17 Know Your Tools

Beware that your CAD program must be setup correctly to define the appropriate pad and via stacks to support different types of vias. The additional process steps may require additional gerber files.

Your CAD manual and board fabricator will be your best friends as your tools are setup, so plan to learn and use all available supporting documentation as well as your extended team members.

Here are some precautions and checks you might consider after your board has been sent through the autorouter:

• Too many vias – Power routing without constraints can result in some traces becoming overly long with too many vias. This can adversely impact the impedance of the subsequent trace, resulting in excessive noise pickup.
• Insufficient trace width – Be sure that power traces have adequate width. In the BeagleBoard example, notice the use of wide traces for all power nets, especially on the inner layers. Inner layer power traces are 10 mils on the BeagleBoard.
• Use the power planes for routing – Although not always mentioned, the power planes can be used for routing. This can be seen on the ground layer of the BeagleBoard.
• Add additional planes on other layers – The BeagleBoard has low routing density on several planes allowing for copper pours for other power planes on these layers.
• Use multiple power planes on the power plane. Three different power rails are routed on the power plane layer and each one is an individual copper pour. Each power rail has its own set of bypass capacitors even though they share the same layer.

NOTE

It is strongly recommended that all vias be covered, especially under BGA packages. In addition, the board vendor should be consulted prior to specifying via fill to establish their manufacturing requirements, required design files, and the needed documentation.