boards (Chapter 18), hybrids (Chapter 19), and multichip modules
(Chapter 20).