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 (Fig. 1).
"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.
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