Hacking the Xbox

Appendix C - Getting Into PCB Layout 231
Circuit Boards Make Fine Heatsinks
When routing high-power components, such as power regulators and highperformance
microprocessors, remember that the copper in a circuitboard is
an excellent conductor of heat. You can save yourself a heat sink under
certain conditions by simply laying out a large region of copper connected to
the heat slug or ground pins of the target part. If you are using a multilayer
board design with power planes, use multiple vias to help conduct heat into
the internal layers.
The heatsinking capabilities of a circuitboard can also be a nuisance during
hand assembly. The good thermal conductivity of copper makes it difficult
to heat up a component pin that is also connected to a large region of
copper. When connecting low-power components to the power planes,
consider using vias with thermal reliefs. A thermal relief is a set of small gaps
in a via connection to a power plane that reduces thermal conductivity
without significantly impacting the connection’s electrical performance. (Note
that a large group of densely packed thermally relieved power vias around a
region of copper can result in unconnected or poorly connected islands of
copper.)
Establish Preferred Routing Directions for
Each Layer
Establishing a dominant routing direction for each layer can simplify the
routing of dense boards. For example, make the top layer the horizontal
routing layer, and the bottom one the vertical routing layer. If you need to
route a signal between two components located diagonally across the board,
first run a horizontal trace on the top and then a vertical trace on the bottom
to connect the two components. The alternative strategy of just running a
trace diagonally across the top layer of the board, for example, reduces the
overall routability between the two halves of the board by one-half: The
only way to get from one half to the other is now to go on the bottom.
Exceptions to this rule are acceptable, especially if you have to make a signal
integrity versus routability trade-off.
Stack a Board with Orthogonal Layers
After establishing the preferred routing directions for each layer, stack
the layers such that no two layers have parallel preferred routing directions.
This orthogonality helps keep the interference of signals between
layers to a minimum. If you have power layers, try to stack them between
layers to help shield interference between signal layers.
On Two-Layer Boards, Use Fingers to Bus
Power
On a two-layer board, it is often tempting to just run power and ground
as a ring around the outside of the board. This is not an ideal situation,