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Guidelines for Ensuring PCB Manufacturability

Rules for designing printed-circuit boards (PCBs) have changed little over the years, although modern circuit boards are being asked to deliver more performance and more functionality in smaller and smaller pieces of real estate. The need for smaller, more-complex circuit boards for smartphones, computer tablets, and other hand-held electronic devices continues to grow and drive the need for smaller and more highly integrated PCBs. As a result, the use of Design for Manufacturability (DFM) plays a more important role than ever in the electronic production process. With the continued miniaturization of products, many designs push the physical limits of PCB manufacturability, inviting poor performance or even failure. But by following some straightforward guidelines, it is possible to apply DFM methods to increase the manufacturability of even these smaller, more complex circuit boards.

Addressing the issue of design tools early in a manufacturing process can help to prevent manufacturability problems. The use of DFM plans ahead for the manufacturing process, considering yield and other manufacturing issues that affect cost and quality.

Designers can use multiple design tool types to create DFM-optimized designs. Each tool type is best suited to a particular usage. When usage requirements call for the best performance with the most thorough rule checking, standalone DFM tools are generally preferred since these tools offer the most extensive capabilities for finding DFM rule-set violations. Unfortunately, such back-end tools can also form bottlenecks that slow manufacturing processes. When use requirements call for the most efficient design process, interactive DFM tools provide increased efficiency, although using such tools can also interrupt the design process. Interactive DFM tools provide insight into how to design for improved yield or manufacturability, helping designers to make more robust design decisions.

Back-End DFM Tools
For a process that does not employ a real-time DFM tool, integrating a back-end DFM tool into the design workflow can be a benefit. A back-end tool can identify what's expected to be a large number of design violations, warnings, and suggestions. To manage the information that is returned from a back-end tool, three common processes are available.

The first option involves the use of fewer full-functionality, full-design checking runs and longer result-review phases. Such an approach is thorough and identifies more production errors overall, but can still suffer the risk of missing a critical error. The second option involves running full-functionality check on portions of a design, with one full-design check at the end of a run for a final check. This approach is commonly used for reviewing modular designs, although the technique is less comprehensive than the first approach and can miss errors and violations. The first option involves the use of more review iterations using focused-functionality runs, with more short design reviews. This third option is the most effective approach of the three options, offering just-in-time rule checking and minimizing chances of performing rework on a design.

Even for a design process that has been built around a DFM philosophy, pitfalls can occur. By better understanding these potential process problems, they can be overcome and invaluable process time can be saved. For example, a common process problem is poor communication with a manufacturer, which can be overcome by establishing close communications between a design firm and its manufacturer. A manufacturing company should be treated as a partner, capable of providing invaluable insight into how design and manufacturing decisions can impact cost, manufacturability, yield, and final product quality.

Mismatched Rule Sets
Another process problem is a mismatch between DFM rule sets at a design firm and its manufacturer. From the beginning, it should be confirmed that a design company and its manufacturer are using consistent DFM rule sets and component footprint files. The same should be done between a design company and its suppliers, since the accuracy of the footprint files on a circuit board can also affect a design's success. Close communication with suppliers helps ensure that a design incorporates up-to-date footprint files.

Yet another potential process problem involves electrical performance problems in completed boards. To avoid costly circuit redesigns, computer simulation tools should be used to check the electrical performance of a design against other target requirements, such as size and yield.

Another potential process problem is when a design that has been successfully implemented with one manufacturer fails when produced by another manufacturer. When using a rapid prototyping service for PCB manufacturing, for example, that service might have specific process requirements. Good manufacturers can help make sure that circuit-board parameters will fit with specifications unique to a particular rapid prototyping service. Another process problem can be with tolerance violations, which can be avoided by checking tolerances at every level, both with parts and with manufacturing processes.

Gauging Manufacturer Capabilities
Not all manufacturers have the same capabilities. The largest number of manufacturers will have the capabilities to handle a relatively simple, conservative circuit design, whereas a more limited number of manufacturers will successfully produce a more challenging, complex circuit design. Knowing the capabilities of different manufacturers can help avoid expensive rework for a project that might be a bad match for a particular manufacturer.

When evaluating different manufacturers, it can help to consider DFM rule set offerings, front-end expertise, and support capabilities at each manufacturer. When evaluating DFM rule sets, for example, a manufacturer should be contacted early in a manufacturing process to confirm that a design firm is using DFM rules matching a manufacturer's specifications. When checking front-end expertise, it can help to learn about a manufacturer's internal process for moving a CAD file to production. Such issues to consider include what kind of DFM checks they perform upon receiving a CAD file, how robust is their quality-assurance (QA) process, how do they resolve design challenges, and what is their scrap rate. Manufacturers that offer a robust DFM process can compensate for gaps in a customer's DFM expertise. Finally, when evaluating support capabilities at a manufacturer, a responsive support staff at a manufacturer can help solve manufacturing issues during the design process rather than after it. A potential manufacturer's support capabilities should be fully assessed before they are selected for a project.

The key to optimizing the manufacturability of a PCB design is to choose tools, processes, and a manufacturing partner with capabilities that meet the specific needs for manufacturing that PCB. Implementing DFM practices earlier in the manufacturing process rather than later can aid the success of a manufacturing process.

Where possible, a PCB design tool with interactive DFM rule-checking capability should be used. If that is not an option, a design process should be defined for multiple partial-functionality checks rather than a single full-functionality check at the end of the design cycle. In addition, parts suppliers and the PCB manufacturer should be treated as members of the design team, with constant communications maintained with these partners throughout the design cycle. The use of computer simulations can help avoid expensive product rework at the production stage.

The right manufacturing partner can make it easier and less expensive to produce a working prototype. Look for a manufacturer that provides free, up-to-date DFM rule sets and product support that meet specific needs for accessibility and expertise. By finding out more about a manufacturer's DFM and QA processes, greater confidence can be placed in that manufacturer's capabilities to produce a PCB with high quality and controlled cost.

 

Contact: Sunstone Circuits, 13626 S. Freeman Road, Mulino, OR 97042 503-829-9108 E-mail: njohnson@sunstone.com Web:
www.sunstone.com

 
 
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