OrCAD Whitepapers

Understanding DFM and Its Role in PCB Layout

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White Paper Understanding DFM and Its Role in PCB Layout Understanding DFM and Its Role in PCB Layout DFM, DRC, DFF, DFA, DFwhat? All terms we hear used daily in the PCB design world regarding manufacturing analysis and often used interchangeably. But what exactly is DFM and why is it an important but often ignored aspect of the PCB design process? Let's start by clarifying some terms. DFM is short for "Design for Manufacturability". It is the process of arranging a PCB layout topology to mitigate problems that could be encountered during the PCB fabrication and assembly processes required to manufacture an electronic system. Addressing fabrication issues is what's known as Design for Fabrication (DFF), and addressing assembly is known as DFA or Design for Assembly. The two together mostly make up DFM analysis. Mostly. In many cases the term DRC, which actually stands for Design Rule Checking also gets used interchangeably and creates further confusion regarding DFM. That's understandable because DRC issues detected in manufacturing can indeed have a direct impact on the manufacturability of a PCB. However, DRC is markedly different from DFF and DFA. Figure 1 These starved thermals pass electrical DRC, but in reality the connection to the actual source is insufficient for a good connection Think of DRC as a hard "pass/fail" detection of a problem in a PCB. Either a problem exists or it doesn't. In engineering, DRC is used to ensure that PCB layout connectivity accurately reflect the connectivity definedin a board's associated schematic diagram. But connectivity is only one aspect of DRC. The "R" stands for "Rules". The "Rules" are used largely to definethe minimum spacing allowed between various PCB objects for the entire PCB or for individual layers, nets or areas on the PCB. In engineering, the spacing may have direct impact on circuit performance. In manufacturing, spacing may play a pivotal role in the ability to fabricate or assemble a PCB. As a result, DRC becomes a subset of DFM, but only if the rules used reflec a manufacturer's requirements 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 specificdiscrep- ancies from the intended interconnect, DFM identifiesissues in the PCB topology that have the "potential" to create manufac- turing 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 matter how many PCBs are produced. By contrast, if the same PCB quantities contain DFM issues, problems may only manifest in some of the PCBs "Good DFM ensures that a design not only performs electrically as expected, but can be manufactured successfully in high volume quantities without increasing cost or risk, or adding unnecessary time to the design process" - Rick Almeida

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