34 DESIGN007 MAGAZINE I SEPTEMBER 2018
Key Points
• Simulation of a PCB design early in the
design process is essential
• Pre-layout simulation leads to an enhanced
perception of what might be a potential
issue once the system is built
• It is imperative to have a constraint system
that is integral to your design flow
• Detailed interconnect routing constraints
should be established before starting
placement and routing
• The prime objective behind setting
constraints up front is to ensure that the
design is going to perform to expectations
• Reuse existing rules when possible to
eliminate the need for re-entry of any
common constraints
• A constraint management system that can
check work in real time as the design
process is executed provides reduced risk
and peace of mind
Further Reading
• Beyond Design: Pre-Layout Simulation by Barry Olney, The
PCB Magazine, July 2012.
• Beyond Design: Split Planes in Multilayer PCBs by Barry
Olney, The PCB Design Magazine, March 2015.
• Beyond Design: Plane Crazy, Part 1 by Barry Olney, The PCB
Design Magazine, December 2015.
• Beyond Design: Interactive Placement and Routing Strate-
gies by Barry Olney, The PCB Design Magazine, December 2012.
• Beyond Design: Controlling Emissions and Improving EMC
by Barry Olney, The PCB Design Magazine, August 2011.
• 5 Ways to Assess PCB Constraint Management Systems
for High-Speed Designs, EMA Design Automation Blog.
• Why impose PCB design constraints? by Steve Hughes of
Mentor, a Siemens Business.
Barry Olney is managing director of
In-Circuit Design Pty Ltd (iCD), Aus-
tralia, a PCB design service bureau
that specializes in board-level simu-
lation. The company developed the
iCD Design Integrity software incor-
porating the iCD Stackup, PDN, and CPW Planner. The
software can be downloaded from www.icd.com.au. To
read past columns or contact Olney, click here.
checks (DRCs), which are adhered to through-
out the design cycle, provide for an error-free,
correct-by-construction design when carefully
planned early in the design phase.
A robust constraint management system
should be unified and integrated into the sche-
matic and layout editors with cross-probing,
including:
• Integrated topology-driven constraint
definition
• Controlled impedance rules, including
spacing and clearance to other nets
• Length or time-based constraints
• Electrical constraints with custom
equation capability
• Physical trace and spacing rules
(class-to-class rules, etc.)
• Region and technology rules
• Same net DRCs for advanced technologies
(HDI)
• PCB fabrication, assembly, and testability
rules (if required)
Although all designs are different, one should
reuse existing rules where possible as a template
to eliminate the need for re-entry of any common
constraints. These include DDRx, PCIe, USB,
SATA, Ethernet, differential pair rules, manufac
-
turing requirements, etc. This also minimizes
the possibility of input errors.
When constraints are defined at the begin-
ning of the design process, designers can be
confident that once the design phase is com-
pleted, the product will move through the pro-
duction phases with fewer errors. A constraint
management system that can check work in
real time as the design process is executed pro-
vides reduced risk and peace of mind that the
constraints will prevent the PCB designer from
violating the rules set up front. Although we
cannot plan for the unexpected, having a reli-
able constraint management system in place
will ensure that the majority of issues are
covered and will alert you if any unforeseen
issues arise.
Stay tuned for next month's column that will
continue to elaborate on the 10 fundamental
rules of high-speed PCB design.
