Chapter 7
Autorouting and Autoplacement
©
National Instruments Corporation
7-7
Understanding the Four Fundamental Routing Functions
The following sections describe the four fundamental routing functions
used by Ultiboard.
Router Preprocessing
Before routing begins, Ultiboard analyzes the entire board, considering
trace widths, clearances, Keep-in, and Keep-out areas. The preprocessor
then adapts the router parameters and algorithms best suited for this
particular board, for example, routing grid size, pin clearance algorithms,
and connection routing order.
Note
The routing algorithms use their own internal routing grid, which is not related to
Ultiboard’s grid.
Initial Routing
Initial routing automatically routes as many traces as possible in a single
pass without any rip-up and retry operations.
Ultiboard uses trace hugging to place traces as close together as possible,
and close to obstacles. It also uses copper sharing where appropriate, for
example, in a T-junction.
Rip-up and Retry Routing
The rip-up and retry router attempts to route all open connections. In some
cases, Ultiboard can place all traces in a single pass without rerouting any
traces. However, if Ultiboard does not achieve 100 percent completion,
then the rip-up and retry router rips up selected traces and reroutes them to
make room for other traces that could not be placed during the first pass.
The rip-up and retry router is guarded by a backtracking algorithm, which
not only prevents routing deterioration or deadlock during rip-up and
optimization, but is able to exploit better routing solutions. The rip-up and
retry router automatically activates an intermediate optimizer if a single
rip-up pass does not achieve 100 percent routing success.
Part of Ultiboard’s rip-up and retry procedure is to analyze dense board
areas and adopt routing strategies in these areas, which decrease
manufacturing costs. The cost factors Ultiboard uses can be adapted to your
current routing problem; thus, you can control the routing strategies that
Ultiboard uses and keep production costs within acceptable limits. It is
strongly recommended that you do not modify these routing strategies
