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24 DESIGN007 MAGAZINE I MAY 2019 To begin with, every designer needs a set of well-established design rules to base the con- straints on. IPC has provided the electronics in- dustry with guidelines for designing and man- ufacturing PCBs compiled over the years with the support of both committee and industry members. The IPC-2220-FAM: Design Standard for Printed Boards series is the bible for PCB de- signers. The series is built around IPC-2221B— the base document that covers all generic re- quirements for PCB regardless of materials. From here, the designer chooses the appropri- ate sectional standard for a specific technology. The IPC-2220-FAM series includes: • IPC-2221B: Generic Standard on Printed Board Design • IPC-2222A: Sectional Design Standard for Rigid Organic Printed Boards • IPC-2223C: Sectional Design Standard for Flexible Printed Boards • IPC-2224: Sectional Standard for Design of PWBs for PC Cards • IPC-2225: Sectional Design Standard for Organic Multichip Modules (MCM-L) and MCM-L Assemblies • IPC-2226A: Sectional Design Standard for High-density Interconnect (HDI) Printed Boards This series provides coverage on material and final finish selection, current-carrying ca- pacity and minimum electrical clearances, test- specimen design, guidelines for V-groove scor- ing, dimensioning requirements, and conduc- tor thickness requirements. Also, several documents apply to HSD and land-pattern design: • IPC-2141A: Design Guide for High-speed Controlled Impedance Circuit Boards • IPC-2251: Design Guide for Electronic Packaging Utilizing High Speed Techniques • IPC-7351B: Generic Requirements for Surface Mount Design and Land Pattern Standard These standards (and their predecessors) have been part of a well-used section of my technical library since 1987. They provide ex- cellent reading and reference material for all PCB designers. These documents are available for purchase from Design rules must keep up with the latest de- vices and fabrication processes without losing sight of design for manufacturability (DFM). DFM is the practice of designing board prod- ucts that can be produced in a cost-effective manner using existing manufacturing process- es and equipment. If you follow the above IPC guidelines, you will be designing for both man- ufacturability and mass production. Howev- er, at times, one must stretch the rules a little to meet the specific requirements of a design. This is fine, providing you can justify the rea- sons and tolerate the consequence of your de- cision. Entry-level EDA tools tend to rely on the skills of the PCB designer to detect possible issues as they arise during the design process. However, these days, a more constraint-driven, correct- by-construction approach is required for com- plex designs. Once the rules are established, they will be followed by downstream tools and validated to conform by the various design rule checkers (DRCs). A spreadsheet format is more efficient if you are dealing with a high volume of constraints, sorting, filtering, and duplicat- ing constraints. Constraint management: • Enables better synchronization between schematic and layout • Streamlines access to relevant PCB data • Eliminates errors due to data integrity issues • Promotes greater reuse of PCB data Figure 2 illustrates typical constraints plan- ning and definition for a high-speed DDR2 and DDR3 design. Constraints should be defined at the schematic level and flow through to the layout process. The advantage of this approach is that the engineer can convey their intent to the PCB designer without misinterpretation. Alternatively, the independent engineer (who does everything) can manage the constraints

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