DfM Navigation Tools for PCB Success


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By Nolan Johnson, Sunstone Circuits

IPC reports that of the key decisions that can improve design for manufacture (DfM) and design for yield (DfY), 75% of the impact of those decisions will be made before the design leaves the CAD tool environment. With the realization and confirmation that designers have so much influence over ultimate yield, it only makes sense to give designers as much useable information as possible to improve the assembly's ultimate success.

The current trend in PCB design tool development points in this direction, bringing manufacturing-related information upstream in the design process. Historically, the process of building out a DfM protocol within a design team has been a largely human endeavor. Senior team leads, for example, work with the junior designers to pull together as much expert knowledge, practical understanding, and collaborative engineering culture as possible. With this collection of knowledge, the team then builds a set of practices a map that help create designs that are predictable in their overall performance. While this human factor in building up a DfM process persists, it no longer is sufficient alone.

Design teams increasingly are moving beyond static process maps, and seeking out the equivalent of an interactive "process GPS" device. Here, DfM tools come into play. Just as the GPS device allows the driver to add more detailed information and alternate views, thereby improving the driving decision-making process, so do interactive DfM tools bring manufacturing details and added information into the PCB design decision-making process.

GPS for PCB DesignersThe role of DfM/DfY is to provide a feedback loop for the engineering and design team. The DfM feedback loop brings manufacturing-side knowledge back to the engineer to be incorporated into engineering's typical exercise in balancing constraints. Just like a GPS navigation system for your design, you can have a prescribed route that the DfM feedback helps you follow. Or, you can use DfM feedback as a general map of keep-out areas. Either way, user of these tools can see more than is visible "on the glass," whether the glass is a windshield or a computer monitor.

It comes as no surprise that, within the world of circuit design, verification tools historically push forward the cutting edge for DfM. What was considered state-of-the-art for DfM 20 years ago now would be considered traditional design rule knowledge: trace-and-space, shorts, opens, etc. This functionality is not more complex than a spell-check utility. Today's complexities require more detailed design analysis. In ICs, for example, the need to close the loop between parasitic effects and the original design's timing specifications requires use of advanced DfM technology. Likewise, in the PCB environment, component information, electrical rules, and thermal requirements play an important role in the current trend.

Tool UsageIn the PCB DfM space, as with ICs, a traditional and common tool solution has been to use a standalone verification tool that delivers post-design checking functionality. While these post-processor-style tools should identify and fix design issues without further involvement from the engineer, they identify design issues only after design completion. Once the layout phase concludes, the time and expense involved in fixing even simple errors can become quite large, and such costs are prompting the trend toward interactive checking functionality. IC designers began the move to an interactive, check-as-you-go model in the early 1990s, and similar functionality increasingly is available in PCB design tools.

Modern standalone DfM tools function best as a pre-check for manufacturing, not necessarily as a GPS-style decision-making tool for designers and engineers. Manufacturing-specific knowledge must make its way into the design tool environment, a feat that requires cooperation between the DFM tool and design tool developers. This sort of development work is underway in IC sectors, particularly within formal verification tools. IC software developers are cultivating more detailed and sophisticated ways to make all design and analysis engines DfM-/DfY-aware.

Online environments are one option, in which PCB designers and manufacturing specialists collaborate to share specialized knowledge to move designs forward. Knowledge, tools, manufacturing, and parts and part libraries are important to making GPS-style designing work. Effective, useful DfM for designers means open communication between all four categories of the design environment. The effect is increased access to information that better informs design decisions.

A DfM Case StudyStratford Digital provides electronic design services (EDS) to a range of industries and customers, and it benefits from a holistic and collaborative DfM approach. Projects range from simple PCBs, to entire embedded systems, to full design from scratch. They use a combination of an appropriate CAD tool and a DfM add-on to enforce design rules and conform to their manufacturing process. "Working through the layout process is simplified overall, if I know where I'll be building the board. If the manufacturer supplies the rules, then I can optimize to their capabilities. I can target the center of their process and avoid the outside edges," according to James Morrison, founder and senior hardware engineer.

In product design projects and layout jobs, Stratford Digital often sends designs to fabrication on behalf of its customers. "Typically, the currency of our customers is time to market. They don't have much spare budget room for quick-turn PCBs, but more importantly, they can't miss their market window. A quick, cost-effective prototype process helps with the monetary cost, and CAD integration ensure that time-to-market costs are minimized. A day lost due to a PCB not passing an incoming DfM check can be a huge hit to many of our customers," Morrison continues.

DfM tool creators are able to move that information back upstream into the design environment by capturing their process in the syntax of the CAD program. Morrison adds, "The more information we have in the design process, the more likely we will hit our first-time-right goals. Saving board spins is the number-one way to reduce time to market. We also can plug in to an assembly house and further shorten the delivery date. I can take projects from schematic design through to a fully populated and assembled prototype.

Emerging TrendsMultiple stakeholders in the PCB manufacture process can share their expert knowledge collaboratively. DfM analysis early in the project lifecycle always will pay dividends in assembly yield, especially with automated systems that greatly reduce the time involved up front.

Not every design team has immediate or sufficient access to design tools, however. Creative solutions such as open-license tools and distributor-supported bill of materials (BOM) checks offer different ways of streamlining the design-to-manufacture transition. Designers can expect chip manufacturing companies to get involved as well. Chip companies are responding to an increase in user pressure to provide parts libraries for design tools in addition to traditional datasheet publications. By supplying the parts, rather than compelling each designer to design their own, chip companies increase a user's design accuracy throughout the process. And, they increase the odds that the chip company's parts will be integrated into more designs. For chip companies, developing library parts as a service for their end users is technologically feasible now, and it offers a good return on a relatively small investment.

An Ongoing MovementA true DfM solution must be holistic. DfM solutions increasingly must include all stakeholders in the process by making meaningful information connections throughout the design flow. By bringing manufacturing and sourcing information into the design process, designers can perform much improved constraint management. In addition, designers are able to concentrate more on functional innovation and less on DfM management.

As with any developing and innovating industry, the movement toward a GPS-style interactive DfM solution for PCB designers is ongoing. As designers continue to adopt interactive DfM methodologies, an increasing palette of capabilities, choices, and even content providers will emerge.

Nolan Johnson, marketing manager, Sunstone Circuits, may be contacted at njohnson@sunstone.com.

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