Printed Circuit Design & Fab - March 2009 - (Page 26) dfm fIGure 2. What is the difference between Design Rule Checking and the full Manufacturing Compliance Rules? DRC is “Global” and represents the “design intent” while MCR are “per layer” and “object-to-object” oriented. An example of this subtle difference is shown in the use of an inset microvia next to a buried via. fIGure 3. Manufacturability is a “Shade of Grey. ” in the Constraint Editor. And like SI/PI analysis, models for MCR need to be established and maintained. Typically, these MCRs are grouped into common categories such as: ■ Proximity and clearances of signal traces ■ Proximity and clearances of holes and vias ■ Proximity and clearances of soldermask and legends ■ Spacing and characteristics of signal traces ■ Spacing and characteristics of planes and copper pours ■ Spacing and characteristics of holes, vias and cutouts ■ Spacing and characteristics of soldermask ■ Spacing and characteristics of silkscreen and legends ■ Spacing and characteristics of solderpaste The organization and maintenance of these MCR files is shown in fIGure 4. Figures 4a to 4l show some of the typical classification of these hazards. fIGure 4. Defining Manufacturing Compliance Rules and Clearances.1 Using Batch DFF as a Design Analysis Tool The design rule checking results are reviewed against the PCB manufacturing capability matrix, with dispositions being made to the acceptability of violations. The PCB customer may be contacted regarding design violations, to correct and retransmit the design or to permit design changes to be performed by the PCB manufacturer. Additionally, the factors reviewed during the initial design review are revisited to confirm the results based on design actuals. If differences are noted, product yields and/or cost predictions may need to be altered. During this step of tooling, several concerns are uppermost in the fabricator’s mind: Does the board meet the intent of the design? Can it be built? Will it function as planned? Does it pass a master rule set? Can it be made cheaper or better? Is there component interconnectivity? Does the IPC-D-356 netlist show any shorted nets or broken nets? Were any multiple plane ties not happening? Did a design work-around fool the system––or allow intentional errors to slip through? facturability of a board and its yield (as taBLe 1 describes and Figure 4.) Most modern CAM-tooling systems can correct these manufacturability issues, but it is better to find them during the layout process so that valuable time is saved! Thus, the manufacturing notes on the fabrication drawings should acknowledge this and allow for the fabricator to implement corrections. Other, less frequent, artwork errors discovered include embedded components and RF shapes. Critical Inspection Points – What To Focus On The manufacturability of the PCB can be improved through design enhancements. Many of the enhancements can be performed on CAD systems (or through specialized postCAD software); other enhancements can be performed at the manufacturing site. Ideally, all design enhancements would be performed at the design site to maintain the consistency of products from multiple manufacturers. Some of the DFM enhancements can impact the CAD system’s capability to provide further design changes. Specifically, the optimization of track-to-feature spacing could result in blocked routing channels, preventing an autorouter from performing. The taBLe 2 shows typical DFM enhancements. When it is appropriate to conduct various manufacturing compliance analysis, they are best performed after: ■ Placement of components. ■ Routing and simulation with signal integrity constraints. MARCH 2009 Bare-Board Analysis. Several physical artwork problems are not discovered by design rule checking and are not the result of design rule violations. Nonetheless, these problems can affect the manu26 PRINTED CIRCUIT DESIGN & FAB
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