Page 338 - A Practical Guide from Design Planning to Manufacturing

308 Chapter Ten

oxide on the metal contacts and allow for a good electrical connection.
Then the board is passed through a wave of molten solder, which only
clings to the board where there is no solder mask. The result is all the
pins being connected by solder to their vias.
The component on the right in step #6 has been mounted using SMT.
SMT uses a stencil to apply a paste mix of solder and flux to contact points
on the surface of the board. Components are then placed with their con-
tacts touching the paste. The board is heated to allow the solder to form
electrical connections between the component and the board. The biggest
advantage of SMT is that it does not require drilling holes through the
board. This reduces the cost of producing the board and allows compo-
nents to be mounted directly on opposite sides of the board. This makes
SMT particularly attractive for mobile or handheld products where pack-
ing components onto the smallest board possible is critically important.
The biggest advantage of THT is its mechanical stability. A component
with pins inserted through the thickness of the board and soldered into
place will not come loose easily. SMT components can tear free of the
board when heated. The materials of the package and PCB all expand
when heated but some expand more than others. The rate of this swelling
is measured as the coefficient of thermal expansion (CTE). If the CTE
of the board and an SMT package are very different, heating the compo-
nent will cause stress to build in the soldered connections to the board.
With enough stress, the component will break free of the board.
The most expensive but most flexible choice is to provide a socket for
the package. This is a connector designed to allow packaged components
to be added without requiring solder. This allows expensive components to
be replaced or upgraded easily. Unused sockets allow functionality to be
added to a PCB after sale. To make sure removing a component will not
pull the socket itself loose, sockets are typically connected to the board
using THT. Sockets also allow cables to be connected to the board when
the end product is assembled. Ideally, any components to be added or con-
nections to be made after the board is manufactured will be provided
sockets to allow solderless assembly of the final product.
Whether a die is attached directly to the main circuit board, packaged
and then attached, or packaged and attached to a daughterboard, some
level of packaging is essential to being able to make use of a semicon-
ductor chip. For microprocessors, which often operate at high frequen-
cies and high-power levels, packaging design choices have as much
impact on their use as silicon design choices.

Package Design Choices

The processor designer must make many choices when selecting a pack-
age. These choices will determine limits on not only the processor die