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12 The PCB Design Magazine • March 2015 UnDERSTAnDInG DFM AnD ITS ROLE In PCB LAyOUT continues feature As a result, DRC becomes a subset of DFM, but only if the rules used reflect a manufacturer's re- quirements for spacing. Otherwise, DRC is used solely for electrical verification. DFM's two primary components, DFF and DFA, are more nuanced than DRC. While DRC detects very specific discrepancies from the in- tended interconnect, DFM identifies issues in the PCB topology that have the potential to create manufacturing problems. What's more, a DRC defect will be present in every copy of the PCB built, so if there is a short missed in DRC, every PCB will contain the short, no mat- ter how many PCBs are produced. By contrast, if the same PCB quantities contain DFM issues, problems may only manifest in some of the PCBs while others perform correctly as expect- ed. For example, a PCB layout containing very thin pieces of copper created in the design tool by rule would be correct per the schematic. And if spaced properly it would pass DRC. However, that same sliver, being so thin, could poten- tially detach on the physical PCB and inadver- tently connect itself to other copper elements during assembly, thus creating shorts on some PCBs but not on others. So, the sliver would pass DRC verification, but in real-world manu- facturing the sliver could cause some PCBs to fail. Without DFM, this problem would go on undetected and would result in scrap or rework. Until recently, DFM analysis was either left to the PCB fabricator or assembly engineer to manage, or it was performed by companies that had the financial resources to purchase high- end DFM analysis software and support a dedi- cated staff to run DFM analysis. Most PCB de- signers would perform only a DRC analysis and visual inspection of the design before submit- ting the design to be manufactured. Manufac- turers know that DFM issues such as acid traps, slivers and starved thermal pad connections can decrease manufacturing yields and increase costs, and they take it upon themselves to ana- lyze the design, often making modifications to ensure that the design can be built with maxi- mum yields and lowest costs. So long as the fin- ished PCB functioned properly, the design engi- neer was content. Figure 1: These starved thermals pass electrical DRC, but in reality the connection to the actual source is insufficient for a good connection.

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