Like many other commodity items that go into
system hardware, printed-circuit boards (PCBs) have
evolved significantly over the years. Since their invention
circa 1936 by Paul Eisler, an Austrian engineer working in
England on radio sets, PCBs have served as the central
nervous system for most electronics assemblies. They
took over when circuit complexity became too much for
earlier point-to-point construction techniques.
Along the way, PCBs became substantially more complex, which is largely a function of the nature of the
devices they harbor. The EDA industry was largely borne
of that growing complexity and the need to automate
the process.
For quite a while, it was sufficient for circuit designers
to do their job in isolation and then toss the finished product "over the wall" to a PCB designer. Then that designer
would toss a Gerber layout file over yet another "wall" to a
board-fabrication house.
With the proliferation of large ball-grid-array (BGA) programmable devices, high-density interconnects (HDIs),
and timing-critical differential-pair signaling links, such
an approach to PCB design is now a roadmap to disaster.
Some broad best practices, though, will help ensure successful design without the delays, expense, and aggravation of respins.
CONCEPTUAL STAGE
The first element of PCB
design is the concept stage. At this point, circuit designers can, and should, collaborate with PCB designers on
technology evaluation. This evaluation will consider such
questions as:
- Which components to use?
- What packages will they be housed in, and what kinds
of pin counts and pinouts would they have?
- What will the PCB's layer stackup consist of; i.e., how
many layers should it have based on cost/performance
tradeoffs?
- What are the performance targets for parameters such
as clock frequency and signaling speeds?
At this stage, designers also must consider elements
such as the board's bus architecture and whether it will be
serial or parallel. They'll also have to consider their impedance-matching strategy, should impedance mismatches
cause reflections, ringing, and other undesirable artifacts.
ENTER CONCURRENCY
Many of these concerns
raise a key point in successful board design, and that's
communication. "PCB design isn't a single-person effort
anymore, but a collaborative team effort between groups of
engineers," says Ed Duranty, senior applications engineer
at Zuken USA.
The theme of communication runs through the PCB
design process. Circuit design teams must clearly communicate their design intent to PCB design teams. They also
must engage in the process with a clear understanding of
what their PCB design tools can and cannot achieve.
"Life can be made much easier, or much harder, for
downstream operations as a result of the circuit designers' understanding of what the tools do and what they
support," says Duranty.
In addition, due to the rising complexity of board routing and increased signaling
rates, PCB design is best
approached concurrently as
opposed to a traditional serial flow ().
"It's been common for part
research and selection to be
an isolated phase from the
rest of the flow, and likewise
the schematic-capture, simulation, and layout stages to
be isolated as well," says
Bhavesh Mistry, product marketing engineer at National
Instruments' Electronic
Workbench Group.
Thus, it behooves designers to seek out tools and
flows that facilitate data sharing. This is the only way geographically dispersed design teams can leverage parallel
efforts and reduce the overall design-cycle time.