50 DESIGN007 MAGAZINE I NOVEMBER 2020
3. Select the critical nets on the schematic,
fan-out, and then route with the auto-
router.
4. Push and shove the traces to the desired
location, move on to the next group of
nets, and repeat. Each group of routed
traces should be verified after completion.
Lock if necessary.
When you drive the router from the sche-
matic, it's possible to see what needs to be
done without entering too many conditional
design rules, and you can later manipulate the
traces as if they were hand-routed.
Once the routing is complete, apart from run-
ning design rule checks (DRCs), run a sanity
check on the board. You can either do this in
the simulation environment or the PCB data-
base. Simply highlight each net one by one.
This is tedious but gets results. You can quickly
see if any nets are longer than the Manhattan
length or spiral around the board before termi-
nation.
Key Points
• The autorouter is guided by design
constraints, and there are only so many
rules that can be practically defined.
Every situation is different, requiring
unique tradeoffs.
• The underlying high-speed issues of the
design need to be translated into corre-
sponding design constraints.
• A pre-layout simulation defines the extent
of placement.
• If the board is difficult to route, it may just
be the result of poor placement.
• The stackup configuration and PDN
needed to be addressed before
commencing routing.
• Cross-probing enables the PCB designer
to build up an extremely dense, complex
route in a couple of hours by controlling
the router from the schematic.
• Cross-probing can also be used as a
powerful search tool, locating parts and
nets on the schematic or PCB.
• One should avoid routing high-speed
signals on the outer microstrip layers of
a multilayer PCB. This can decrease
radiation by up to 10 dB. DESIGN007
Further Reading
• B. Olney, "Beyond Design: High-Speed PCB Design
Constraints," Design007 Magazine, May 2019.
• B. Olney, "Beyond Design: Embedded Signal Routing,"
The PCB Magazine, September 2011.
• B. Olney, "Beyond Design: Interactive Placement and
Routing Strategies," The PCB Design Magazine, December
2012.
• B. Olney, "Beyond Design: Routing Techniques for Com-
plex Designs," The PCB Design Magazine, January 2013.
• B. Olney, "Beyond Design: Critical Placement," The
PCB Magazine, September 2012.
Barry Olney is managing director of In-
Circuit Design Pty Ltd (iCD), Australia,
a PCB design service bureau that spe-
cializes in board-level simulation. The
company developed the iCD Design
Integrity software incorporating the
iCD Stackup, PDN, and CPW Planner. The software can be
downloaded at icd.com.au. To read past columns or con-
tact Olney, click here.
